USA Biology Olympiad (USABO) Practice Exam

Which type of mutation results in a codon that codes for a different amino acid?

1. Missense mutation

2. Nonsense mutation

3. Silent mutation

4. Frameshift mutation

The correct answer is: Missense mutation

A missense mutation occurs when a single nucleotide change in the DNA sequence leads to the substitution of one amino acid for another in the resulting protein. This change can alter the protein's structure and function, depending on the properties of the new amino acid and its location within the protein. Notably, since codons are triplet sequences of nucleotides that specify amino acids, a change in a single nucleotide can result in a different codon that encodes a different amino acid, distinguishing missense mutations from other types. On the other hand, a nonsense mutation introduces a premature stop codon, leading to incomplete protein formation rather than a change in an amino acid. A silent mutation does not alter the amino acid sequence at all, even though the nucleotide sequence may change, meaning it does not fit the criteria of changing the amino acid specified by the original codon. Frameshift mutations result from insertions or deletions of nucleotides that disrupt the reading frame, potentially leading to extensive changes in the amino acid sequence downstream, but they do not specifically substitute one amino acid for another in the way a missense mutation does.