Nucleic Acids

Finally, nucleic acids are the largest macromolecules existing in living organisms. They consist of a series of interlinked nucleotides forming a long polymerized chain. The nucleotides contain a sugar molecule, one or more phosphates, and a nitrogen compound known as a base.

Two nucleic acids play a particularly important role in the formation of living things. Deoxyribonucleic acid (DNA) consists of four nucleotides, all containing the same sugar and phosphate groups, but with different bases (adenine, thymine, guanine, and cytosine, Fig. 9.4). In 1953, James Watson and Francis Crick discovered the double helix of DNA, which, by separating the two halves of its helix, is able to produce an identical copy of itself, the bases being associated in pairs (adenine— thymine and guanine—cytosine). The duplication of DNA molecules (Fig. 9.5) is the basis of the reproduction mechanism of living creatures; the sequence of bases in DNA defines the genetic code.

nh2 N N

adenine

NH NH, guanine

NH NH, guanine

<nno

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