Mutations Practice Test

A cytosolic enzyme is encoded by a gene whose coding sequence includes codons 45–47: 5'-GAA CCT TTT-3', producing mRNA 5'-GAA CCU UUU-3' and amino acids Glu–Pro–Phe. In a mutant, one nucleotide is deleted from the DNA within codon 45, changing the sequence to 5'-GAC CTT TT…-3' from that point onward. No other changes occur in the gene. The enzyme’s active site depends on amino acids encoded downstream of codon 45.

Which outcome is most likely from this mutation?

A gene encodes an enzyme whose active site includes a lysine residue. A mutation changes the mRNA codon for that residue from 5′-AAA-3′ (lysine) to 5′-AAU-3′ (asparagine). The rest of the mRNA sequence is unchanged, and the protein is produced at normal levels. Lysine is positively charged at cellular pH, while asparagine is polar but uncharged. Which outcome is most likely for the enzyme’s function?