Table 15-1 lists all 64 permutations, with the left-hand column indicating the base at the 5' end of the triplet, the row across the top specifying the middle base and the right-hand column identifying the base in the 3' position. One of the most striking features of the code is that 61 of the 64 possible triplets specify an amino acid, with the remaining three triplets being chain-terminating signals (see below). This means that many amino acids are specified by more than one codon, a phenomenon called degeneracy, Codons specifying the same amino acid are synonyms. For example, fJLIU and (JUG are synonyms for phenylalanine, whereas serine is encoded by the synonyms LICU, UCC, UCA, UCG, ACU, and ACC- In fact, when the" first two nucleotides are identical, the third nucleotide can be either cytosine or uracil and the codon will still code for the same amino acid. Often, adenine and guanine are similarly interchangeable. However, not all degeneracy is based on equivalence of the first two nucleotides. Leucine, for example, is coded by LfUA and UUG, as well as by CULf, CUC, CUA, and CUG (Figure 15-1). Codon degeneracy, especially the Frequent third-place equivalence of cytosine and uracil or gnanine and adenine, explains how there can be great variation in the AT/GC ratios in the DMA of various organisms without correspondingly large changes in the relative proportion of amino acids in their proteins, (For example, the genomes of certain bacteria display vastly different AT/GC ratios, and yet are closely related enough to encode proteins of highly similar amino acid sequences.)
TABtE 15-1 The Genetie Code
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