Question

mutaions types:

“synonymous” “nonsynonymous” mutations. “silent” “noisy”. “missense”, “nonsense”, and “sense” mutations.

if mutations (and evolutionary base substitutions) were truly random, then which of these categories of mutations would you expect to occur most often in protein coding sequences? Second most often?If you restricted your attention to the third codon position, then which category of mutation would you expect to occur most often?

and why ?

Answer 1

Synonymous mutations are the most common mutations occur most often in protein-coding sequences. Synonymous mutations are may silent or may noisy. If synonymous mutations are silent, then the result specifies the same amino acid as the original codon. If synonymous mutations are noisy, then it may result in a new spliced junction of a coding sequence, which can alter the whole translation process.

Nonsynonymous mutations are the second most common mutations occur on protein-coding sequences. Nonsynonymous or amino acid-altering mutations are further classified into missense, nonsense, and sense mutations. Where missense mutation changes the affected codon into a codon that specifies a different amino acid from the one previously encoded. A nonsense mutation changes a sense codon into a termination codon, thus prematurely ending the translation process and ultimately resulting in the production of a truncated protein. A sense mutation changes a stop codon into a sense codon thereby resulting in an elongated protein.

If any changes occur in third codon position, then the chances of Synonymous mutations expected to occur most often. For example, GGT, GGA, GGC, and GGG all code for glycine. Any change in the third position of the codon (e.g. A->G), will result in the same amino acid being incorporated in the protein sequence at that position. Thus, the chances of occurrence of Synonymous mutations are much greater than the other ones.