Observed frequencies of amino acid replacements between closely related proteins. Dayhoff, M.O. (1978) Atlas of Protein Sequence and Structure, 5, suppl. 3; National Biomedical Research Foundation, Washington D.C.

This table is compiled from 1572 substitutions between closely related proteins. These replacements are caused by mutations in the gene sequence (a linear sequence of nucleotides). Due to the nature of the genetic code for amino acids, an amino acid residue affected by such a single genetic mutation is more likely to be replaced by some amino acids than others. This will be explained in the later section on protein synthesis. But for now, the most significant deviations from the frequencies expected from random genetic mutations are shown in bold above. What is the relationship between these frequencies, and the chemical nature of the amino acid residues involved?