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universe-review.catitle>Mono-cell Organismsuniverse-review.ca/title>
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universe-review.caa href="form.htm" target="default.htm">universe-review.catd width="10%" align="center">universe-review.cab>Contactuniverse-review.ca/b>universe-review.ca/td>universe-review.ca/a>universe-review.ca/tr>universe-review.ca/table>
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universe-review.cah2>universe-review.cafont color="blue">Unicellular Organismsuniverse-review.ca/font> universe-review.ca/h2>
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universe-review.cah3>Contentsuniverse-review.ca/h3>
universe-review.caa href="universe-review.ca#origin/default.htm#origin/">Origin of Lifeuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#virus/default.htm#virus/">Prion and Virusesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#organic/default.htm#organic/">Organic Chemistryuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#carbohydrates/default.htm#carbohydrates/">Carbohydratesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#lipids/default.htm#lipids/">Lipidsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#nucleotides/default.htm#nucleotides/">Nucleotidesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#amino/default.htm#amino/">Amino Acidsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#macromolecules/default.htm#macromolecules/">Energy Requirementuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#DNA/default.htm#DNA/">DNAuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#RNA/default.htm#RNA/">RNAuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#proteins/default.htm#proteins/">Proteins and Enzymesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#cells/default.htm#cells/">Cellsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#Ychromosome/default.htm#Ychromosome/"> The Y Chromosomeuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#genomes/default.htm#genomes/">Genomesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#microbiology/default.htm#microbiology/">Microbiologyuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#archaebacteria/default.htm#archaebacteria/">Archaebacteria (Ancient Bacteria)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#bacteria/default.htm#bacteria/">Bacteriauniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#protista/default.htm#protista/">Protista (Unicellular Eukaryotes)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#footnotes/default.htm#footnotes/">Footnotesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#references/default.htm#references/">Referencesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#Index/default.htm#Index/">Indexuniverse-review.ca/a>universe-review.cabr />
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universe-review.cah3 align="center">universe-review.caa name="origin">Origin of Lifeuniverse-review.ca/a>universe-review.ca/h3>
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universe-review.catd width="70%">There is no unanimous agreement on a theory about the origin of life.   Any record would have been erased during the long history of turmoil on the Earth's surface.  However, it is known that it must have happened between 4.5 - 3.7 billion years ago when the Earth's crust solidified and 
flecks of universe-review.caA NAME="bio-carbon">universe-review.ca/A>bio-carbon (organisms favour Cuniverse-review.casup>12universe-review.ca/sup> over Cuniverse-review.casup>13universe-review.ca/sup>) was found in universe-review.caa href="I10-20-Isua.jpg">Isua, Greenlanduniverse-review.ca/a>.  An artist's impression about the events leading to the origin of life and thereafter is drawn in Figure 11-01.universe-review.cabr />universe-review.cabr />
Some relevant information about this subject is presented in the followings:universe-review.cabr />universe-review.cabr />
universe-review.cali>The Environment - Four and a half billion years ago, the universe-review.caA NAME="proto-Earth">universe-review.ca/A>universe-review.caa href="F09-earth.htm#beginning">proto-Earthuniverse-review.ca/a> was completing its formation.  It was still covered with a thick layer of molten lava.  A grazing collision occurred with the subsequent appearance of a transient ring around the Earth that rapidly become the universe-review.caa href="F07-planets.htm#Moon">Moonuniverse-review.ca/a>.  Small cometary impacts persisted until about four billion years ago.  The still dense atmosphere had time to stabilize and slowly cool, above oceans which were at first very hot, then tepid.  The number of lagoons and shallows, alternately covered and uncovered by the ebb and flow of the seas, was very great, because the tides were gigantic (with the Moon universe-review.ca/li>
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universe-review.catr>universe-review.catd width="30%">universe-review.cah4>Figure 11-01 Origin of Life universe-review.caa href="R11-01-originoflife.htm">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
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at least three times closer than at present). Figure 11-02a shows the Earth's atmospheric composition from 4.5  to 1 billion years ago.  The oxygen concentration started to rise at about 3.5 billion years ago with the proliferation of life.universe-review.caa name="i1102">universe-review.ca/a>universe-review.cabr />universe-review.cabr />
universe-review.catable border="0" width="100%">universe-review.catr>universe-review.catd width="30%">universe-review.caa href="I11-02-atmocompo.jpg">universe-review.caimg src="I11-02-atmocompo.jpg" name="Atmospheric Composition" alt="Atmospheric Composition" align="left" width="240"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="30%">universe-review.caa href="I11-02-materials.jpg">universe-review.caimg src="I11-02-materials.jpg" name="Prebiotic Materials" alt="Prebiotic Materials" align="left" width="210"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="40%">universe-review.cali>The Materials - The raw materials in the atmosphere of early Earth consisted mainly of nitrogen and traces of other molecules as shown in Figure 11-02a. This composition is markedly different from those exist in the atmosphere of the outer planets (see universe-review.caa href="F07-planets.htm#data">Table 07-01universe-review.ca/a>) and in the interstellar molecular clouds where hydrogen is the dominant constituent. Figure 11-02b shows       
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universe-review.catr>universe-review.catd width="30%">universe-review.cah4>Figure 11-02a Atmospheric Composition universe-review.caa href="I11-02-atmocompo.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="30%">universe-review.cah4>Figure 11-02b Prebiotic Materialsuniverse-review.caa href="I11-02-materials.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="40%">the progression from inorganic molecules to simple organic molecules, to more complex organic compunds and eventually toward life.
universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
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In 1953 universe-review.caA NAME="StanleyMiller">universe-review.ca/A>universe-review.caa href="I11-40-SMiller.jpg">Stanley Milleruniverse-review.ca/a> mixed substances such as water, molecular hydrogen, methane, and ammonia in a flask.  After passing electrical discharge as input energy to this mixture, the assembly rearranged into a host of universe-review.caa href="universe-review.ca#organic/default.htm#organic/">organic moleculesuniverse-review.ca/a> as shown in Figure 11-03 including amino and nucleic acids - the building blocks of life.  However, the result cannot be reproduced if carbon dioxide or molecular oxygen is added to the experiment.  Since the experimental environment is not exactly the same as the atmosphere of the early Earth (note the presence of COuniverse-review.casub>2universe-review.ca/sub>), it seems that those organic 
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universe-review.catr>universe-review.catd width="30%">universe-review.cah4>Figure 11-03 Prebiotic Chemistry universe-review.caa href="I11-02-prebiotic.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="70%">molecules were produced in either localized spots on Earth where the chemical composition may be different from the global environment, or they might come from outer space.  For example, the universe-review.caA NAME="Murchison">universe-review.ca/A>Murchison meteoriteuniverse-review.casup>universe-review.caa href="universe-review.ca#one/default.htm#one/">1universe-review.ca/a>universe-review.ca/sup> contains universe-review.caa href="I11-02-prebiotic2.jpg">similar organic matteruniverse-review.ca/a> universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
as produced in the experiment.  Note that glycine and alanine are the most abundant amino acids in both cases.  They are the simplest amino acids produced by the most stable codons GGC and GCC (see universe-review.caa href="universe-review.ca#genetic/default.htm#genetic/">genetic codeuniverse-review.ca/a>).  Recent research indicates that they are probably the earliest building blocks for life.  
universe-review.cap>Another essential ingredient in the Miller's experiment is water (the ocean in the flask).  It is the fluid that transports the molecular components from one place to another and facilitates the chemical reactions that keep life going. Water serves as a supporting and cleansing fluid, bearing nutrients to where they are needed and taking away wastes.  Furthermore, water is the only general purpose solvent for dissolving organic molecules.  If molecules are to be broken down and reconstructed in a controlled way, if information codes are to be translated into working molecules and if information storages are to persist over a long period, only water can satisfy the requirement of providing such a solution in the ranges of temperature and pressure on Earth.  
The high water content in our body has suggested to many biologists that life on Earth arose in the oceans. In fact, there is a rough correspondence between the content of such elements as calcium and potassium in seawater and in blood and tissues.  It is thought that living systems tend to incorporate the primitive environment, so that their internal surroundings would tend to resemble the familiar conditions of the early history of life, a possibility first glimpsed by the 19th century French physiologist Claude Bernard.universe-review.ca/p>
universe-review.cap>Now, even if water and the basic organic molecules are somehow available in the early Earth's environment, there were no universe-review.caa href="universe-review.ca#proteins/default.htm#proteins/">enzymesuniverse-review.ca/a> (themselves made of proteins) available in the primordial soup.  Usually these simple organic molecules cannot be strung together to form biological universe-review.caA NAME="polymers">universe-review.ca/A>polymers (such as proteins and nucleic acids) without the assistance from the enzymes.  But it has been shown that polymerization of amino acids can occur when exposed to dry heat.  Another possible way around this problem is to assume that inorganic catalysts such as the surface of some mineral can perform similar function. universe-review.ca/p>  
universe-review.cap>One of the diagrams in Figure 11-04a shows the universe-review.caA NAME="prebiotic">universe-review.ca/A>prebiotic world just before the occurrence of polymerization with pieces of universe-review.caa href="universe-review.ca#amino/default.htm#amino/">amino acidsuniverse-review.ca/a> (in red segments), universe-review.caa href="universe-review.ca#lipids/default.htm#lipids/">fatty acidsuniverse-review.ca/a> (in green rib-like shapes), universe-review.caa href="universe-review.ca#carbohydrates/default.htm#carbohydrates/">ribose sugarsuniverse-review.ca/a> (the pentagons), and the universe-review.caa href="universe-review.ca#nucleotides/default.htm#nucleotides/">nucleotidesuniverse-review.ca/a> (the hexagons) mixed together; and occasionally linked up to form primitive polymers.  The universe-review.caA NAME="pre-RNA">universe-review.ca/A>pre-RNA world in the next diagram shows a short strand of nucleic acids (a nucleotide chain) undergoing binary fission with mutation.  However, it is estimated that even a short strand will take too long to form with random encounters.  universe-review.ca/p>
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universe-review.catd width="30%">universe-review.cali>The Transition - There is no record on the transition from lifeless chemical activity to organized biological metabolismuniverse-review.casup>universe-review.caa href="universe-review.ca#two/default.htm#two/">2universe-review.ca/a>universe-review.ca/sup>. It is only known that there are several components essential to the process:     
 universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="70%">universe-review.cah4>Figure 11-04a Transition from Non-life to Life universe-review.caa href="I11-07-transition.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4> universe-review.ca/td>
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universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>   
universe-review.caul>universe-review.caul>
universe-review.cali>The universe-review.caa href="I11-05-emzyme.jpg">Enzymesuniverse-review.ca/a> - Enzymes are required to accelerate the chemical reactions necessary for organizing the random population of molecules into self-sustaining metabolic cycles.universe-review.ca/li>
universe-review.cali>The universe-review.caa href="universe-review.ca#gene/default.htm#gene/">Genesuniverse-review.ca/a> - RNA molecules capable of replicating themselves have been synthesized in sterilized laboratory.  The same process is actually very difficult to achieve in the primordial soup where other closely related nucleotide analogues are present.  They tend to join the polymers and render a cluttered product.  The results of thirty years of intensive chemical experimentation have shown that the prebiotic synthesis of amino acids is easy to simulate in a reducing environment (meaning the composition does not contain oxygen or hydrogen is present in the composition), but the prebiotic synthesis of nucleotides is difficult in all environments.universe-review.ca/li>
universe-review.cali>The universe-review.caa href="universe-review.ca#cells/default.htm#cells/">cellsuniverse-review.ca/a> - A space enclosed by a wall is a prerequisite to assemble the necessary chemicals in a confined volume.  It would keep the complex molecules inside but allow smaller molecules such as waste and nutrition to pass through. There are some macromolecules such as phospholipid, which can naturally form a tiny sphere bound by a membrane in water (the droplet is  called universe-review.caa href="I11-07-protocell.jpg">liposomeuniverse-review.ca/a>).  Various substances can be incorporated into the droplet until the right mixture is acquired to start up metabolism and reproduction.universe-review.ca/li> 
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universe-review.cap>Considerable debate in origin-of-life studies has revolved around which of these fundamental components came first - the original chicken-or-egg question.universe-review.ca/p>
universe-review.cap>An earlier scenario suggested the cells - enzymes - genes evolutionary sequence.  It proposed that life began by the successive accumulation of more and more complicated molecular populations within the droplets (the proto-cells).  It is a world of little "garbage bags" that only metabolize and reproduce themselves statistically.  Once the framework has been established, natural selection will operate to improve the quality of the catalysts and the accuracy of the reproduction.universe-review.ca/p>
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universe-review.catd width="30%">universe-review.caa href="I11-07-RNAworld.jpg">universe-review.caimg src="I11-07-RNAworld.jpg" name="RNA World" alt="RNA World" align="left" width="300"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="70%">A modern version has the order of the events reversed, e.g., genes - enzymes - cells.  It places self-replicating RNA at the beginning, enzymes appearing soon afterwards to start up the metabolic cycles, and cells appearing later to give the process cohesion.  This theory is closely related to the concept of "RNA world" (see Figure 11-04b).universe-review.cabr />universe-review.cabr />
The universe-review.caA NAME="RNAworld">universe-review.ca/A>RNA world postulates that in the beginning the universe-review.caa href="universe-review.ca#RNA/default.htm#RNA/">RNAuniverse-review.ca/a> molecules also performed the catalytic activities necessary to assemble themselves from a nucleotide soup.  The RNA molecules evolved in self-replicating patterns, using recombination and mutation to  explore new niches.  They then developed an entire range of enzymic activities.  At the next stage, RNA molecules began to synthesize the first proteins, which would simply be better enzymes than their RNA counterparts.  Finally, universe-review.caa href="universe-review.ca#DNA/default.htm#DNA/">DNAuniverse-review.ca/a> appeared on the scene, the ultimate holder of information copied from the genetic RNA molecules by reverse transcription.  RNA is then relegated to the intermediate role it has today - no longer at the center of the stage, displaced by DNA and the more effective protein enzymes. 
 universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="30%">universe-review.cah4>Figure 11-04b RNA World universe-review.caa href="I11-07-RNAworld.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4> universe-review.ca/td>
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universe-review.caul>universe-review.caa name="protocell">universe-review.ca/a>Starting in 1991,  biochemist universe-review.caa href="I11-07-Szostak.jpg">Jack Szostakuniverse-review.ca/a> (and his graduate students) has embarked on a project to recreate life in the laboratory from where Stanley Miller left off.  Followings is a summary of their accomplishments up till 2004:universe-review.cabr />universe-review.cabr />
universe-review.cali>By a process of artificial selection, a ribozyme (an enzyme made from RNA instead of protein) was created to copy up to 14 nucleotides from an RNA with an accuracy of roughly 97%.universe-review.ca/li>
universe-review.cali>Fatty acids were used to form bubbles (for cell membranes) known as vesicles.  These vesicles could grow and divide as they were forced through 100-nanometer-wide pores - an "early earth" simulation of the pores in rocks around hydrothermal vents.  It is also found that by adding a kind of clay known as montmorillonite to the solution of fatty acids, it sped up the rate of vesicle formation 100-fold.universe-review.ca/li>
universe-review.catable border="0" width="100%">universe-review.catr>universe-review.catd width="20%">universe-review.caa href="I11-07-origin.jpg">universe-review.caimg src="I11-07-origin.jpg" name="Proto-cell" alt="Proto-cell" align="left" width="190"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="80%">universe-review.cali>It was discovered that in a mixture of RNA, fatty acids and clay, the clay can cause nucleotides to spontaneously assemble themselves into RNA, which is then automatically trapped inside the fatty acid bubbles.  The result is something that resembles a cell: It has genetic material and water contained within a waterproof fatty-acid pouch.  Figure 11-04c is an image of these makeshift cells taken with an optical microscope and enhanced using fluorescent dye, reveals yellow bits of RNA inside spherical green vesicles.  They have not created life yet.  It will be their next step to assemble a system of ribozyme, RNA and vesicle.  This system has to grow, divide, and evolve  in order to make the transition to life.universe-review.ca/li>
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universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 11-04c Proto-cell universe-review.cabr />universe-review.caa href="I11-07-origin.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="80%">Once there's one example of a lab system that's evolving by itself, then the challenge is to build systems that can evolve under different conditions such as the high-pressure liquid hydrogen in Jupiter and Saturn.  universe-review.ca/p>
universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>universe-review.ca/ul>
Recent research in 2004 suggests that microbes have a better chance to survive (in their cosmic journey) on universe-review.caa href="I11-43-exobio.jpg">specks of dustuniverse-review.ca/a> than on boulders or stones.  The solar system could be surrounded by a large "bio-sphere" of frozen organisms floating on grains of rock all of which can wander in and out (of the solar system) quite easily.  In such scenario, the seeds of life on Earth came from outer space.  (The idea is similar to a 1969 science fiction: "universe-review.caa href="I11-44-Astrain.jpg">The Andromeda Strainuniverse-review.ca/a>" in which a satellite brought back deadly microbes from outer space. ...)  However, the question about the origin of life is still unanswered.  It only shifts the problem to somewhere else in the universe.universe-review.caa name="i1104">universe-review.ca/a>
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
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universe-review.cah3 align="center">universe-review.caa name="virus">Prion and Virusesuniverse-review.ca/a>universe-review.ca/h3>
We may not know exactly the kind of organisms existing at the beginning of life.  However, there are some organisms living today on the border line between non-life and life.
There is a protein called prion with about 250 units of amino acid sequences.  Its normal form is harmless.  However, when the prion protein is converted into the "wrong" conformation it acts as a template and induces the same conformational change in other 'healthy" prion proteins. Thus the prion can reproduce without the genes. They become "infectious" agents and cause many kinds of diseases including the "mad-cow" disease.  The normal 
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and misfolded configurations are shown in Figure 11-05.  In a healthy individual, the normal prion molecule (left) typically resides on the surfaces of cells, including neurons in the brain.  In an infected person or animal, the normal protein is converted into the misfolded prion, which accumulates in plaques that clutter the diseased brain.  The structure of the normal protein has been confirmed by nuclear magnetic resonance, whereas the structure of the misfolded protein is predicted from moleculear modeling techniques.  Validity of this "protein only" (no transmissible nucleic acids) hypotheses has been demonstrated by research in universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="25%">universe-review.cah4>Figure 11-05 Prion universe-review.caa href="I11-08-prion.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> 
universe-review.catd width="75%">2004 (Nature, 265, 319, 323; 18 March 2004). The existence of different universe-review.caa href="I11-45-prion.jpg">prion strainsuniverse-review.ca/a> has also been confirmed.  universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="20%">universe-review.caa href="I11-09-virus1.jpg">universe-review.caimg src="I11-09-virus1.jpg" name="Virus" alt="Virus" align="left" width="138"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="80%">The structure of viruses consists of a protein capsule containing DNA or RNA with 1000 - 200000 base pairs.  Figure 11-06a shows the virus known as bacteriophage that preys exclusively on bacteria.    In the spring of 2003 a new strain ofuniverse-review.caA NAME="coronavirus">universe-review.ca/A> universe-review.caa href="R11-08-coronavirus.htm">coronavirusuniverse-review.ca/a> (see Figure 11-06c) causes the "Severe Acute Respiratory Syndrome" (SARS), which is much harder to control than universe-review.caA NAME="influenza">universe-review.ca/A>influenza (Orthomyxovirus infection of the upper respiratory tract and lungs) or universe-review.caA NAME="commoncold">universe-review.ca/A>common cold (Rhinovirus infection of the upper respiratory tract).  universe-review.caA NAME="viroids">universe-review.ca/A>universe-review.caa href="I11-45-viroid.jpg">Viroidsuniverse-review.ca/a> are even simpler organism consisting only of a short chain of naked RNA containing 240 - 375 bp, there is no capsid to house the genetic material.  Viruses survive and reproduce by infecting a cell and commandeering the cellular synthetic machinery to make more viruses.  Then the viruses lyse (destroy) the cell and start the cycle over again. Figure 11-06b shows the replication process for the DNA virus.  After entering by endocytosis, the virus becomes uncoated.  The DNA then replicates more of its kind and 
universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 11-06a Virus universe-review.caa href="I11-09-virus1.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> 
universe-review.catd width="80%">  universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="20%">universe-review.caa href="I11-09-virus2.jpg">universe-review.caimg src="I11-09-virus2.jpg" name="DNA Virus" alt="DNA Virus" align="left" width="250"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="20%">universe-review.caa href="R11-08-coronavirus.htm">universe-review.caimg src="I11-26-coronavirus.jpg" name="RNA Retrovirus" alt="RNA Retrovirus" align="left" width="180"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="60%">simultaneously making new coating proteins.  These parts assemble to form more viruses, which exit from the host to infect more cells.  The RNA retrovirus (such as the coronavirus) does it somewhat differently because the genetic material is in the form of RNA. It has to undergo a reverse transcription to form cDNA (DNA copied off from the RNA), which is then integrated into the host DNA.  It commandeers the host's replication mechanism to make more RNAs, which in turn make more coating proteins for the final assembly of new viruses (see Figure 11-06c).
universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 11-06b DNA Virus universe-review.cabr />universe-review.caa href="I11-09-virus2.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> 
universe-review.catd width="20%">universe-review.cah4>Figure 11-06c RNA Retro- virus universe-review.caa href="R11-08-coronavirus.htm">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>
universe-review.catd width="60%">  universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="15%">universe-review.caa href="R11-15-evolution.htm">universe-review.caimg src="I11-09-virus3a.jpg" name="Virus Evolution" alt="Virus Evolution" align="left" width="170"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="85%">Since these simple organisms are parasites depending on a host to reproduce, it seems that they cannot be the living relic left over from the beginning of life.  It is difficult to imagine that a primeval organism, after lurking in odd corners of the Earth for billions of years, would miraculously find itself pre-adapted to the chemistry of modern cells. There are three theories of virus evolution including relatively recent origin from "running away" host RNA, and the very ancient existence related to the RNA world in the origin of life.universe-review.cabr />universe-review.cabr />
universe-review.caul>Theories of virus evolution (see Figure 11-06d):
universe-review.cali>Regressive Theory - It proposes that viruses arise from free-living organisms like bacteria that have progressively lost genetic information to the point where they become intracellular parasites dependent upon a host to supply the functions they have lost.universe-review.ca/li>
universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 11-06d Virus Evo- lution universe-review.caa href="R11-15-evolution.htm">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> 
universe-review.catd width="80%">universe-review.cali>Run-away RNA - It proposes that viruses arise from the host-cell RNA or DNA, which gain a self-replicative but parasitic existence.  One or a few genes (or the mRNA) acquires the ability to replicate and evolve independently of its host gene.universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.cali>Coevolution - This theory proposes that viruses originated and evolved along with the most primitive molecules that first contained self-replicating abilities.  While some of these molecules were eventually collected into units of organization and duplication termed cells, other molecules were packaged into virus particles that coevolved with cells and parasitized them.universe-review.ca/li>universe-review.ca/ul>
Recently in 2004, another theory proposes that the cell nucleus itself is of viral origin.  The advent of the nucleus, which differentiates eukaryotes from prokaryotes, cannot be satisfactorily explained solely by the gradual adaptation of prokaryotic cells until they became eukaryotic.  Rather the nucleus may have evolved from a persisting large DNA virus that made a permanent home within prokaryotes.  Some support for this idea comes from sequence data showing that the gene for a DNA polymerase (a DNA copying enzyme) in the virus called T4 (Figure 11-06a), which infects bacteria, is closely related to other DNA polymerase genes in both eukaryotes and the viruses that infect them.  This theory implies that virus has been in existence before the emergence of eukaryotic cells.  Indeed, huge numbers of viruses are constantly replicating and mutating.  They would be very much adoptive to the ancient as well as the modern environment.
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="organic">Organic Chemistryuniverse-review.ca/a>universe-review.ca/h3>
The basic units to build the modern cells involve organic chemicals such as carbohydrates, lipids, nucleotides, and amino acids.  The macromolecules for supporting life such as DNA, RNA, and proteins are assembled from these basic units.  The organized sum total of these molecules becomes a cell - a living entity.  Therefore, it is very difficult to understand the work of a cell without the knowledge of organic chemistry.
universe-review.catable border="0" width="100%">universe-review.catr>universe-review.catd width="30%">
universe-review.caimg src="I11-03-organic.jpg" name="Organic" alt="Organic" align="left" width="300" />universe-review.ca/td>
universe-review.catd width="70%">universe-review.cap>The advent of organic chemistry is often associated with the discovery in 1828 by the German chemist Friedrich W�hler that the inorganic, or mineral, substance called ammonium cyanate could be converted in the laboratory to urea, an organic substance found in the urine of many animals. Before this discovery, chemists thought that intervention by a so-called life force was necessary for the synthesis of organic substances. universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
W�hler's experiment broke down the barrier between inorganic and organic substances. Modern chemists consider organic compounds to be those containing carbon and one or more other elements, most often hydrogen, oxygen, nitrogen, sulfur, or the halogens, but sometimes others as well. universe-review.ca/p>
universe-review.catable border="0" width="100%">universe-review.catr>universe-review.catd width="55%">
universe-review.cap>The ability to form universe-review.caA NAME="covalent">universe-review.ca/A>universe-review.caa href="F12-molecule.htm#covalent">covalent bondsuniverse-review.ca/a> with other carbon atoms in long chains and rings distinguishes carbon from all other elements. This property of carbon, and the fact that carbon nearly always forms four bonds to other atoms, accounts for the large number of known compounds. At least 80 percent of the 5 million chemical compounds registered as of the early 1980s contain carbon.  The affinity of carbon for the most diverse elements does not differ very greatly - so that even the most diverse derivatives need not vary very much in energy content.  This ability allows the organic world to exist in a special form of thermodynamic stability. universe-review.cabr />universe-review.cabr /> 
universe-review.caa name="noneq">universe-review.ca/a>The electron configuration of the normal carbon atom has 2 electrons in energy level 2S and 2P respectively.  By supplying about 2 ev to a carbon atom, the 4 electrons in the 2S and 2P states are rearranged to the SPuniverse-review.casup>3universe-review.ca/sup> state (Figure 07a).  The four electrons in the SPuniverse-review.casup>3universe-review.ca/sup> state form the tetrahedral arrangement (Figure 07b) of universe-review.caa href="F12-molecule.htm#orbitals">orbitalsuniverse-review.ca/a> (probability distribution of electrons), 
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="25%">universe-review.caa href="I11-03-carbon1.jpg">universe-review.caimg src="I11-03-carbon1.jpg" name="SPuniverse-review.casup>3universe-review.ca/sup> State" alt="SPuniverse-review.casup>3universe-review.ca/sup> State" align="left" width="340"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="25%">universe-review.caa href="I11-03-carbon2.jpg">universe-review.caimg src="I11-03-carbon2.jpg" name="Tetrahedral Structure" alt="Tetrahedral Structure" align="left" width="190"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="50%">
which can form stable covalent bonds with other atoms.  This is the basic reason for pumping energy into biological system to maintain metabolism and cellular structure.  Therefore,  the biological system is said to be in a universe-review.caA NAME="non-equilibrium">universe-review.ca/A>non-equilibrium state.  The universe-review.caa href="universe-review.ca#StanleyMiller/default.htm#StanleyMiller/">electrical dischargeuniverse-review.ca/a> in Stenley 
universe-review.catr>universe-review.catd width="25%">universe-review.cah4>Figure 11-07a SPuniverse-review.casup>3universe-review.ca/sup> State universe-review.caa href="I11-03-carbon1.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> universe-review.catd width="25%">universe-review.cah4>Figure 11-07b Tetrahedral Structure universe-review.caa href="I11-03-carbon2.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>
universe-review.catd width="50%">Miller's experiment represents the enrgy input required to move the molecular configuration into the non-equilibrium state.  Note thatuniverse-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
this kind of hybrid orbitals do not exist in an isolated atom, but arise while that atom is interacting with others to form a molecule.  The tetrahedral configuration will dissolve once an associated constituent is removed.
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="carbohydrates">Carbohydratesuniverse-review.ca/a>universe-review.ca/h3>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="15%">universe-review.caa href="I11-10-sugar.jpg">universe-review.caimg src="I11-10-sugar.jpg" name="Sugars" alt="Sugars" align="left" width="140"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="30%">universe-review.caa href="I11-10-sugars.jpg">universe-review.caimg src="I11-10-sugars.jpg" name="Synthesis" alt="Synthesis" align="left" width="290"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="55%">
Carbohydrate is characterized by the presence of the atomic grouping H-C-OH, in which the ratio of H to O is approximately 2:1.  Because water has this same ratio of hydrogen atoms to oxygen atoms, hence the name carbohydrate, which 
universe-review.catr>universe-review.catd width="15%">universe-review.cah4>Figure 11-08a Sugars universe-review.cabr />universe-review.caa href="I11-10-sugar.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> universe-review.catd width="30%">universe-review.cah4>Figure 11-08b Synthesis and Hydrolysis universe-review.cabr />universe-review.caa href="I11-10-sugars.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>
universe-review.catd width="55%">means hydrates of carbon, was given to them. If the number of carbon atoms in a compound is low (from 3 to 7), then the carbohydrate is a simple universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.catable border="0" width="100%">universe-review.catr>universe-review.catd width="49%">
sugar, or monosaccharide. Larger carbohydrates are created by joining monosaccharides as shown in Figure 11-08b. Figure 11-08a shows a 5-carbon sugar called universe-review.caA NAME="ribose">universe-review.ca/A>ribose, which is a component of RNA (universe-review.caA NAME="deoxyribose">universe-review.ca/A>deoxyribose has one less oxygen atom attached to the second carbon atom, hence the name DNA); and a 6-carbon sugar called universe-review.caA NAME="glucose">universe-review.ca/A>glucose. The small numbers count the carbon atoms, which is important in specifying the carbon atom linkage (to other atom or group of atoms). Figure 11-08b shows the synthesis and hydrolysis (dissociation) of glucose.  Polysaccharide is a carbohydrate that contains a large number of universe-review.ca/td>universe-review.catd width="2%"> universe-review.ca/td>universe-review.catd width="49%">monosaccharide molecules.  There are 3 polysaccharides that are common in organisms: starch, glycogen, and cellulose.  Glucose is used as an energy source in cells.  universe-review.caA NAME="starch">universe-review.ca/A>Starch and universe-review.caA NAME="glycogen">universe-review.ca/A>glycogen are storage form of glucose in plant and animal cells, respectively, and universe-review.caA NAME="cellulose">universe-review.ca/A>cellulose is found in plant cell walls.  Naturally occurring sugars are all right-handed. Its mirrored version, i.e., the universe-review.caa href="I11-10-Lglucose.gif">left-handed sugaruniverse-review.ca/a> can be produced artificially, but cannot be digested by living organism (making it a good but expensive dietary sugar).  They are called universe-review.caA NAME="chiral">universe-review.ca/A>chiral objects that cannot be superimposed on each other. universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="lipids">Lipidsuniverse-review.ca/a>universe-review.ca/h3>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="25%">universe-review.caa href="I11-11-fatty.jpg">universe-review.caimg src="I11-11-fatty.jpg" name="Fatty Acids" alt="Fatty Acids" align="left" width="210"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="35%">universe-review.caa href="I11-11-fat.jpg">universe-review.caimg src="I11-11-fat.jpg" name="Fat" alt="Fat" align="left" width="290"/>universe-review.ca/td>
universe-review.catd width="40%">
Lipids are a heterogeneous collection of compounds that share only one property: they are easily dissolved in organic solvents but can only hardly or not at all be dissolved in water. They include fats and oils, phospholipids, steroids, glycolipids, and waxes. 
universe-review.catr>universe-review.catd width="25%">universe-review.cah4>Figure 11-09 Fatty Acids universe-review.cabr />universe-review.caa href="I11-11-fatty.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> universe-review.catd width="35%">universe-review.cah4>Figure 11-10 Fat universe-review.caa href="I11-11-fat.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>
universe-review.catd width="40%"> universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
The basic units for fat are universe-review.caA NAME="fatty">universe-review.ca/A>fatty acids either saturated (in solid form) or unsaturated (in liquid form, the good one to prevent the deposits of cholesterol and fat on the lining of blood vessels; unsaturated compounds can undergo addition reactions with various reagents that cause the double or triple bonds to be replaced with single bonds).  Each fatty acid has a long chain of carbon atoms with hydrogens attached, and it ends in an acid group (COOH) as shown in Figure 11-09.  A universe-review.caA NAME="fats">universe-review.ca/A>fat (or an oil and sometimes also called a triglyceride) is formed when one molecule of glycerol reacts with 3 fatty acids as shown in Figure 11-10.  universe-review.caA NAME="glycerol">universe-review.ca/A>Glycerol is a compound with 3 hydrates of carbon.  A fat is nonpolar, i.e., the molecule has no groups that can be ionized and become charged.  It is the long-term energy source.  Since it contains more C-H bonds and less oxygen than carbohydrates, lipids can store twice as much energy.  This is why all animals (and some plants) use them for energy storage and respited after supplies of carbohydrates are exhausted.universe-review.cabr />universe-review.cabr />
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="40%">universe-review.caa href="I11-11-phospholipids.jpg">universe-review.caimg src="I11-11-phospholipids.jpg" name="Phospholipids" alt="Phospholipids" align="left" width="350"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="60%">universe-review.cap>universe-review.caA NAME="phospholipids">universe-review.ca/A>Phospholipids, as their name implies, contain a phosphate group POuniverse-review.casub>4universe-review.ca/sub>universe-review.casup>-universe-review.ca/sup>.  Essentially, phospholipids are constructed as fats are, except that in place of the third fatty acid, there is a phosphate group or a grouping that contains both phosphate and nitrogen (Figure 11-11).  These molecules are not electrically neutral as are the fats because the phosphate group can be ionized.  
universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="40%">universe-review.cah4>Figure 11-11 Phospholipids universe-review.caa href="I11-11-phospholipids.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>
universe-review.catd width="60%">universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
Therefore, the phospholipids have a nonpolar region that is not soluble in water and a polar region that is soluble.  Most of the lipids in the cell membrane are phospholipids.  Each phospholipid molecule has a polar head and 2 nonpolar tails.  When surrounded by water, universe-review.caa href="I11-11-lipolayer.jpg">phospholipid moleculesuniverse-review.ca/a> form a bilayer naturally.  The heads, being polar, are attracted to the water (universe-review.caA NAME="hydrophillic">universe-review.ca/A>hydrophillic), which is also polar; therefore, the heads face outward. The nonpolar tails face inside, away from the water (universe-review.caA NAME="hydrophobic">universe-review.ca/A>hydrophobic).  Some of the lipids in the cell membrane are glycolipids.  universe-review.caA NAME="glycolipids">universe-review.ca/A>Glycolipids are constructed similarly to phospholipids except the polar head consists of a chain of sugar molecules.  Glycolipids only occur in the outer half of the bilayer, where they function in cell-to-cell recognition.  Different types of cells have different glycolipids. universe-review.cabr />universe-review.cabr />
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="30%">universe-review.caa href="I11-12-steroids.gif">universe-review.caimg src="I11-12-steroids.gif" name="Steroids" alt="Steroids" align="left" width="300"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="70%">
universe-review.caA NAME="steroids">universe-review.ca/A>Steroids are lipids that have entirely different structures than fats (see Figure 11-12).  Molecules such as hormones, vitamin D, bile acids, and cholesterol are examples of steroids in the body.  Steroids are found in plant and animal food sources; however, cholesterol is derived only from animal sources. Hormones are used to regulate chemical in body, vitamin D is important for bone and teeth formation, bile acids is digestive fluid for the absorption of fats, and cholesterol is important to the body as a constituent of cell membranes, and is involved in the formation of bile acid and some hormones. Cholesterol is associated with heart and blood vessel diseases because it collects on the inside of vessel walls and restricts blood flow.
universe-review.ca/td>universe-review.ca/tr> 
universe-review.catr>universe-review.catd width="30%">universe-review.cah4>Figure 11-12 Steroids universe-review.caa href="I11-12-steroids.gif">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.cabr />universe-review.caA NAME="waxes">universe-review.ca/A>Waxes are found in many plants and animals.  Coatings of carnauba wax on fruits and the leaves and stems of plants help to prevent loss of water and damage from pests.  Waxes on the skin, fur, and feathers of animals and birds provide a water-proof coating.  Properties of some waxes are listed in Table 11-01 below.universe-review.ca/li>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="100%">
universe-review.caimg src="I11-13-waxes.jpg" name="Waxes" alt="Waxes" align="left" width="700" />universe-review.cabr />universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd>universe-review.cah4 align="center">Table 11-01 Properties of Waxesuniverse-review.ca/h4>universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="nucleotides">Nucleotidesuniverse-review.ca/a>universe-review.ca/h3>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="20%">universe-review.caa href="I11-14-DNAbases.jpg">universe-review.caimg src="I11-14-DNAbases.jpg" name="DNA Bases" alt="DNA Bases" align="left" width="150"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="20%">universe-review.caa href="I11-14-DNAsugars.jpg">universe-review.caimg src="I11-14-DNAsugars.jpg" name="DNA Sugars" alt="DNA Sugars" align="left" width="180"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="20%">universe-review.caa href="I11-14-nucleotide.jpg">universe-review.caimg src="I11-14-nucleotide.jpg" name="Nucleotide" alt="Nucleotide" align="left" width="145"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="40%">Nucleotide - The basic unit for DNA and RNA is the nucleotides, which consist of three components: the nitrogen bases, the ribose sugars, and the phosphates.   The nitrogen universe-review.caA NAME="bases">universe-review.ca/A>bases, which include the two purines, adenine (A) and guanine (G); and the two pyrimidines, cytosine (C) and thymine(T). RNA contains the same 
universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 11-13 DNA Bases universe-review.cabr />universe-review.caa href="I11-14-DNAbases.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>universe-review.catd width="20%">universe-review.cah4>Figure 11-14 DNA Sugars and Phosphate universe-review.cabr />universe-review.caa href="I11-14-DNAsugars.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>universe-review.catd width="20%">universe-review.cah4>Figure 11-15 Nucleotideuniverse-review.cabr />universe-review.caa href="I11-14-nucleotide.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4>universe-review.ca/td>universe-review.catd width="40%">bases, except thymine is replaced by uracil (U) (Figure 11-13).  universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.catable border="0" width="100%">universe-review.catr>universe-review.catd width="49%">
In RNA, the sugar is ribose while in DNA, the sugar is deoxyribose (no oxygen is bonded in the 2' carbon) (Figure 11-14).  And finally, there is the phosphate which forms part of the backbone (of the helix).  The combination of the base and sugar is called universe-review.caA NAME="nucleoside">universe-review.ca/A>nucleoside with the correponding products called (deoxy)adenosine, (deoxy)guanosine, (deoxy)cytidine, dexoythymidine and uridine.  The product is called nucleotide with the additional element of phosphate (Figure 11-15); the naming convention is to add universe-review.cabr />"5'-monophosphate" (5' indicates the 5universe-review.casup>thuniverse-review.ca/sup> carbon) at the end, e.g., "adenosine 5'-monophosphate".  The abbreviations are (d)AMP, (d)GMP, (d)CMP, (d)TMP, and UMP.  Any of the nucleotide such as AMP can bond to additional phosphate groups.  For example, universe-review.caA NAME="AMP">universe-review.ca/A>adding another phosphate to AMP gives ADP (adenosine 5'-diphosphate) and ATP (adenosine 5'-triphosphate) when there are a total of three phosphates.  ATP is a nucleotide that is used as a carrier of energy in cells.  Energy is released when ATP is universe-review.ca/td>universe-review.catd width="2%"> universe-review.ca/td>universe-review.catd width="49%">broken down to ADP and  phosphate.  As it will be explained further later, the energy package stored in the ATP serves to weld together the amino acid units in proteins and the nucleotide units in DNA and RNA, as well as the units in sugar and phospholipid molecules that abound in cells. The universe-review.caA NAME="cAMP">universe-review.ca/A>universe-review.caa href="I11-18-cAMP1.gif">cAMPuniverse-review.ca/a> (c for cyclic) used by universe-review.caa href="R10-18-slimemoulds.htm">slime moulduniverse-review.ca/a> as molecular signal is a compound made from ATP.  It is still used by more complex organisms for the same purpose. cAMP is widespread in animal cells as a universe-review.caa href="I11-18-cAMP2.gif">second messengeruniverse-review.ca/a> in many biochemical reactions induced by hormones. Upon reaching their target cells, the hormones activate adenylate cyclase, the enzyme that catalyses cyclic AMP production. Cyclic AMP activates a cascade of enzymes, which results in a thousand-fold response just from the binding of a single hormone molecule to a receptor on the cell membrane. Cyclic AMP is also involved in controlling gene expression, cell division, immune responses, and nervous transmission. universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="amino">Amino Acidsuniverse-review.ca/a>universe-review.ca/h3>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="25%">universe-review.caimg src="I11-15-aminoacid.jpg" name="Amino Acid" alt="Amino Acid" align="left" width="200"/>universe-review.ca/td>
universe-review.catd width="75%">The building blocks of proteins are amino acids.  An amino acid contains an universe-review.caA NAME="aminogroup">universe-review.ca/A>amino group (-NHuniverse-review.casub>2universe-review.ca/sub>), a universe-review.caA NAME="carboxylic">universe-review.ca/A>carboxylic acid group (-COOH), and a universe-review.caA NAME="side">universe-review.ca/A>side chain (R).  The carbon at the center is called the alpha-carbon (Figure 11-16).  Although there are many amino acids, only 20 different amino acids are present in humans.  The unique characteristics of the 20 amino acids are due to the side chain.universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="25%">universe-review.cah4>Figure 11-16 Amino Acid universe-review.ca/h4>universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="25%">universe-review.caa href="R11-04-aminoacids.htm">universe-review.caimg src="I11-15-aminoacids.jpg" name="Amino Acid 20" alt="Amino Acid 20" align="left" width="225"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="75%">Figure 11-17 shows the 20 amino acids.  universe-review.caA NAME="nonpolar">universe-review.ca/A>Nonpolar amino acids are not soluble in water, which makes them hydrophobic. universe-review.caA NAME="polar">universe-review.ca/A>Polar amino acids have hydrophilic side chain, which forms universe-review.caa href="F12-molecule.htm#hydrogenbond">hydrogen bondsuniverse-review.ca/a>universe-review.casup>universe-review.caa href="universe-review.ca#three/default.htm#three/">3universe-review.ca/a>universe-review.ca/sup> with water.  Acidic amino acids have side chains that can ionize as a weak acid.  The side chains of the basic amino acids contain an amino group that can ionize as a weak base.  The numbers at the bottom of each graph is the value of isoelectric point (pI).  The universe-review.caA NAME="isoelectric">universe-review.ca/A>isoelectric point is a value of universe-review.caa href="I11-16-pH.jpg">pHuniverse-review.ca/a> at which the amino acid gives an overall charge of zero and not accepting or donating any Huniverse-review.casup>+universe-review.ca/sup> ion in a solution.  The hexagon is the benzene ring Cuniverse-review.casub>6universe-review.ca/sub>Huniverse-review.casub>6universe-review.ca/sub>.  Amino acids on earth are all left-handed with the NHuniverse-review.casub>2universe-review.ca/sub> group to the left.  Essential (E in Figure 11-17) amino acids cannot be synthesized by the human body and must be provided through diet, while non-essential (NE) amino acids are synthesized by the body from carbon, nitrogen, hydrogen, oxygen, and sulphur.  
universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="25%">universe-review.cah4>Figure 11-17 The 20 Amino Acidsuniverse-review.ca/h4> universe-review.ca/td>universe-review.catd width="75%">universe-review.caa href="R11-04-aminoacids.htm">universe-review.cah4>[view large image]universe-review.ca/h4>universe-review.ca/a> universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>     
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="60%">universe-review.caa href="I11-15-peptide.jpg">universe-review.caimg src="I11-15-peptide.jpg" name="Peptide" alt="Peptide" align="left" width="500"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="40%">The bond that joins 2 amino acids is called a universe-review.caA NAME="peptide">universe-review.ca/A>peptide bond.  The NHuniverse-review.casub>2universe-review.ca/sub> and OH group at the end of the peptide are available for adding more amino acids to the chain. (Figure 11-18)universe-review.cabr />universe-review.caa href="I11-15-peptide.jpg">universe-review.cah4>[view large image]universe-review.ca/h4>universe-review.ca/a>
universe-review.catr>universe-review.catd width="60%">universe-review.cah4>Figure 11-18 Peptide Formation universe-review.ca/h4>universe-review.ca/td>universe-review.catd width="40%"> universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>     
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="macromolecules">Energy Requirementuniverse-review.ca/a>universe-review.ca/h3>
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Before we continue on to the subject of macromolecules, it is necessary to clarify the way the molecules become organized (gain of information) and how energy is used to drive the system to a non-equilibrium state as mentioned in universe-review.caa href="F01-introduction.htm#noneq">topic 1universe-review.ca/a>.
universe-review.cap>When energy (such as in a photon) is pumped into a chemical system, the energy partitions into thermal and electronic components.  The thermal component makes the molecules move faster, and the electronic component increases the number of "high-energy" electronic states.  Both energy components will foster molecular organization: the faster the molecules vibrate, rotate, and translate, and the more of them that are in electronic states above ground level, the higher is the probability that the molecules will interact and the more work can be done in organizing them.  However, there is a limit to that.  At very high energy levels all chemical bonds become inherently unstable, the molecular structures eventually fall to pieces.  This property draws the line to the energy input; it is impossible to make a macromolecule in one run from scratch.  It has to be made by supplying the required energy little by little.  The aggregates (such as DNA or protein) are created by joining the units one at a time.  This way each step of molecular synthesis could be driven by a separate and tolerable energy input.universe-review.ca/p>
Living organisms store photon energy in chemical form, and then trickle it down molecular chains to the individual molecular bonding sites.  The energy flux that organizes all living matter on our planet is so channeled as to first pump COuniverse-review.casub>2universe-review.ca/sub> and H in the atmosphere and water up to the level of universe-review.ca/td>universe-review.catd width="2%"> universe-review.ca/td>universe-review.catd width="49%">carbohydrate, namely glucose, and then to drop the level gradually from that reservoir back to the ground again.  This gradient drives nearly all work in the biomass, not just the making of macromolecules.  The smallest unit for this chemical (energy) currency is stored in the third phosphate bond of the ATP. universe-review.cabr />universe-review.cabr />
The flow starts with the capture of photons by certain molecules, such as the universe-review.caA NAME="chlorophyll">universe-review.ca/A>chlorophyll of plants and similar pigments of microorganisms according to the universe-review.caA NAME="photosynthesis">universe-review.ca/A>universe-review.caA NAME="respiration">universe-review.ca/A>photosynthesis reaction:universe-review.cabr />universe-review.cabr />
6COuniverse-review.casub>2universe-review.ca/sub> + 6Huniverse-review.casub>2universe-review.ca/sub>O + energy  ===>   Cuniverse-review.casub>6universe-review.ca/sub>Huniverse-review.casub>12universe-review.ca/sub>Ouniverse-review.casub>6universe-review.ca/sub> + 6Ouniverse-review.casub>2universe-review.ca/sub>
universe-review.cap>Respiration runs in the reversed direction.  While the energy input is carried by photons in photosynthesis, the energy output in respiration is distributed among a maximum of 38 ATPs.universe-review.ca/p> 
The photon energy is stored in the covalent bondsuniverse-review.casup>universe-review.caa href="universe-review.ca#three/default.htm#three/">3universe-review.ca/a>universe-review.ca/sup> of glucose -- about 6 quanta of photon in one glucose molecules.  From this reservoir, energy then flows along various pathways, nursing everything, all organization and all work.
The chemical energy chains that nurse macromolecular organization commonly use ATP as their final link.  Each package contains an energy of  7.3 kilocalories per mole (~ 0.3 ev/ATPuniverse-review.casup>universe-review.caa href="universe-review.ca#four/default.htm#four/">4universe-review.ca/a>universe-review.ca/sup>).  It is given off at the sites of amalgamation of the molecular building blocks -- one package for each site with spatial precision to where it is needed.universe-review.cabr />universe-review.cabr />universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.cap>The following chemical formula shows a simple case of molecular synthesis.  The 7.3 kilocalorie package of ATP is fed into the site of elongation, the last member of the carbon chain.  NHuniverse-review.casub>2universe-review.ca/sub> is added to the glutamate using the energy from the hydrolysis of ATP (into ADP and Pi, the phosphate).  The glutamine becomes an energized molecular system stably links a NHuniverse-review.casub>2universe-review.ca/sub> group to the chain.universe-review.ca/p> 
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universe-review.catd>universe-review.caimg src="I11-17-ATP.jpg" name="ATP" alt="ATP" align="left" width="700"/>universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.cap>Since aerobic respiration (energy-producing process with air) requires oxygen, the energy carrier ATP cannot be manufactured in the absence of this substance.  The aerobic cells and organism will soon expire because the metabolism will stop without energy supply.  The energized universe-review.caa href="universe-review.ca#covalent/default.htm#covalent/">covalent bondsuniverse-review.ca/a> would break down due to a variety of causes such as thermal agitation, chemical corrosion, biological degradation, and damage by radiations.  In addition, water would tends to hydrolysis many of the organic compounds.  Eventually, the organism would return to dust (the basic chemical components) just like an old house crumbling down to ruin.universe-review.ca/p>
universe-review.cabr />
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="DNA">DNAuniverse-review.ca/a>universe-review.ca/h3>
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universe-review.catd width="50%"> 
DNA is formed by joining together nucleotides with the phosphate groups link to the sugars at the 3' and 5' carbons.  This is the backbone held up by covalent bonds. The nitrogen bases are attached to the 1' carbon in the sugar.  The complementary DNA strand has the same kind of construction but running in opposite direction (with the 5-sugar pointing upside down).  The two strands are joined by weaker hydrogen bonds (H-O or H-N).  The pairing of the bases can occur only between universe-review.caA NAME="bases2">universe-review.ca/A>Adenine (A) and Thymine (T) or Guanine (G) and Cytosine (C). (See Figure 11-19.)universe-review.cabr />universe-review.cabr />
universe-review.cap>universe-review.caA NAME="replication">universe-review.ca/A>DNA replication occurs when the complementary strands of DNA break apart and unwind. This is accomplished with the help of enzymes called helicases. Additional enzymes and proteins attach to the individual strands, holding them apart and preventing them from coiling upon universe-review.ca/p>
universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="50%">universe-review.cah4>Figure 11-19 DNA Structureuniverse-review.ca/h4>universe-review.ca/td>universe-review.catd width="50%">themselves.  universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>universe-review.ca/li>
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universe-review.catd width="40%">universe-review.caimg src="I11-20-DNAreplication.jpg" name="DNA Replication" alt="DNA Replication" align="left" width="425"/>universe-review.ca/td>
universe-review.catd width="60%">
The point at which the double helix separates is called the replication fork, because of the shape of the molecule. At this site enzymes called DNA polymerases move along each of the separated DNA strands, adding nucleotides to the exposed bases according to the base pairing rules. The ribose-phosphate bonds form between the new nucleotides to hold the new strand together. The synthesis acquires energy via the removal of two phosphates from the triphosphate. The process continues until the original double helix is completely unwound and two new double helices have been formed. Each new double helix is composed of one old DNA strand and one new strand. This is described as semi-conservative replication. (See Figure 11-20.)
There is a small variation for the processing on the other strand and is lagging behind the leading strand.  The polymerase on the lagging strand adds bases to one section of the strand at one place, jumps ahead to add bases to a different section of the lagging strand. Then it may jump behind to add more. It jumps all over the place on the lagging strand to make base pairs. These small fragments 
universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="40%">universe-review.cah4>Figure 11-20 DNA Replicationuniverse-review.ca/h4>universe-review.ca/td>universe-review.catd width="60%">are joined together by DNA ligase.universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>universe-review.cabr />universe-review.cabr />
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="49%"> 
A universe-review.caA NAME="mutation">universe-review.ca/A>mutation is a change in the DNA nucleotide sequence that alters the sequence of amino acids, which would alter the structure and function of a protein in a cell.  Some mutations are known to result from X-rays, UV light, chemicals called mutagens, and possibly some viruses.  If a change in DNA occurs in a somatic cell, the altered DNA will be limited to that cell and its daughter cells.  If there is universe-review.ca/td>universe-review.catd width="2%"> universe-review.ca/td>universe-review.catd width="49%">uncontrolled growth, the mutation could lead to cancer.  If the mutation occurs in germ cell DNA, then all the DNA produced in a new individual will contain the same genetic change.  If the genetic change greatly affects the catalysis of metabolic reactions or the formation of important structural proteins, the new cells may not survive or the person may exhibit a genetic disease.universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="RNA">RNAuniverse-review.ca/a>universe-review.ca/h3>
universe-review.caul>RNA is similar to DNA with several important differences:universe-review.cabr />universe-review.cabr />
universe-review.cali>The sugar in RNA is ribose rather than the deoxyribose found in DNA.universe-review.ca/li>
universe-review.cali>The nitrogen base uracil replaces thymine.universe-review.ca/li>
universe-review.cali>RNA molecules are single, not double stranded.universe-review.ca/li>
universe-review.cali>RNA molecules are much smaller than DNA molecules.universe-review.ca/li>universe-review.ca/ul>
There are three major types of RNA  in the cells: messenger RNA (mRNA), which makes up about 75% of RNA; transfer RNA (tRNA), which makes up about 15% of the total; and ribosomal RNA (rRNA), which makes up the rest of 10%. 
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="30%">universe-review.caa href="I11-21-mRNA1.jpg">universe-review.caimg src="I11-21-mRNA1.jpg" name="mRNA Transcription" alt="mRNA Transcription" align="left" width="250"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="30%">universe-review.caa href="I11-21-mRNA2.jpg">universe-review.caimg src="I11-21-mRNA2.jpg" name="mRNA Construction" alt="mRNA Construction" align="left" width="250"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="40%">universe-review.caA NAME="mRNA">universe-review.ca/A>mRNA carries genetic information from the DNA in the nucleus to the ribosomes in the cytoplasm for protein synthesis.  Each gene, a segment of DNA, produces a separate mRNA molecule when a certain protein is needed in the cell, the mRNA is broken down quickly after translation.  The size of an mRNA depends on the number of nucleotides in the gene. 
universe-review.ca/td>
universe-review.catr>universe-review.catd width="30%">universe-review.cah4>Figure 11-21 mRNA Transcription universe-review.caa href="I11-21-mRNA1.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="30%">universe-review.cah4>Figure 11-22 mRNA Construction universe-review.caa href="I11-21-mRNA2.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
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universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
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universe-review.catd width="49%"> 
In the initiation stage, RNA polymerase binds to universe-review.caa href="R11-11-operon.htm">promotersuniverse-review.ca/a> and starts to unwind the DNA strands. In the elongation stage, RNA polymerase reads the DNA template stand from 3� to 5� and produces the RNA transcript from 5� to 3�. The nucleotides are always added to the 3� end of the growing RNA. In the final stage, the RNA polymerase reaches the termination site and the RNA universe-review.caA NAME="transcription">universe-review.ca/A>transcription, i.e., the messenger RNA is released from the template. (Figure 11-21.)  In eukaryotes, the genes contain sections known as exons that code for proteins, are mixed with sections universe-review.ca/td>universe-review.catd width="2%"> universe-review.ca/td>universe-review.catd width="49%">
called introns that do not code for protein.  A newly formed mRNA is called a pre-mRNA because it is a copy of the entire DNA template including the noncoding introns.  Before the newly synthesized pre-mRNA leave the nucleus, it undergoes processing to remove the intron sections.  The splicing of the pre-mRNA produces a mature, functional mRNA that leaves the nucleus to deliver the genetic information to the ribosomes for the synthesis of protein. (Figure 11-22)universe-review.cabr />universe-review.cabr />universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table> 
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="30%">universe-review.caa href="I11-21-tRNA1.jpg">universe-review.caimg src="I11-21-tRNA1.jpg" name="tRNA Illustration" alt="tRNA Illustration" align="left" width="250"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="30%">universe-review.caa href="I11-21-tRNA2.jpg">universe-review.caimg src="I11-21-tRNA2.jpg" name="tRNA Structure" alt="tRNA Structure" align="left" width="250"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="40%">universe-review.caA NAME="tRNA">universe-review.ca/A>tRNA, the smallest of the RNA molecules, interprets the genetic information in DNA and brings specific amino acids to the ribosome for protein synthesis.  Only the tRNA can translate the genetic information into amino acids for proteins.  There are one or more different tRNAs for each of the 20 amino acids.  The structures of the transfer RNAs are similar, consisting of 70-90 nucleotides.  Hydrogen bonds between some of the complementary bases in the chain produce loops that give some double-stranded regions. (See Figure 11-23.)
universe-review.ca/td>
universe-review.catr>universe-review.catd width="30%">universe-review.cah4>Figure 11-23 tRNA Illustration universe-review.cabr />universe-review.caa href="I11-21-tRNA1.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="30%">universe-review.cah4>Figure 11-24 tRNA Structure universe-review.cabr />universe-review.caa href="I11-21-tRNA2.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
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universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.cap>The actual structure of a tRNA is a three-dimensional L shape, (See Figure 11-24.) but it is often drawn as a cloverleaf to illustrate its features.  All tRNA molecules have a 3' end with the nucleotide sequence -- ACC, which is known as the acceptor stem.  An enzyme attaches an amino acid by forming an ester bond with the free -- OH at the end of the acceptor stem.  Each tRNA contains an anticondon, which is a series of three bases that complements the three bases on a mRNA.universe-review.ca/p>
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universe-review.catd width="20%">universe-review.caa href="I11-21-rRNA1.jpg">universe-review.caimg src="I11-21-rRNA1.jpg" name="rRNA Translation" alt="rRNA Translation" align="left" width="200"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="80%">universe-review.caA NAME="rRNA">universe-review.ca/A>rRNA makes up 65% of the structural material of the universe-review.caa href="I11-21-rRNA2.jpg">ribosomesuniverse-review.ca/a>; the other 35% is protein.  Ribosomes, which are the sites for protein synthesis, consist of two subunits, a large subunit and a small subunit.  Protein synthesis requires mRNA, tRNA, amino acids, ribosomes, ATP, and various protein factors.  These pieces come together at the beginning of translation, in a stage called translation initiation.universe-review.cabr />universe-review.cabr />universe-review.ca/td>
universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 11-25a rRNA Translation universe-review.caa href="I11-21-rRNA1.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="80%">
universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.caA NAME="translation">universe-review.ca/A>Translation begins when an mRNA molecule binds to a segment of rRNA that is part of a small ribosomal subunit.  The anticodon of a tRNA bearing methionine (met) bonds to the initiation codon (AUG) on the mRNA.  These bound structures form the initiation complex.  Next, a large ribosomal subunit binds to the complex, and a tRNA bearing a second amino acid bonds between its and the second mRNA's codon.  The amino acid brough in by the first tRNA bonds with the amino acid brought in by the second tRNA, and the first tRNA detaches and floats away.  The ribosome moves down the mRNA by 
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="20%">universe-review.caa href="I10-11-geneticode.jpg">universe-review.caimg src="I10-11-geneticode.jpg" name="Genetic Code" alt="Genetic Code" align="left" width="197"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="80%">one codon, and a third tRNA arrives, carrying another amino acid, (See Figure 11-25a.) ... and the process continues until a termination codon is reached.  Each of the three bases (the codon) in the mRNA is translated into an amino acid according to the universe-review.caA NAME="genetic">universe-review.ca/A>genetic code (see Figure 11-25b).  For example, an tRNA with bases CCG in the anticodon and amino acid Glycine (Gly) in the attachment site would bind to the condon GGC in the mRNA, the amino acid Glycine (Gly) would be added to the growing protein chain in the ribosome as a result of this combination.
universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 11-25b Genetic Code universe-review.caa href="I10-11-geneticode.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="80%">
universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.caA NAME="genexp">universe-review.ca/A>The process of gene expression starts from transcription of a gene to the production of a protein.  However, some genes inside the cell are harmful and should never be expressed.  The mobile genetic elements (jumping genes) migrate from spot to spot on the DNA; its expression will cause cancer or other diseases.  Similarly, the genes from viruses will hijack the cell's protein production facilities to crank out viral proteins.  Cells have ways of fighting back.  For example, the mammalian cells would deploy interferon response when viral genes enter a cell.  This response produces an enzyme known as PKR, which blocks translation of all mRNAs (normal and viral), and the enzyme RNAse L, which indiscriminately destroys all mRNAs.
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="40%">universe-review.caa href="R11-12-RNAi.htm">universe-review.caimg src="I11-38-RNAi.jpg" name="RNAi" alt="RNAi" align="left" width="350"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="60%">
In the past several years, scientists have discovered a more precise and - for the purposes of research and medicine - more powerful security apparatus built into nearly all plant and animal cells.  universe-review.caA NAME="RNAi">universe-review.ca/A>This system is called RNA interference, or RNAi, which acts like a censor.  When a threatening gene is expressed, the RNAi machinery silences it by intercepting and destroying only the offender's mRNA, without disturbing the mRNAs for the other genes.  RNAi also regulates the activity of normal genes during growth and development.
universe-review.ca/td>
universe-review.catr>universe-review.catd width="40%">universe-review.cah4>Figure 11-26 RNAi universe-review.caa href="R11-12-RNAi.htm">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="60%">
universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.cap>The gene-censoring mechanism is thought to have emerged about a billion years ago to protect some common ancestor to plants, animals and fungi against viruses and mobile genetic elements.  RNAi appears to work like this (as shown in Figure 11-26): Inside a cell, double-stranded RNA encounters an enzyme dubbed Dicer.  Using the chemical process of hydrolysis, Dicer cleaves the long RNA into pieces, known as short interfering RNAs, or siRNAs.  Each siRNA is about 22 nucleotides long.  The siRNA duplex is then unwound, and one strand of the duplex is loaded into an assembly of proteins to form the RNA-induced silencing complex (RISC).universe-review.ca/p>
universe-review.cap>Within the RISC, the siRNA molecule is positioned so that mRNAs can slide into it.  The RISC will encounter thousands of different mRNAs that are in a typical cell at any given moment.  But each siRNA of the RISC will adhere well only to a mRNA that closely complements its own nucleotide sequence.  So, unlike the interferon response, the silencing complex is highly selective in choosing its target mRNAs.universe-review.ca/p>
universe-review.cap>When a matched mRNA finally docks onto the siRNA, an enzyme know as Slicer cuts the captured mRNA strand in two.  The RISC then releases the two mRNA pieces (now rendered incapable of directing protein synthesis) and moves on.  The RISC itself stays intact, free to find and cleave another mRNA.  In this way, the RNAi censor uses bits of the double-stranded RNA as a "blacklist" to identify and mute corresponding mRNAs.universe-review.ca/p>
universe-review.cap>When the RNAi machinery is not defending against attack, it apparently pitches in to help silence normal cellular genes during developmental transitions needed to form disparate cell types, such as neurons and muscle cells, or different organs, such as the brain and heart.  The triggers are "microRNAs" - small RNA fragments that resemble siRNAs but differ in origin.  Whereas siRNAs come from the same types of genes or genomic regions that ultimately become silenced, microRNAs come from genes whose sole mission is to produce these tiny regulatory RNAs.  The RNA molecule initially transcribed from a microRNA gene - the microRNA precursor - folds back on itself.  With the help of Dicer, the middle section is chopped out of the microRNA, and the resulting piece typically behaves very much like an siRNA - with the important exception that it does not censor a gene with any resemblance to the one that produced it but instead censors some other gene altogether.universe-review.ca/p>
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="proteins">Proteins and Enzymesuniverse-review.ca/a>universe-review.ca/h3>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="25%">universe-review.caa href="I11-22-protein1.jpg">universe-review.caimg src="I11-22-protein1.jpg" name="Protein" alt="Protein" align="left" width="250"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="25%">universe-review.caimg src="I11-22-protein2.jpg" name="Protein" alt="Protein" align="left" width="165"/>universe-review.ca/td>
universe-review.catd width="50%">Proteins commonly have 3 levels of organization in their structure (Figure 11-27), but they can combine to form the fourth level (Figure 11-28).  The primary structure is the linear sequence of the amino acids joined by peptide bonds.  Any number of the 20 different amino acids can be joined in any sequence (only a certain sequences are useful to the organisms).  Any given protein has a characteristic sequence of amino acids.
universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="25%">universe-review.cah4>Figure 11-27 Protein universe-review.cabr />universe-review.caa href="I11-22-protein1.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> 
universe-review.catd width="25%">universe-review.cah4>Figure 11-28 Quarternary Structureuniverse-review.ca/h4>universe-review.ca/td>
universe-review.catd width="50%"> universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="49%"> 
The secondary structure of a protein comes about when the polypeptide chain takes a particular orientation in space.  One common arrangement of the chain is the alpha helix, or right-handed coil, with 3.6 amino acids per turn.  Hydrogen bonding between amino acids stabilizes the helix.  Another type of secondary structure is known as the universe-review.caa href="I11-22-protein4.jpg">beta-pleated sheetuniverse-review.ca/a>.  Such polypeptide chains are held together side by side by hydrogen bonds between the peptide chains.  A protein can consist of  alpha helix, beta-pleated sheet, or a universe-review.caa href="I11-22-protein5.jpg">mixtureuniverse-review.ca/a> of the two types.  The amino acids Alanine, Cysteine, Glutamic Acid, Glutamine, Histidine, Leucine, universe-review.ca/td>universe-review.catd width="2%"> universe-review.ca/td>universe-review.catd width="49%">Lysine, and Methionine are found in alpha helix region; while Arginine, Aspartic Acid, Asparagine, Proline, Serine, and Valine are found in beta-pleated sheets.
universe-review.cap>The tertiary structure of a protein is its final three-dimensional shape.  The tertiary shape of a protein is maintained by various types of bonding between the R groups.  Covalent, ionic, and hydrogen bonding are all seen.universe-review.ca/p>
When two or more polypeptide chain interweave to form one molecule the protein has a quarternary structure.universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.cabr />universe-review.cabr />universe-review.caa name="folding">universe-review.ca/a>The protein folds to the state of minimum energy.  (See Figure 11-29a)  Its sequence has to produce an unique configuration to be useful for living organism. The final shape of a protein is very important to its function.  When proteins are exposed to extreme heat and pH, they undergo an irreversible change in shape called denaturation.  The change occurs because the normal bonding between the R groups has been disturbed.  Once a protein loses its normal shape, it is no longer able to perform its usual function. universe-review.cabr />universe-review.cabr />
It is known that even if the gene can code a correct sequence of amino acids and the ribosome can translate the coding without error, the resulting protein can misfold and cause serious problem for the organism.  As shown in Figure 11-29, it seems that the repulsion between some key residues (the remainder of two or more amino acids combine to form a peptide) 
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="20%">universe-review.caa href="I11-22-protein6.jpg">universe-review.caimg src="I11-22-protein6.jpg" name="Protein Folding" alt="Protein Folding" align="left" width="200"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="20%">universe-review.caa href="I11-22-protein7.jpg">universe-review.caimg src="I11-22-protein7.jpg" name="Protein Misfolding" alt="Protein Misfolding" align="left" width="130"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="60%"> 
such as the hydrophobic and polar residues is essential to establish a rudimentary native-like architecture (the saddle point in the diagram).  Once the correct topology has been achieved, the native structure (the natural conformation of a protein) will then almost invariably be generated during the final stages of folding.  There are molecular chaperones in the cell to weed out the misfolded proteins as shown in Figure 11-29.  Failure of this quality-control system entails a variety of diseases including cancer, diabetes, universe-review.caa href="universe-review.ca#virus/default.htm#virus/">BSEuniverse-review.ca/a>, cystic fibrosis, Alzheimer, and Parkinson.  These "protein-misfolding diseases" share the common pathological feature of aggregated misfolded protein deposits.
universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="20%">universe-review.cah4>&nbsp;&nbsp;Figure 11-29 Protein Folding universe-review.cabr />&nbsp;&nbsp;universe-review.caa href="I11-22-protein6.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>
universe-review.catd width="20%">universe-review.cah4>and Misfolding universe-review.caa href="I11-22-protein7.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>
universe-review.catd width="75%">universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table> 
A strand of RNA such as the tRNA also trends to fold into a structure similar to a protein or enzyme.  This ability of the RNA has inspired the hypothetical RNA world in considering the origin of life.  The single-strand RNA can fold up to various shapes, depending on the sequence of its bases.  The three-dimensional structure results from hydrogen bonding between the complementary bases and between other bases. These forces twist the strand into a partial double helix with a tertiary structure.  When certain strategic bonds are broken, this usually stable structure untwists to a one-dimensional form, which is more suitable for information transfer.universe-review.cabr />universe-review.cabr />
universe-review.caa name="enzyme">universe-review.ca/a>An enzyme is a special kind of protein that can accelerate chemical reaction while retaining its own structure. A chemical reaction is about two molecules coming together and altering their structures.  Firstly they need a chance to approach each other, the frequency of encounter depends on the concentration of the reactants. Then they should have enough kinetic energy to overcome the potential barrier (activation energy), this energy is related to the temperature.  Finally, there is a special orientation of the reactants such that the reaction would proceed much faster, sometimes a million folds faster.  Such favourable condition can be created with a special material called universe-review.caa href="I11-05-emzyme.jpg"">enzymeuniverse-review.ca/a> or catalyst.  The enzyme forces the reactants into a position most suitable to execute the reaction.  The enzyme itself does not change and can be re-used again and again.  For inorganic chemical reactions, enzyme may not be necessary since the inorganic molecules have high degree of symmetry.  For organic chemical reaction, the symmetry for the molecules involved is much lower or none at all; therefore, most chemical processes in life depend on the assistance of the enzyme. universe-review.cabr />universe-review.cabr />
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="cells">Cellsuniverse-review.ca/a>universe-review.ca/h3>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="25%">universe-review.caa href="I10-05-cellorganelle.jpg">universe-review.caimg src="I10-05-cellorganelle.jpg" name="Cell" alt="Cell" align="left" width="250"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="25%">universe-review.caa href="I10-04-cellnucleus.jpg">universe-review.caimg src="I10-04-cellnucleus.jpg" name="Cell Nucleus" alt="Cell Nucleus" align="left" width="250"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="50%">universe-review.cap>There are two types of cells: prokaryotic and eukaryotic.  Prokaryotic cells have no nucleus and form unicellular  organisms such as bacteria.  The cells in protista, fungi, plants and animals are eukaryotic cells, which have a nucleus.universe-review.ca/p>
universe-review.cap>In a universe-review.caA NAME="eukaryotic">universe-review.ca/A>eukaryotic cell, the plasma membrane is a lipid bilayer that separates the materials inside the cell from the environment surrounding it.  The outer surface of the membrane universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="25%">universe-review.cah4>Figure 11-30 Eukaryotic Cell universe-review.cabr />universe-review.caa href="I10-05-cellorganelle.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td> 
universe-review.catd width="25%">universe-review.cah4>Figure 11-31 Cell Nucleus universe-review.cabr />universe-review.caa href="I10-04-cellnucleus.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>
universe-review.catd width="50%">contains structures that allow cells to communicate with each other.  universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
The nucleus contains the genes that control DNA replication and protein synthesis of the cell (Figure 11-31).  The cytoplasm consists of all the materials between the nucleus and the plasma membrane.  The cytosol, which is the fluid part of the cytoplasm, is an aqueous solution of electrolytes and enzymes that catalyze many the cell's chemical reactions.universe-review.ca/p>
universe-review.cap>Within the cells are specialized structure called universe-review.caA NAME="organelles">universe-review.ca/A>organelles that carry out specific functions in the cell.  The cell structure is shown in Figure 11-30,  the functions of the cell are shown in Table 11-02 below.universe-review.ca/p>   
universe-review.catable bgcolor="EBFBFB" border="1" width="100%" align="center">
universe-review.catr>
   universe-review.cath>STRUCTUREuniverse-review.ca/th>
   universe-review.cath>DESCRIPTIONuniverse-review.ca/th>
   universe-review.cath>FUNCTIONuniverse-review.ca/th>   
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">STRUCTURAL ELEMENTSuniverse-review.ca/td>
   universe-review.catd align="center"> universe-review.ca/td>
   universe-review.catd align="center"> universe-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">Cytosketetonuniverse-review.ca/td>
   universe-review.catd align="center">Network of protein filamentsuniverse-review.ca/td>
   universe-review.catd align="center">Structural support; cell movementuniverse-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">Flagella(cilia, microvilli)universe-review.ca/td>
   universe-review.catd align="center">Cellular extensionsuniverse-review.ca/td>
   universe-review.catd align="center">Motility or moving fluids over surfacesuniverse-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">Centriolesuniverse-review.ca/td>
   universe-review.catd align="center">Hollow microtubulesuniverse-review.ca/td>
   universe-review.catd align="center">Moving chromosomes during cell divisionuniverse-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">ENDOMEMBRANE SYSTEMuniverse-review.ca/td>
   universe-review.catd align="center"> universe-review.ca/td>
   universe-review.catd align="center"> universe-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">Plasma membraneuniverse-review.ca/td>
   universe-review.catd align="center">Lipid bilayer in which proteins are embeddeduniverse-review.ca/td>
   universe-review.catd align="center">Regulates what passes into and out of cell; cell-to-cell communicationuniverse-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">Endoplasmic reticulumuniverse-review.ca/td>
   universe-review.catd align="center">Network of internal membranes; forms compartments and vesiclesuniverse-review.ca/td>
   universe-review.catd align="center">Rough type processes proteins for secretion and synthesizes phospholipids; smooth type synthesize fats and steroidsuniverse-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">Nucleusuniverse-review.ca/td>
   universe-review.catd align="center">Structure bounded by double membrane; contains chromosomesuniverse-review.ca/td>
   universe-review.catd align="center">Control center of cell; directs protein synthesis and cell reproductionuniverse-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">Golgi complexuniverse-review.ca/td>
   universe-review.catd align="center">Stacks of flattened vesiclesuniverse-review.ca/td>
   universe-review.catd align="center">Modifies and packages proteins for export from cell; forms secretory vesiclesuniverse-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">Lysosomesuniverse-review.ca/td>
   universe-review.catd align="center">Vesicles derived from Golgi complex that contain hydrolytic digestive enzymesuniverse-review.ca/td>
   universe-review.catd align="center">Digest worn-out mitochondria and cell debris; play role in cell deathuniverse-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">ENERGY-PRODUCTING ORGANELLESuniverse-review.ca/td>
   universe-review.catd align="center"> universe-review.ca/td>
   universe-review.catd align="center"> universe-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">Mitochondriauniverse-review.ca/td>
   universe-review.catd align="center">Bacteria-like elements with inner membraneuniverse-review.ca/td>
   universe-review.catd align="center">Power plant of the cell; site of oxidative metabolism; synthesis of ATPuniverse-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">ORGANELLES OF GENE EXPRESSIONuniverse-review.ca/td>
   universe-review.catd align="center"> universe-review.ca/td>
   universe-review.catd align="center"> universe-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">Chromosomesuniverse-review.ca/td>
   universe-review.catd align="center">Long threads of DNA that form a complex with proteinuniverse-review.ca/td>
   universe-review.catd align="center">Contain hereditary informationuniverse-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">Nucleolusuniverse-review.ca/td>
   universe-review.catd align="center">Site of rRNA synthesisuniverse-review.ca/td>
   universe-review.catd align="center">Assembles ribosomesuniverse-review.ca/td>
universe-review.ca/tr>
universe-review.catr>
   universe-review.catd align="center">Ribosomesuniverse-review.ca/td>
   universe-review.catd align="center">Small, complex assemblies of protein, often bound to ERuniverse-review.ca/td>
   universe-review.catd align="center">Site of protein synthesisuniverse-review.ca/td>
universe-review.ca/tr>
universe-review.ca/table>
universe-review.cah4 align="center">Table 11-02 Cell Organizationuniverse-review.ca/h4>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="30%">universe-review.caa href="I11-25-chromosome.jpg"">universe-review.caimg src="I11-25-chromosome.jpg" name="Chromosome" alt="Chromosome" align="left" width="200"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="70%">
The universe-review.caA NAME="nucleus">universe-review.ca/A>nucleus is of primary importance in the cell because it is the control center that oversees the metabolic functioning of the cell and ultimately determines the cell's characteristics. Within the nucleus, there are masses of threads called chromatin, which is indistinct in the non-dividing cell, but it condenses to chromosomes at the time of cell division. Figure 11-32 shows the packed universe-review.caA NAME="chromosome">universe-review.ca/A>chromosome unwinding to a DNA strand.  The nucleolus is the specialized part of chromatin in which the ribosomal RNA (rRNA), is produced (Figure 11-31).universe-review.cabr />universe-review.cabr />
The universe-review.caA NAME="telomeres">universe-review.ca/A>universe-review.caa href="I11-27-telomere.jpg">telomeresuniverse-review.ca/a> lie at the tips of the chromosome.  They have hundreds to thousands of repeats of a specific 6-nucleotide DNA sequence.  The telomeres lose 50 to 200 of these nucleotides at each mitosis; gradually shortening the chromosome.  After about 50 divisions, a critical amount of telomere DNA is lost, which somehow signals the cell to stop mitosis.  The cell may remain alive for a while but is unable to divide further.  This is the universe-review.caA NAME="clock">universe-review.ca/A>cellular clock, which pre-determines the life span of the cell.universe-review.cabr />universe-review.cabr />
universe-review.caa href="I11-25-chromosome.jpg"">universe-review.cah4>[view large image]universe-review.ca/h4>universe-review.ca/a>
universe-review.ca/td>
universe-review.catr>universe-review.catd width="30%">universe-review.cah4>Figure 11-32 Chromosome, DNA  universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="70%"> universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="Ychromosome">The Y Chromosomeuniverse-review.ca/a>universe-review.ca/h3>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="25%">universe-review.caimg src="I11-35-Y1.jpg" name="X-Y Chromosomes" alt="X-Y Chromosomes" align="left" width="300"/>universe-review.ca/td>
universe-review.catd width="70%">
The 24 chromosome types in human cells are numbered from largest to smallest - 1 to 22.  Each type occurs in universe-review.caa href="R10-14-alleles.htm">allelic pairuniverse-review.ca/a>, with length ranged from 279 Mb (megabase) for chromosome 1 to 48 Mb for chromosome 22.  The exceptions are the X-Y chromosomal pair (see Figure 11-33); while the X chromosome has a length of 163 Mb, the Y chromosome is only 51 Mb long.  These two chromosomes determine gender (male or female) in birds and mammals.  There was a time, around 300 million years ago, when there was not a Y chromosome.  Instead, most animals had a pair of identical Xs and gender was determined by other factors, such as temperature (in some amphibians and reptiles, eggs still hatch out as males above a certain temperature and as females below it).  Then, in one of those dramatic evolutionary transformations that created the Y, a gene on an X chromosome in a particular mammal mutated.  It endowed a special feature to the carrier we now called male and survived by putting a block universe-review.ca/td>universe-review.ca/tr> 
universe-review.catr>universe-review.catd width="25%">universe-review.cah4>Figure 11-33 The X-Y Chromosomes universe-review.ca/h4>universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>  
on the process of swapping genes (universe-review.caa href="R10-12-meiosis.htm">crossoveruniverse-review.ca/a>) with the other X of its pair (otherwise it would have been weeded out).  Gradually, the X with the rogue gene was able to do less and less trading with its unaltered partner, and took on an identity of its own, as the Y chromosome.  Thereafter, the carrier of two X chromosomes is developed into female, while the one with a X-Y pair becomes male (for some reasons the reverse is true for birds).  In humans, the sexes look alike until the sixth week of prenatal development.  All embryos contain two-sided, unspecialized gonads (organs that will become either testes or ovaries) and two sets of tubes.  At the sixth week, one of two events occurs: cascades of a hormone (by the Y chromosome) steer development along a male route, or in the absence of this hormonal exposure, development continues along a female pathway (the default).  The human Y chromosome has been sequenced in 2003, a summary of the new genetic information can be found in the appendix - "universe-review.caa href="R11-14-Ychromosome.htm">Y chromosomeuniverse-review.ca/a>".
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="30%">universe-review.caa href="I11-35-Y3.jpg">universe-review.caimg src="I11-35-Y3.jpg" name="Y Chromosome 2" alt="Y Chromosome 2" align="left" width="312"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="70%">
Long before the sequencing of the Y chromosome in 2003, geneticist Ms. Jane Gitshier of the University of California, San Francisco  had already come up with her own map of the human Y chromosome as shown in Figure 11-34 (published in the August 1993 issue of Science).  It purported to have located genes for such stereotypically male traits as flipping between TV channels, interest in the sports pages of newspapers and an inability to express affection over the phone - among others.  The only thing wrong with the diagram is that these male behaviours come not from specific genes for each of them, but from the general masculinisation of the brain by hormones such as testosterone, which results in a tendency to behave this way in the modern environment. Boys are more competitive, more interested in machines, weapons and deeds.  Girls are more interested in people, clothes and words. Thus, in a sense, many masculine habits are all the products of the SRY gene itself, which sets in train the series of events that lead to the masculinisation of the brain as well as the body.  The evidence from zoology has always pointed that way: male behaviour is systematically different from female behaviour in most species and the difference has an innate component.  The brain is an organ with innate gender.universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="30%">universe-review.cah4>Figure 11-34 Y Chromosome, A Woman's View universe-review.ca/h4>universe-review.ca/td>
universe-review.catd width="70%">universe-review.cah4>universe-review.caa href="I11-35-Y3.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="genomes">Genomesuniverse-review.ca/a>universe-review.ca/h3>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="40%">universe-review.caa href="R11-05-genomes.htm">universe-review.caimg src="I11-23-genomes.jpg" name="Genomes" alt="Genomes" align="left" width="388"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="60%">universe-review.caa name="gene">universe-review.ca/a>A gene is the part of a DNA sequence containing information about the amino acid sequence of one protein.  Genes used to be studied one at a time, but with the invention of DNA sequencing machines it has become possible to consider the total DNA of an organism, usually referred to as its genomeuniverse-review.casup>universe-review.caa href="universe-review.ca#five/default.htm#five/">5universe-review.ca/a>universe-review.ca/sup>.  
The genomes of many bacteria consist of a single, circular chromosome.  Human and other animal cells have linear chromosomes.  An important feature of animal genomes is that much of the DNA does not code for genes.  The universe-review.caA NAME="junk">universe-review.ca/A>non-coding DNA, also known as junk DNA, consists mostly of the same few sequences repeated over and over again. They are often inserted within a region of coding gene.  The purpose of the noncoding DNA, if any, is not understood.  As much as 97% of human DNA is noncoding. Some research show that they might be used as testing site for genetic mutation; 
universe-review.ca/td>
universe-review.catr>universe-review.catd width="40%">universe-review.cah4>Figure 11-35a Genomes Size universe-review.caa href="R11-05-genomes.htm">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="60%">other suggests that they might have a controlling function.universe-review.cabr />universe-review.cabr />universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
Recent research in 2003 has found that many of these non-coding (RNA-only) genes play major roles in the health and development of plants and animals.  Active forms of RNA also help to regulate a separate "epigeneticuniverse-review.casup>universe-review.caa href="universe-review.ca#six/default.htm#six/">6universe-review.ca/a>universe-review.ca/sup>" layer of heritable information that resides in the chromosomes but outside the DNA sequence.  Lately a new kind of RNA has been discovered.  Dubbed universe-review.caa href="I11-39-riboswitch.jpg">riboswitchesuniverse-review.ca/a>, these long RNAs are both coding and non-coding at once.  They produce protein only when activated by target chemical.  These precision genetic switches have been identified from species in all five kingdoms of life.  This implies that they were probably present in the last common ancestor, not long after the dawn of evolution.  They may be the living relic from the universe-review.caa href="universe-review.ca#pre-RNA/default.htm#pre-RNA/">RNA worlduniverse-review.ca/a> 3.8 billion years ago.
universe-review.cap>There is an unexpected application of these non-coding DNA in modern life.  Since the number of repeats is highly variable among individuals, universe-review.caA NAME="profile">universe-review.ca/A>universe-review.caa href="I11-36-DNAprofile.jpg">DNA profilesuniverse-review.ca/a> has been compiled to replace fingerprints as personal identification or for paternity testing.universe-review.ca/p>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="50%">universe-review.caa href="I11-24-chromosomes.jpg">universe-review.caimg src="I11-24-chromosomes.jpg" name="Chromosome Number" alt="Chromosome Number" align="left" width="400"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="50%">
universe-review.caa name="same">universe-review.ca/a>Bacterial genomes are far more compact than eukaryotic genomes.  They have very little noncoding DNA (introns).  The number of genes and base pairs for some organisms are shown in Figure 11-35a. The mouse genome sequence reveals about 30,000 genes, with 99% having direct counterparts in humans.  It seems to indicate that complexity is not solely determined by the number of genes, it may also be related to the regulation of these genes universe-review.casup>universe-review.caa href="universe-review.ca#seven/default.htm#seven/">7universe-review.ca/a>universe-review.ca/sup>.  Figure 11-35b shows that the number universe-review.ca/td>
universe-review.catr>universe-review.catd width="50%">universe-review.cah4>Figure 11-35b Chromosome Number universe-review.caa href="I11-24-chromosomes.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="50%">of chromosomes is unrelated to complexity. It could be just the error in chromosome segregation during cell  universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
division.  An extra pair of chromosomes was retained by mistake.  Over time, mutations would accumulate in the duplicated pair until they were so divergent that they were clearly distinct. universe-review.cabr />universe-review.cabr />
universe-review.caa name="junk2">universe-review.ca/a>The role of junk DNA becomes clearer by 2004.  It is found that it may serve the function of gene regulation.  The introns are not merely discarded after separating from the mRNA (the exons in the gene).  Some of them are processed into MicroRNAs, which regulate the gene expression similar to some of the proteins translated by the mRNAs (see Figure 11-26a).  Aside from introns, the other great source of presumed genomic junk - accounting for about 40% of the human genome - comprises 
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="20%">universe-review.caa href="I11-46-intron.jpg">universe-review.caimg src="I11-46-intron.jpg" name="Intron" alt="Intron" align="left" width="200"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="80%">transposons (jumping genes) and other repetitive elements.  These sequences are widely regarded as molecular parasites that, like introns, colonized our genomes in waves at different times in evolutionary history.  Evidence suggests that transposons contribute to the evolution and genomic regulation of higher organisms and may play a key role in epigenetic inheritance (the modification of genetic traits).  The A-to-I (adenosine-to-inosine) editing process, in which a RNA sequence changes at a very specific site, occurs in repeat sequences call Alu elements that reside in noncoding RNA sequences.  It is particularly active in the brain, and is two orders of magnitude more widespread in humans than was previously thought.  What was dismissed as junk because it was not understood may well turn out to hold the secrets to human complexity.  universe-review.ca/td>
universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 11-36a RNA Regulation universe-review.caa href="I11-46-intron.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="80%">universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
The creation of complex objects, whether houses or horses, demands two kinds of specifications: one for the components and one for the system that guides their assembly.  The component molecules that make up different organisms are fundamentally alike: around 99% of the proteins in humans have recognizable equivalents in mice, and vice versa; many of those proteins are also conserved in other animals, and those involved in basic cellular processes are conserved in all eukaryotes.  So it must be the architectural information that accounts for the diversity of animals.  Since the amount of regulation increases as a nonlinear function of complex and protein regulation has its limitation, it is suggested that the rise of multicellular organisms over the past billion years was a consequence of the transition to a new control mechanism based 
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="30%">universe-review.caa href="I11-46-complexity.jpg">universe-review.caimg src="I11-46-complexity.jpg" name="Complexity" alt="Complexity" align="left" width="300"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="70%">largely on RNA regulatory signals from the junk DNAs.  The evolution of complexity (in term of new regulatory system) helps to explain the phenomenon of the Cambrian explosion about 52.5 million years ago, when invertebrate animals evolved, seemingly abruptly, from much simpler life (see Figure 11-26b). universe-review.cabr />universe-review.cabr /> universe-review.ca/td>
universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 11-36b Evolution of Complexity universe-review.cabr />universe-review.caa href="I11-46-complexity.jpg">[view large image]universe-review.ca/a> universe-review.ca/h4> universe-review.ca/td>
universe-review.catd width="80%">universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
Another study in 2004 shows that when two large non-coding "gene deserts" were removed from the mouse genome, not only were the resulting mice viable, but their morphology, reproductive fitness, growth and longevity were indistinguishable from normal litter mates.  Though some of the deleted sequences may encode functions not yet identified, the good health of these mice does suggest that there is disposable DNA in the genomes of mammals.  This finding is in contrary to those just mentioned above.  A possible explanation for the contradiction could be that there are so many copies of the non-coding sequence, deletion of one or two million such base pairs does not affect the biological functions.
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="microbiology">Microbiologyuniverse-review.ca/a>universe-review.ca/h3>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="20%">universe-review.caa href="R11-07-microbiology.htm">universe-review.caimg src="I11-28-microbiology.jpg" name="microbiology" alt="microbiology" align="left" width="205"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="80%">
Microbiology is the study of microorganisms composed of one cell, which carries out all life functions including feeding, digestion, excretion and reproduction.  They are called microorganisms or microbes because they are only visible under the microscope (see "universe-review.caa href="R11-13-microscopes.htm">Microscopeuniverse-review.ca/a>" in the appendix for detail).  While some can be harmful, most are harmless, and many are beneficial and essential for the ecosystem.  Bacteria and cyanobacteria are ubiquitous.  They are found in arctic conditions, in all waters, and in the upper strata of the atmosphere.  Species distribution in these places is generally similar to that in soils.  Because of their low mass, microorganisms can be transported by air currents.  They can be classified into archaebacteria, bacteria, and protista as shown in Figure 11-37.  The bacteria are sometimes classified as gram-positive and gram-negative according to the cell wall structure.  Gram-positive bacteria are more susceptible to the treatment of antibiotic such as lysozyme and penicillin.

universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 11-37 Evolutionary Tree universe-review.cah4>universe-review.ca/td>
universe-review.catd width="80%">universe-review.cah4>universe-review.caa href="R11-07-microbiology.htm">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="archaebacteria">Archaebacteria (Ancient Bacteria)universe-review.ca/a>universe-review.ca/h3>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="20%">universe-review.caa href="I11-29-archaea1.jpg">universe-review.caimg src="I11-29-archaea1.jpg" name="archaea" alt="archaea" align="left" width="175"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="20%">universe-review.caa href="I11-29-archaea2.jpg">universe-review.caimg src="I11-29-archaea2.jpg" name="archaea env." alt="archaea env." align="left" width="225"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="60%">
Many years ago archaebacteria were believed to be the earliest prokaryotes (cells without nucleus, i.e., the bacteria). Molecular evidence now indicates an extremely ancient separation between Bacteria and Archaea. Though they lack a nuclear membrane and are therefore prokaryotes, archaea resemble universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 11-38 Archaea universe-review.caa href="I11-29-archaea1.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>
universe-review.catd width="20%">universe-review.cah4>Figure 11-39 Environment for Archaea universe-review.caa href="I11-29-archaea2.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
eukaryotes (cells with nucleus) in several aspects of their genetic system, including an intron / exon gene structure, and membrane infolding.   Eukaryotic cells were derived from the archaean branch approximately 1.7 billion years ago. (see universe-review.caa href="I10-02-TreeOfLife.jpg">Figure 10-02universe-review.ca/a>.)  Modern archaea are found in extreme environments requiring universe-review.caA NAME="xenv">universe-review.ca/A>methanogenic, halophilic, or thermophilic metabolisms. While they are able to live elsewhere, they are usually not found there because outside of extreme environments they are competitively excluded by other organisms.  Figure 11-38 shows some of the archaebacteria and Figure 11-39 shows the various environments where the archaea are thriving: (1) Halophiles in salty lakes, (2) Thermoproteus in deep-sea hydrothermal vents, (3) Sulfolobus in hot sulfur springs, (4) Methanobacterium in swamps and marshes, and (5) Acidianus in acidic ponds.
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="bacteria">Bacteriauniverse-review.ca/a>universe-review.ca/h3>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="20%">universe-review.caa href="I11-30-bacteria.gif">universe-review.caimg src="I11-30-bacteria.gif" name="bacteria" alt="bacteria" align="left" width="215"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="20%">universe-review.caa href="I11-30-cyanobacteria.jpg">universe-review.caimg src="I11-30-cyanobacteria.jpg" name="cyanobacteria" alt="cyanobacteria" align="left" width="200"/>universe-review.ca/a>universe-review.ca/td>
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Bacteria were among the first life forms on Earth. They are very small one-celled organisms that lack a nucleus (size ~ 10universe-review.casup>-4universe-review.ca/sup> cm). Despite their small size, bacteria are the most abundant of any organism on Earth. They are highly adaptable. Their normally rapid reproduction rate (by asexual binary fission) and high capacity for spontaneous mutation allows universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 11-40 Bacteria Shapes universe-review.cabr />universe-review.caa href="I11-30-bacteria.gif">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>
universe-review.catd width="20%">universe-review.cah4>Figure 11-41 Cyanobacteria universe-review.cabr />universe-review.caa href="I11-30-cyanobacteria.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>universe-review.ca/tr>universe-review.ca/table>
them to respond to changing environments readily. This has made them ubiquitous in the biosphere, both as free-living forms and as parasites in multicellular forms of life.  They're everywhere; they can be found in the air, soil, water, on you, and inside you. In fact, there are more bacterial cells inside your gut and on your skin than there are cells in your entire body - no matter how many times you try to wash them off.  Bacteria often get a bad reputation because certain types are responsible for causing a variety of illnesses, including many types of food poisoning. However, most bacteria are completely harmless and many even perform beneficial functions, such as turning milk into yogurt or cheese and helping scientists produce drugs (such as penicillin) to fight disease. 
universe-review.cap>The cells of all bacteria are classified as "prokaryotic", the simplest and most ancient of the cell types. Prokaryotes lack many of the structures found in the more complex, eukaryotic cells. Bacteria occur in 3 basic shapes (Figure 11-40): rod (bacillus), spherical or round (coccus), and spiral (spirillum).  The bacilli and the cooci may form chains of a length typical of the particular bacterium.  When faced with unfavorable environmental conditions, some bacteria form endospores.  During the formation process, the cell shrinks, rounds up within the former cell membrane, and secretes a new, thicker wall inside the old one.  Endospores are amazingly resistant to extreme temperatures, drying out, and harsh chemicals, including acids and bases.  When conditions are suitable for growth, the spore absorbs water, breaks out of the inner shell, and becomes a typical bacterial cell again.universe-review.ca/p>
universe-review.cap>Some bacteria are obligate universe-review.caA NAME="anaerobes">universe-review.ca/A>anaerobes and are unable to grow in the presence of oxygen.  Some other bacteria are able to grow in either the presence or absence of oxygen.  Most bacteria, however, are aerobic and like animals require a constant supply of oxygen to carry out cellular respiration.  universe-review.ca/p>
universe-review.cap>Every type of nutrition is found among bacteria except holozoism (eating whole food).  Some autotrophic bacteria are photosynthetic.  Some are chemosynthetic bacteria, which oxidize inorganic compounds to obtain necessary energy to produce their own food.  The majority of bacteria are free-living universe-review.caA NAME="aerobic">universe-review.ca/A>aerobic heterotrophs and feed on dead organic matter  by secreting digestive enzymes and absorbing the products of digestion. They are needed to complete the elementary cycles of nature (the carbon cycle, the nitrogen cycle, the phosphate cycle, and the sulphur cycle) by degrading the wastes and the corpses from higher organisms back to inorganic and mineral compounds.universe-review.ca/p> 
universe-review.cap>Bacteria are often universe-review.caA NAME="symbiotic">universe-review.ca/A>symbiotic; they live in association with other organisms.  The nitrogen-fixing bacteria in the nodules of legumes are mutualistic, as are the bacteria that live within our own intestinal tract.  We provide the bacteria with a home, and they provide us with certain vitamins.universe-review.ca/p>
universe-review.cap>universe-review.caA NAME="cyanobacteria">universe-review.ca/A>Cyanobacteria, formerly called blue-green algae, are the most prevalent of the photosynthetic bacteria.  They are believed to be responsible for first introducing oxygen into the primitive atmosphere.  Cyanobacteria can be unicellular, filamentous (see Figure 11-41), or colonial.  The filaments and colonies are not considered multicellular because each cell is independent of the others.  Cyanobacteria lack any visible means of locomotion.  They are common in fresh water, in soil, and on moist surfaces but also are found in inhospitable habitats, such as hot springs.  They also form symbiotic relationships with a number of organisms, such as ferns and even at times invertebrates, like corals.  In association with fungi, they form universe-review.caa href="I11-34-lichen.jpg">lichensuniverse-review.ca/a>, which can grow on rock.  Therefore, cyanobacteria may have been among the first organisms to colonize land.universe-review.ca/p>
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="protista">Protista (Unicellular Eukaryotes)universe-review.ca/a>universe-review.ca/h3>
universe-review.catable border="0" width="100%">universe-review.catr>
universe-review.catd width="20%">universe-review.caa href="I11-31-diatoms.jpg">universe-review.caimg src="I11-31-diatoms.jpg" name="diatom" alt="diatom" align="left" width="200"/>universe-review.ca/a>universe-review.ca/td>
universe-review.catd width="20%">universe-review.caa href="I11-31-amoeba.jpg">universe-review.caimg src="I11-31-amoeba.jpg" name="amoeba" alt="amoeba" align="left" width="215"/>universe-review.ca/a>universe-review.ca/td>
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Protista are the simplest of the eukaryotes. Protists are an unusual group of organisms that were put together because they don't really seem to belong to any other group. Some protists perform photosynthesis like plants such as the diatoms (see Figure 11-42.) while others move around and act like animals such as the amoeba (see Figure 11-43), but protists are neither plants nor animals. 
universe-review.ca/td>universe-review.ca/tr>
universe-review.catr>universe-review.catd width="20%">universe-review.cah4>Figure 11-42 Diatoms universe-review.cabr />universe-review.caa href="I11-31-diatoms.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>
universe-review.catd width="20%">universe-review.cah4>Figure 11-43 Amoeba universe-review.cabr />universe-review.caa href="I11-31-amoeba.jpg">[view large image]universe-review.ca/a>universe-review.ca/h4>universe-review.ca/td>
universe-review.ca/tr>universe-review.ca/table>
universe-review.cap>As with all eukaryotic cells, protists contain membrane-bound nuclei and endomembrane systems, as well as numerous organelles. Movement is often provided by one or more flagella, and cilia are often present on the plasma membrane as sensory organelles.  Unlike prokaryotes, protistan nuclei contain multiple DNA strands, though the total number of nucleotides is significantly less than that in more complex eukaryotes. Protists can reproduce mitotically, and some are capable of meiosis for sexual reproduction. Cellular respiration in the kingdom is primarily an aerobic process, but some protists, including those that live in mud below ponds or in animal digestive tracts, are strict or facultative anaerobes.universe-review.ca/p>
universe-review.cap>Protists represent an important step in early evolution, evolving from prokaryotes and eventually giving rise to the entire line of eukaryotes. The first protists probably evolved 1.7 billion years ago, 2.3 billion years after the origin of life, from simple communities of prokaryotic cells. Membrane infolding was one of the defining processes in this evolution: in some prokaryotic cells, parts of the plama membrane folded into the cell to create the nuclear envelope and the other organelles of the endomembrane system. The second major step in the evolution of protists from bacteria was the process of universe-review.caA NAME="endosymbiosis">universe-review.ca/A>endosymbiosis, which introduced the mitochondrion and chloroplast as organelles of eukaryotic cells. Small prokaryotic cells capable of cellular respiration or photosynthesis entered eukaryotic cells, either as parasites or indigestible food, and these prokaryotes evolved into mitochondria and chloroplasts as they developed a symbiotic relationship with the host cell. (Because mitochondria are present in all eukaryoptic cells, this process probably happened to mitochondria first.) As a result of these two processes, protists evolved as sucessful organisms. Eventually, colonial protists evolved into plants, fungi, and animals, of the eukaryotic kingdom, which came to dominate the earth.universe-review.ca/p>
universe-review.cap>Protista is divided into four major groups by lifestyle: the protozoans, the slime molds, the unicellular algae, and the multicellular algae. Protozoans include all protists that ingest their food, and thus they live primarily in aquatic habitats, such as ponds, drops of water in soil, or the digestive tracts of animals. In the latter capacity, a small number of protozoans function as parasites. The slime molds in the second group are unique in having both unicellular and multicellular stages. When sufficient bacteria (food) are present, cellular slime molds are single amoeboid cells; however, when food becomes scarce, they aggregate into slug-like colonies, which become large reproductive structures. Plasmodial slime molds also exist as single cells when nutrients are plentiful, but each cell can grow into a large, branching plasmodium with many nuclei. This differentiates into reproductive structures when food is scarce. The third and fourth groups of protists, the algae, contain chloroplasts and photosynthesize like plants; these can be unicellular, colonial, or multicellular. Multicellular marine algae, the seaweeds, are similar to marine plants, and many biologists support moving seaweed into the plant kingdom.universe-review.ca/p>
universe-review.cap>universe-review.caA NAME="diatoms">universe-review.ca/A>Diatoms (see Figure 11-42.) have a golden brown accessory pigment in their chloroplasts that can mask the color of chlorophyll.  The structure of a diatom often is compared to a box because the cell wall has 2 halves, or valves, with the larger valve acting as a "lid" for the smaller valve.  The cell wall of the diatom has an outer layer of silica, a common ingredient of glass.  Diatoms are among the most numerous of all unicellular algae in the oceans.  As such, they serve as an important source of food for other organisms.universe-review.ca/p>
universe-review.cap>universe-review.caA NAME="amoeba">universe-review.ca/A>Amoeba proteus (see Figure 11-43.) are a small mass of cytoplasm without any definite shape.  They move about and feed by means of cytoplasmic extensions called universe-review.caA NAME="pseudopodia">universe-review.ca/A>pseudopodia, or false feet.  A pseudopodium forms when the cytoplasm streams forward in the particular direction.  The organelles within an amoeba include food or digestive universe-review.caA NAME="vacuoles">universe-review.ca/A>vacuoles and contractile vacuoles (for expelling waste).universe-review.ca/p>  
Further details of micro-organisms evolution are described in an appendix - universe-review.caa href="R10-23-plants.htm">Evolution of Micro-organisms and Plantsuniverse-review.ca/a>.
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3 align="center">universe-review.caa name="footnotes">Footnotesuniverse-review.ca/a>universe-review.ca/h3>
universe-review.cap>universe-review.caA NAME="one">universe-review.ca/A>universe-review.casup>1universe-review.ca/sup>The universe-review.caa href="I11-06-Murchison.jpg">Murchison meteoriteuniverse-review.ca/a> was recovered in Australia in 1969.  Analysis of the meteorite found over 90 types of amino acids as well as some left-handed sugar that does not exist naturally on Earth.  This rare form of substance tends to prove the extraterrestrial origin of the rest of the contents.  The Murchison meteorite contains the same amino acids obtained by Stanley Miller in his laboratory, and even in the same relative proportions universe-review.ca/p> 
universe-review.cap>universe-review.caA NAME="two">universe-review.ca/A>universe-review.casup>2universe-review.ca/sup>Metabolism is the sum of all chemical activities occurring inside a living cell.  Metabolic cycle (pathway) begins with a particular reactant and terminate with an end product.universe-review.ca/p>
universe-review.cap>universe-review.caA NAME="three">universe-review.ca/A>universe-review.casup>3universe-review.ca/sup>The bonding energy for the various universe-review.caa href="F12-molecule.htm#covalent">chemical bondsuniverse-review.ca/a> are roughly in the ratio -  Van der Waals : Hydrogen Bonds : Covalent Bonds = 1 : 10 : 100.  Formation of hydrogen bonds releases 3 - 10 kcal/mole (~ 0.1 - 0.4 ev).  Hydrogen bonds are found between only a few elements of the periodic table.  The most common are those in which H connects two atoms from the group F, O, N, and, less commonly, Cl.   The hydrogen bond in water has the configuration: H-O-H(+)��..(-)O=Huniverse-review.casub>2universe-review.ca/sub>.  Covalent bonds are created with sharing electrons in between two atomic nuclei.  A stable configuration can be achieved by sharing up to three pairs of electrons.  Van der Waals forces are the intermolecular attractions produced by temporary dipoles (shifting of electrons).universe-review.ca/p> 
universe-review.cap>universe-review.caA NAME="four">universe-review.ca/A>universe-review.casup>4universe-review.ca/sup>The ev is an energy unit called electron volt.  It is defined as the energy acquired by a particle of one electronic charge e, accelerated through a potential difference of 1 volt.  Approximately 1 ev ~ 1.6 x 10universe-review.casup>-19universe-review.ca/sup> joule.  Photosynthesis peaks at a wavelength of around 700 nm (red light), which carries an energy of about 5 ev.universe-review.ca/p>
universe-review.cap>universe-review.caA NAME="five">universe-review.ca/A>universe-review.casup>5universe-review.ca/sup>The human genome selected the most common alleles over a number of individuals or from just one person.  DNA sequence variations among individuals do occur, it is called polymorphisms.  Alleles are detectable variations occurring at a single genetic locus (location).  Where allelic variation is frequently found (say that at least 10% of chromosomes have an allele other than the most commonly occurring one) one refers to "a polymorphism". If variation is rare one is more likely to speak of "a mutation".  SNP (Single Nucleotide Polymorphisms) refers to variation of just one nucleotide.  The SNP consortium (TSC) is a public/private collaboration that has to date discovered and characterized nearly 1.8 million SNPs, which are important in tracing the evolution of the human race and controlling human diseases.universe-review.ca/p>
universe-review.cap>universe-review.caA NAME="six">universe-review.ca/A>universe-review.casup>6universe-review.ca/sup>Epigenetics is the study of heritable changes in gene function that occur without a change in the DNA sequence. Epigenetic mechanisms includes DNA methylation (replacing H with CHuniverse-review.casub>3universe-review.ca/sub>), histone deacetylation (removing the acetyl group CHuniverse-review.casub>3universe-review.ca/sub>COuniverse-review.casup>-universe-review.ca/sup>), and universe-review.caa href="universe-review.ca#RNAi/default.htm#RNAi/">RNA interferenceuniverse-review.ca/a>.  DNA methylation and histone deacetylation result in the condensation of chromatin into a compact state that is inaccessible by transcription factors.  Open chromatin is characterized by non-methylated DNA and histones with acetylated tails. This allows the assembly of transcription factors and transcription by RNA polymerase.  Their effects in gene inactivation and activation are increasingly understood to be very important in phenotype transmission and embryonic development. universe-review.ca/p>
universe-review.cap>universe-review.caA NAME="seven">universe-review.ca/A>universe-review.casup>7universe-review.ca/sup>The birth of universe-review.caa href="I11-33-CopyCat.jpg">CC (a.k.a. Copy Cat)universe-review.ca/a>, the cloned cat,  shows that the characteristics of the clone can be very different from its genetic parent.  Recent work in pig cloning found that some attributes - such as the levels of albumin and calcium in blood - varied less in clones than in a control group of naturally bred pigs.  Yet a surprising variety of other traits - including blood glucose and globulins, hair type, number of teats and weight - fluctuate as much in clones as in controls.  These characteristics, like the pattern of CC's coat, are influenced by environmental factors and "epigenetic" controls that affect universe-review.caa href="R11-09-genexp.htm">gene expressionuniverse-review.ca/a>.universe-review.ca/p>
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" />  
universe-review.caul>universe-review.cah3>universe-review.caa name="references">References:universe-review.ca/a>universe-review.ca/h3>
universe-review.cali>Origin of Life -- universe-review.caa href="http://www.chelt.ac.uk/gdn/origins/life/index.htm">http://www.chelt.ac.uk/gdn/origins/life/index.htmuniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Origin of Life -- universe-review.caa href="http://www.resa.net/nasa/origins_life.htm#precursors">http://www.resa.net/nasa/origins_life.htm#precursorsuniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Origin of Life, RNA World -- universe-review.caa href="http://www.postmodern.com/~jka/rnaworld/nfrna/nf-index.html">http://www.postmodern.com/~jka/rnaworld/nfrna/nf-index.htmluniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Origin and Evolution of Life -- universe-review.caa href="http://www.geocities.com/CapeCanaveral/Lab/2948/orgel.html">http://www.geocities.com/CapeCanaveral/Lab/2948/orgel.htmluniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Evolution -- universe-review.caa href="http://www.as.wvu.edu/biology/bio115/Garbutt_notes/Evol_Lect_10.htm">http://www.as.wvu.edu/biology/bio115/Garbutt_notes/Evol_Lect_10.htmuniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Prion -- universe-review.caa href="http://www.as.ua.edu/ant/bindon/ant570/Papers/McGrath/McGrath.htm">http://www.as.ua.edu/ant/bindon/ant570/Papers/McGrath/McGrath.htmuniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Viruses -- universe-review.caa href="http://www.ucmp.berkeley.edu/alllife/virus.html">http://www.ucmp.berkeley.edu/alllife/virus.htmluniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Organic Chemistry -- universe-review.caa href="http://www.chemguide.co.uk/orgmenu.html">http://www.chemguide.co.uk/orgmenu.htmluniverse-review.ca/a>universe-review.ca/li>
universe-review.cali> Biological Energy, ATP -- universe-review.caa href="http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookATP.html">http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookATP.htmluniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Macromolecules -- universe-review.caa href="http://www.sc2000.net/~czaremba/explanations/macromolecules.html">http://www.sc2000.net/~czaremba/explanations/macromolecules.htmluniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Molecular Biology -- universe-review.caa href="http://www.rothamsted.bbsrc.ac.uk/notebook/courses/guide/">http://www.rothamsted.bbsrc.ac.uk/notebook/courses/guide/universe-review.ca/a>universe-review.ca/li>
universe-review.cali>Cells -- universe-review.caa href="http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap2.html">http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap2.htmluniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Microbiology, Archaebacteria -- universe-review.caa href="http://www.ucmp.berkeley.edu/archaea/archaea.html">http://www.ucmp.berkeley.edu/archaea/archaea.htmluniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Molecular Biology, Bacteria and Yeast -- universe-review.caa href="http://www.ces.uga.edu/pubcd/b817-w.html">http://www.ces.uga.edu/pubcd/b817-w.htmluniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Microbiology, Eukaryotes -- universe-review.caa href="http://fhis.gcal.ac.uk/BIO/micro/drjrattray/nutmicro/euk.html#Introduction">http://fhis.gcal.ac.uk/BIO/micro/drjrattray/nutmicro/euk.html#Introductionuniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Yeast -- universe-review.caa href="http://biochemie.web.med.uni-muenchen.de/Yeast_Biology/">http://biochemie.web.med.uni-muenchen.de/Yeast_Biology/universe-review.ca/a>universe-review.ca/li>
universe-review.cali>Genome -- universe-review.caa href="http://www.ornl.gov/TechResources/Human_Genome/publicat/primer2001/index.html">http://www.ornl.gov/TechResources/Human_Genome/publicat/primer2001/index.htmluniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>SNP  -- universe-review.caa href="http://snp.cshl.org/">http://snp.cshl.org/universe-review.ca/a>universe-review.ca/li>
universe-review.cali>Sex Chromosomes -- universe-review.caa href="http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/SexChromosomes.html">http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/SexChromosomes.htmluniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Y Chromosome -- universe-review.caa href="http://www.marshill.org/Y%20Chromosome%20Error%20Correction.htm">http://www.marshill.org/Y%20Chromosome%20Error%20Correction.htmuniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>General, Basic Biology -- universe-review.caa href="http://www.mcb.mcgill.ca/~hallett/GEP/Lecture1/Lecture1.html">http://www.mcb.mcgill.ca/~hallett/GEP/Lecture1/Lecture1.htmluniverse-review.ca/a>universe-review.ca/li>
universe-review.cali>Future of Genomics Research -- Francis S. Collins, et al. Nature 422, 835-847 (2003).
universe-review.ca/ul>
universe-review.caa href="default.htm#top/">universe-review.cah4>[Top]universe-review.ca/h4>universe-review.ca/a>
universe-review.cahr align="center" width="15%" /> 
universe-review.cah3>universe-review.caa name="Index">Indexuniverse-review.ca/a>universe-review.ca/h3>
universe-review.catable border="0" width="100%">
universe-review.catr>
universe-review.catd width="49%">
universe-review.caa href="universe-review.ca#bases/default.htm#bases/">Adenine (A)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#aerobic/default.htm#aerobic/">Aerobic heterotrophsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#amino/default.htm#amino/">Amino acidsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#aminogroup/default.htm#aminogroup/">Amino groupuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#amoeba/default.htm#amoeba/">Amoeba proteusuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#AMP/default.htm#AMP/">AMP, ADP, ATPuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#anaerobes/default.htm#anaerobes/">Anaerobesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#archaebacteria/default.htm#archaebacteria/">Archaebacteria (ancient bacteria)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#bacteria/default.htm#bacteria/">Bacteriauniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#bio-carbon/default.htm#bio-carbon/">Bio-carbonuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#cAMP/default.htm#cAMP/">cAMPuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#carbohydrates/default.htm#carbohydrates/">Carbohydratesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#carboxylic/default.htm#carboxylic/">Carboxylic acid groupuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#cells/default.htm#cells/">Cellsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#clock/default.htm#clock/">Cellular clockuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#cellulose/default.htm#cellulose/">Celluloseuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#chiral/default.htm#chiral/">Chiral objectsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#chlorophyll/default.htm#chlorophyll/">Chlorophylluniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#chromosome/default.htm#chromosome/">Chromosomeuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#commoncold/default.htm#commoncold/">Common Colduniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#coronavirus/default.htm#coronavirus/">Coronavirus (SARS)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#covalent/default.htm#covalent/">Covalent bonduniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#cyanobacteria/default.htm#cyanobacteria/">Cyanobacteriauniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#bases/default.htm#bases/">cytosine (C)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#deoxyribose/default.htm#deoxyribose/">Deoxyriboseuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#diatoms/default.htm#diatoms/">Diatomsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#DNA/default.htm#DNA/">DNAuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#junk/default.htm#junk/">DNA, junk (noncoding)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#profile/default.htm#profile/">DNA profileuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#replication/default.htm#replication/">DNA replicationuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#endosymbiosis/default.htm#endosymbiosis/">Endosymbiosisuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#six/default.htm#six/">Epigeneticsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#eukaryotic/default.htm#eukaryotic/">Eukaryotic celluniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#fats/default.htm#fats/">Fatsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#fatty/default.htm#fatty/">Fatty acidsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#genomes/default.htm#genomes/">Gene and genomesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#genexp/default.htm#genexp/">Gene expressionuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#genetic/default.htm#genetic/">Genetic codeuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#glucose/default.htm#glucose/">Glucoseuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#glycerol/default.htm#glycerol/">Glyceroluniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#glycolipids/default.htm#glycolipids/">Glycolipidsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#glycogen/default.htm#glycogen/">Glycogenuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#bases/default.htm#bases/">Guanine (G)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#xenv/default.htm#xenv/">Halophilicuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#hydrophillic/default.htm#hydrophillic/">Hydrophillicuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#hydrophobic/default.htm#hydrophobic/">Hydrophobicuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#influenza/default.htm#influenza/">Influenzauniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#isoelectric/default.htm#isoelectric/">Isoelectric pointuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#lipids/default.htm#lipids/">Lipidsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#macromolecules/default.htm#macromolecules/">Macromolecules and molecular biologyuniverse-review.ca/a>universe-review.cabr />
universe-review.ca/td>universe-review.catd width="2%"> universe-review.ca/td>universe-review.catd width="49%">
universe-review.caa href="universe-review.ca#xenv/default.htm#xenv/">Methanogenicuniverse-review.ca/a>universe-review.cabr /> 
universe-review.caa href="universe-review.ca#microbiology/default.htm#microbiology/">Microbiologyuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="R11-13-microscopes.htm">Microscopesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#Murchison/default.htm#Murchison/">Murchison meteroriteuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#mutation/default.htm#mutation/">Mutationuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#non-equilibrium/default.htm#non-equilibrium/">Non-equilibrium stateuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#nonpolar/default.htm#nonpolar/">Nonpolar amino acidsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#nucleoside/default.htm#nucleoside/">Nucleosidesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#nucleotides/default.htm#nucleotides/">Nucleotidesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#nucleus/default.htm#nucleus/">Nucleus, celluniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#organelles/default.htm#organelles/">Organellesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#organic/default.htm#organic/">Organic chemistryuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#origin/default.htm#origin/">Origin of lifeuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#peptide/default.htm#peptide/">Peptide bonduniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#phospholipids/default.htm#phospholipids/">Phospholipidsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#photosynthesis/default.htm#photosynthesis/">Photosynthesisuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#polar/default.htm#polar/">Polar amino acidsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#polymers/default.htm#polymers/">Polymersuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#prebiotic/default.htm#prebiotic/">Prebiotic worlduniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#pre-RNA/default.htm#pre-RNA/">Pre-RNA worlduniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#virus/default.htm#virus/">Prion and virusesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#proteins/default.htm#proteins/">Proteins and enzymesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#folding/default.htm#folding/">Protein folding and misfoldinguniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#protista/default.htm#protista/">Protista (unicellular eukaryotes)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#proto-Earth/default.htm#proto-Earth/">Proto-Earthuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#pseudopodia/default.htm#pseudopodia/">Pseudopodiauniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#respiration/default.htm#respiration/">Respirationuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#ribose/default.htm#ribose/">Riboseuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#RNA/default.htm#RNA/">RNAuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#RNAi/default.htm#RNAi/">RNA, interferenceuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#mRNA/default.htm#mRNA/">RNA, messenger (mRNA)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#rRNA/default.htm#rRNA/">RNA, ribosomal (rRNA)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#tRNA/default.htm#tRNA/">RNA, transfer (tRNA)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#RNAworld/default.htm#RNAworld/">RNA worlduniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#side/default.htm#side/">Side chainuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#starch/default.htm#starch/">Starchuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#StanleyMiller/default.htm#StanleyMiller/">Stenley Milleruniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#steroids/default.htm#steroids/">Steroidsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#symbiotic/default.htm#symbiotic/">Symbioticuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#telomeres/default.htm#telomeres/">Telomeresuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#xenv/default.htm#xenv/">Thermophilicuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#bases/default.htm#bases/">Thymine (T)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#transcription/default.htm#transcription/">Transcriptionuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#translation/default.htm#translation/">Translationuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#bases/default.htm#bases/">Uracil (U)universe-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#vacuoles/default.htm#vacuoles/">Vacuolesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#viroids/default.htm#viroids/">Viroidsuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#waxes/default.htm#waxes/">Waxesuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#Ychromosome/default.htm#Ychromosome/">Y chromosomeuniverse-review.ca/a>universe-review.cabr />
universe-review.caa href="universe-review.ca#yeast/default.htm#yeast/">Yeastuniverse-review.ca/a>universe-review.cabr />
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