Evolution of the amino acid code: inferences from mitochondrial codes.

The amino acid code is usually presented as a table of 64 codons. Actually the code results from the action of tRNA molecules that carry amino acids to codons in mRNA by means of codon-anticodon pairing. The tRNA molecules are transcribed from genes that undergo evolution and the number of anticodons can therefore increase during evolution, but the number of codons is fixed at 64. Mammalian mitochondrial codes contain only 22 anticodons for 20 amino acids as compared with 54 anticodons for 20 amino acids in the universal code. It is proposed that an archetypal code containing 16 anticodons for 15 amino acids evolved into the universal code by gene duplication, followed by mutations that modified the anticodons and amino acid acceptor sites. In substantiation of this proposal, it is noted that the mammalian mitochondrial code is simplified by comparison with the universal code. For example, single anticodons are used for each of eight amino acids in the mammalian mitochondrial code. This simplification may represent an evolutionary retrogression towards the proposed archetypal code.