Question

Consider an mRNA strand that has a sequence of base pairs of GUU CUA ACU.
Suppose a substitution mutation occurred in the DNA, resulting in the last
base in the first codon of the mRNA to be replaced by Cytosine. Using the codon chart below, a)
identify which amino acid will now be formed, and b)
explain why this mutation would not result in a phenotypic change

Answer 1

a) The mRNA sequence GUU CUA ACU corresponds to the codon GUA GAU UGA. The substitution mutation replaces the last base (A) of the codon with Cytosine, resulting in the codon GUA GAC UGA.

Referring to the codon chart, the codon GUA codes for the amino acid Valine (Val), while the codon GAC codes for the amino acid Aspartic acid (Asp). Therefore, the substitution mutation changes the amino acid that will be formed from Valine to Aspartic acid.

b) This mutation would not result in a phenotypic change because both Valine and Aspartic acid are nonpolar amino acids. Nonpolar amino acids generally have similar properties, such as their hydrophobic nature. Therefore, substituting Valine with Aspartic acid would not significantly alter the structure or function of the resulting protein.