Consider an mRNA strand that has a sequence of b ade pairs of GUU CUA ACU. Suppose a substitution mutation occurred in the DNA, the resulting in the last base in the first codon chart below, a) identify which amino acid will now be formed, and b) explain why the mutation would not result in a phenotypic change.
a) The original mRNA sequence was "GUU CUA ACU". The last base in the first codon is a "U". If there is a substitution mutation in the DNA resulting in a different base at this position, it could be any of the other three bases (A, G, or C). Let's consider each possibility:
1. If the substitution is A, the new mRNA sequence would be "GUA CUA ACU". The codon is GUA, which codes for the amino acid valine.
2. If the substitution is G, the new mRNA sequence would be "GUG CUA ACU". The codon is GUG, which also codes for the amino acid valine.
3. If the substitution is C, the new mRNA sequence would be "GUU CUA ACC". The codon is GUU, which codes for the amino acid valine.
In all three cases, the amino acid formed is valine.
b) The mutation would not result in a phenotypic change because valine is a nonpolar amino acid, and substitutions between similar amino acids may not significantly affect the structure or function of the resulting protein. Additionally, since the mutation occurs in the third base of the codon, it corresponds to the wobble position, which means it may have less impact on the final amino acid sequence and may be synonymous with the original codon.