Reproduction coded molecules

There may not be any alternative way for life to begin or to develop than through the reduction and then condensation reactions of organic molecules aided by inorganic ions which require energy and the use of a mutatable reproductive code. Certainly, none is known on Earth. If we look at the molecular machines involved, the principles of their construction and activity appears to have remained virtually unchanged, to this day (see Section 4.7 and Appendix 4C). 

The fidelity of cellular repair and reproduction is determined by a coding system based on polynucleotides - deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). In general (with some inevitable exceptions of course), the information flow is from DNA molecules (genes) which are transcribed to yield RNA molecules which in turn are translated on complex macromolecular protein-RNA assemblies called ribosomes to yield proteins (polymers of amino acids linked by peptide bonds). 

Fig, 5. Amplified production of silicatein protein from recombinant DNA cloned in bacteria. DNA coding for the silicatein a protein was integrated enzymically into a double-stranded circular DNA vector molecule to produce a recombinant DNA. When introduced into bacteria, the recombinant DNA is replicated by intracellular enzymes to produce many copies in each cell. These DNA molecules serve as templates, directing the synthesis of the silicatein protein they encode. Rapid reproduction of the bacteria greatly amplifies this synthesis. 

How do nucleic acids determine cell reproduction Each time a cell (or virus) reproduces itself, it transmits the ability to continue the reproduction. The reliability of this transmittal accounts for the continuity of species. It is therefore reasonable to assume that some sort of durable pattern is preserved or reconstructed with each duplication. A pattern is a physical entity whose size, shape, and makeup carry information, such as a punched card, a dress pattern, or a perforated metal plate that establishes the positions of boltholes. In the cell nucleus, the reproduction pattern is the DNA molecule. The sequence of bases embodies the pattern, or code, for the synthesis of proteins. For brevity, let us designate the bases by their initials, A, C, C, and T, as shown in Fig. 26.10. Now if we imagine that a single DNA strand were isolated, and stretched out along a straight line, we would have a structure such as... 

How do arene oxides cause cancer We have seen that nucleophiles react with epoxides to form addition products. 2"-Deoxyguanosine, a component of DNA (Section 26.1), has a nucleophilic NH2 group that is known to react with certain arene oxides. Once a molecule of 2"-deoxyguanosine becomes covalently attached to an arene oxide, the 2 -deoxyguanosine can no longer fit into the DNA double helix. As a result, the genetic code will not be properly transcribed (Section 26.7), which can lead to mutations Aat cause cancer. Cancer results when cells lose their ability to control their growth and reproduction. 

Replication of the DNA molecule by the complementary duplication of polynucleotide chains satisfies at least the fundamental condition for existence of living matter in all its forms reproduction and preservation of genetic information, recording nature by means of a nucleotide alphabet or code. The elementary units of this code are known as codons, groups of three nucleotides, each of which codes the position of one amino acid residue in the polypeptide chain. 

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