Evolutionary nucleotide replacements in DNA.
With the increasing availability of analytical information on mRNA molecules, it is now possible to compare homologous nucleotide sequences from different organisms and to draw conclusions about their evolution. Such comparisons have shown that silent changes in codons occur more frequently than nucleotide replacements that produce changes in amino acid sequences (code-altering changes). Furthermore, there is an important difference between amino acid sequence comparisons and nucleotide sequence comparisons. The former show only differences in amino acid residues, but the latter show several types of differences when corresponding codons are compared. Single-base replacements may be degenerate (silent) or expressed as amino acid replacements. Two-base codon changes may be degenerate, single-base changes, or be visible as such. Three-base codon changes may be degenerate (involving serine), simulate either single-base or two-base changes or be visible as such. All nine types of change are found in comparisons of genes from the viruses phi X174 and G4. The relative numbers of these nine types as based on all possible interchanges between all 61 amino acid codons were listed by Holmquist et al. and are shown in Table 1. We discuss these results in the light of the significance of nucleotide changes in molecular evolution.