Evolution of Structural Gene Sequences

Abstract

A nucleotide substitution would result in an amino acid substitution only if a missense mutation were produced in a DNA sequence which is transcribed and translated. Nonsense mutations could produce only shorter polypeptides by premature chain termination, while samesense mutations will be unnoticed, unless they modify the rate of translation of the messenger RNA transcribed from the mutated cistron. Frame-shift mutations (which are due to deletions or insertions of nucleotides that do not result in deletions or insertions of entire codons) could result in totally different proteins. Most probably, a frame-shift mutation would not be tolerated, but if it were, it would be most difficult to trace by any of the biochemical methods used in protein analysis. This is because a frame-shift mutation may result in an amino acid sequence entirely different from that of the original protein until the two of them would have practically nothing in common. Thus, they would lack the minimum degree of “homology”, or structural and functional similarity, necessary to make a comparison between the original and the mutated protein. Therefore, unless a frame-shift mutation is somehow traced at the DNA level, it is unlikely to be detected by comparing proteins of different organisms and assigning them to the same structural cistron. Excluding tRNA mutations that may cause ambiguous coding, the amount of amino acid substitutions between homologous proteins of different species is therefore indicative of the amount of missense mutations that they have undergone at their structural cistrons.