Some protein sequences evolve more rapidly than others. But how can this be demonstrated? One approach is to compare several genes from the same two species, as shown for rat and human in the table. Two measures of rates of nucleotide substitution are indicated in the table.

Nonsynonymous changes refer to single-nucleotide changes in the DNA sequence that alter the encoded amino acid (for example, ATC → TTC, which gives isoleucine → phenylalanine). Synonymous changes refer

to those that do not alter the encoded amino acid (ATC→ ATT, which gives isoleucine → isoleucine, for example).

(As is apparent in the genetic code, Figure 7−27, there are many cases where several codons correspond to the same amino acid.)

A. Why are there such large differences between the synonymous and nonsynonymous rates of nucleotide substitution?

B. Considering that the rates of synonymous changes are about the same for all three genes, how is it possible for the histone H₃ gene to resist so effectively those nucleotide changes that alter its amino acid sequence?

C. In principle, a protein might be highly conserved because its gene exists in a “privileged” site in the genome that is subject to very low mutation rates. What feature of the data in the table argues against this possibility for the histone H₃ protein?

Transcribed Image Text:

Gene Amino Rates of Change Acids Nonsynonymous 0.0 0.6 3.1 Synonymous 4.5 4.4 5.5 Histone H3 Hemoglobin a Interferon y Rates were determined by comparing rat and human sequences and are expressed as nucleotide changes per site per 10⁹ years. The average rate of nonsynonymous changes for several dozen rat and human genes is about 0.8. 135 141 136