Christine V
New member
Some genes evolve rapidly, whereas others are highly conserved. But how can we tell whether a gene has evolved rapidly of has simply had a long time to diverge from its relatives? The most reliable approach is to compare several genes from the same two species, as shown for rat and human in the table below. Two measures of rates of nucleotide substitution are indicated in the table. Non synonymous changes refer to single nucleotide changes in the DNA sequence that alter the encoded amino acid (ATC-TTC, which is I(soleucine) to F(p)henylalanine for example). Synonymous changes refer to those that do not alter the encoded amino acid (ATC-ATT, which is I to I, for example). Individual amino acids are typically encoded by multiple codons.
a) why are there such large differences between the synonymous and the 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 amino acid sequences?
c) In principle, a gene might be highly conserved because it exists in a privileged site in the genome that is subject to very low mutation rates. What feature of the data in the table below argues against this possibility for the histone H3 gene?
GENE AMINO ACID NON SYNONYMOUS SYNONYMOUS
Histone H3 135 0.0 4.5
Hemoglobin alpha141 0.6 4.4
Interferon gamma136 3.1 5.5
a) why are there such large differences between the synonymous and the 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 amino acid sequences?
c) In principle, a gene might be highly conserved because it exists in a privileged site in the genome that is subject to very low mutation rates. What feature of the data in the table below argues against this possibility for the histone H3 gene?
GENE AMINO ACID NON SYNONYMOUS SYNONYMOUS
Histone H3 135 0.0 4.5
Hemoglobin alpha141 0.6 4.4
Interferon gamma136 3.1 5.5