  The previously formulated model for the evolution of the genetic code was
shown to clarify why base triplets of some precursor amino acids differ by a
single base from product amino acid codons, while others show less homology.
First, the model indicated that the direction of code evolution changed on
expansion from the N-fixers code (stage 2). Growth of the code from 16 codons
in the NAN column (N, any standard nucleotide) proceeded by assignment of
codons in the GNN, ANN, CNN and UNN rows. Expansion phase (stage 4 to 7)
precursor/product pairs that spanned this shift included aspartate/threonine,
aspartate/methionine and glutamate/proline. Both 5' and mid-base differ in the
codons of each of these pairs. Second, post-expansion additions (stage 9 to 14)
required codon reassignment, eliminating initial correlations. Codons for the
post-expansion pair, aspartate (glutamate)/arginine, also differ at both 5' and
mid-base sites. Third, the distribution of core structure groups among
acceptors indicated that variant tRNA specific for a sibling, rather than
precursor, commonly participated as cofactors in early amino acid synthesis.
Sibling pairs, rather than precursor/product pairs, then exhibit codon
correlations. On removing these sources of variation, highly significant
correlations emerged between codons assigned to biosynthetically related amino
acids.
