It has been suggested that the present-day triplet genetic code evolved from a doublet code when there were fewer amino acids available for primitive protein synthesis.
(a) Can you find any support for the doublet code notion in the existing coding dictionary?
(b) The amino acids Ala, Val, Gly, Asp, and Glu are all early members of biosynthetic pathways (Taylor and Coates, 1989) and are more evolutionarily conserved than other amino acids (Brooks and Fresco, 2003). They therefore probably represent “early” amino acids. Of what significance is this information in terms of the evolution of the genetic code? Also, which base, of the first two, would likely have been the more significant in originally specifying these amino acids?
(c) As determined by comparisons of ancient and recently evolved proteins, cysteine, tyrosine, and phenylalanine appear to be late-arriving amino acids. In addition, they are considered to have been absent in the abiotic earth (Miller, 1987). All three of these amino acids have only two codons each, while many others, earlier in origin, have more. Is this mere coincidence, or might there be some underlying explanation?
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