As earlier pointed out and as can be seen from Figure 30.4, there are more then one codons for each of several amino acids (see Table 30.5). The frequency of different amino acids in proteins also agrees with the number of codons representing each amino acid (Fig. 30.5), thus suggesting that the synonym codons have been retained in the genetic code for a definite purpose and is not a wasteful phenomenon. It may also be seen that codons representing same amino acid are clustered in the genetic code dictionary (Fig. 30.4), and that the synonym codons usually differ in the third base only, a phenomenon sometimes called third base degeneracy
Fig. 30.4. Genetic code dictionary.
Fig. 30.5. Relationships of frequencies of codons for different amino acids in the genetic code (this gives expected frequencies of amino acids in proteins) and the observed frequencies of amino acids in proteins of living systems (drawn from Lewin's"Genes IV").