Question

b) a random mutation occurs and the gga codon is changed to ggu. what effect does this substitution have on the amino acid being called for? explain your answer.
c) a random mutation occurs and the gga codon is changed to cga. what effect does this substitution have on the amino acid being called for? explain your answer.
d) based on your above answers, do all substitutions cause a change in the amino acid sequence of a protein? explain.

Explanation:

Part b)

To determine the effect, we refer to the genetic code. The codon GGA codes for glycine. The codon GGU also codes for glycine (due to the degeneracy of the genetic code, where multiple codons can code for the same amino acid). So, this substitution is a silent mutation as the same amino acid (glycine) is still called for.

First, find the amino acid for GGA (glycine) and CGA. The codon CGA codes for arginine. So, this substitution changes the codon from one that codes for glycine to one that codes for arginine, resulting in a different amino acid being called for (a missense mutation).

From part b, we saw a substitution (GGA to GGU) that did not change the amino acid (silent mutation). From part c, a substitution (GGA to CGA) that did change the amino acid (missense mutation). So, not all substitutions cause a change in the amino acid sequence because of the degeneracy of the genetic code (some codon substitutions still code for the same amino acid, like in silent mutations).

Answer:

This substitution has no effect on the amino acid called for (it remains glycine). GGA and GGU are both codons that code for glycine (due to genetic code degeneracy), so the amino acid is unchanged.

Part c)