Alternative codes and templates

Genetic Code: Emergence of a Symmetrical Pattern

The triplets of nucleotides which apparently specify the detailed structure of proteins fall into a regular pattern: the 64 combinations of four nucleotides taken three at a time, are resolved into 32 pairs. The second member of each pair is identical with the first, except that in one position a purine is replaced by the other purine or a pyrimidine by the other pyrimidine. Almost all of the reported triplets fit into this pattern, and from it one can predict which amino acids will be found to correspond to the remaining 19 unidentified triplets. This pattern accounts for several of the observations concerning regularities in the data, partially determines the order of the nucleotides in each triplet, and suggests a structural basis for transfer RNA specificity.

Amino acid codes in mitochondria as possible clues to primitive codes

Differences between mitochondrial codes and the universal code indicate that an evolutionary simplification has taken place, rather than a return to a more primitive code. However, these differences make it evident that the universal code is not the only code possible, and therefore earlier codes may have differed markedly from the previous code. The present universal code is probably a "frozen accident." The change in CUN codons from leucine to threonine (Neurospora vs. yeast mitochondria) indicates that neutral or near-neutral changes occurred in the corresponding proteins when this code change took change took place, caused presumably by a mutation in a tRNA gene.