Two types of chemically similar nucleic acids, DNA (deoxyri-bonucleic acid) and RNA (ribonucleic acid), are the principal information-carrying molecules of the cell. The monomers from which DNA and RNA are built, called nucleotides, all have a common structure: a phosphate group linked by a phosphoester bond to a pentose (a five-carbon sugar molecule) that in turn is linked to a nitrogen- and carbon-containing ring structure commonly referred to as a "base" (Figure 2-14a). In RNA, the pentose is ribose; in DNA, it is deoxyribose (Figure 2-14b). The bases adenine, guanine, and cytosine are found in both DNA and RNA; thymine is found only in DNA, and uracil is found only in RNA.
Adenine and guanine are purines, which contain a pair of fused rings; cytosine, thymine, and uracil are pyrimidines, which contain a single ring (Figure 2-15). The bases are often abbreviated A, G, C, T, and U, respectively; these same singleletter abbreviations are also commonly used to denote the entire nucleotides in nucleic acid polymers. In nucleotides,
NH2 Adenine |
NH2 Adenine |
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