Question

On the one hand, Luria and Delbrück’s famous fluctuation test has demonstrated that point mutations are random. As a corollary, we expect that the mutation rates are the same for the nucleotides at all three positions within a codon. On the other hand, we detect a higher rate of evolution for synonymous than for nonsynonymous nucleotide substitutions. Explain how this apparent discrepancy can be resolved.

Answer 1

A permanent change in the sequence of nucleotides of the genome is called mutation, which results in altered or absence of corresponding protein synthesis. The change in single nucleotide results in point mutation, which is of different types such as silent mutations, missense mutations, nonsense mutations, and frame-shift mutations.

The frequency of mutations varies from a gene to gene (based on its size and complexity).   The frequency of synonymous nucleotide substitutions is because as the genetic code is redundant, a single amino acid may have more than one codon.

For example, the amino acid glutamic acid can be coded by two different codons such as GAA and GAG. As both the triplets code for the same amino acid, no functional change in the protein synthesis occurs. These type of mutations are called as silent mutations (a type of point mutation), and it commonly occurs in several cells, but the phenotypic characters are not expressed out.

The nonsynonymous nucleotide substitutions are rare. For example, missense mutations occur as a result of base substitutions, but the results are not much disruptive like nonsense mutations. Therefore, due to nonsynonymous nucleotide substitutions the extinction of species takes place. Therefore, they would not stay in the population to characterize evolution.