A mutation that changes a single nucleotide but does not affect the amino acid sequence is called what?

• A. Frameshift mutation
• B. Missense mutation
• C. Nonsense mutation
• D. Silent mutation

Answer: (D)

This question asks about a nucleotide change that doesn’t alter the protein sequence, highlighting how the genetic code’s redundancy preserves the amino acid sequence despite DNA variation. When a single base is changed but the codon still codes for the same amino acid, the primary structure of the protein remains unchanged. This is a silent (synonymous) mutation. The genetic code is degenerate, meaning multiple codons can encode the same amino acid; for example, some codons ending in different nucleotides still specify the same amino acid, so the translation outcome stays the same. While silent mutations can sometimes influence translation efficiency or mRNA structure, they do not change the amino acid sequence.

In contrast, a mutation that shifts the amino acid sequence would be a missense mutation (one amino acid replaced by another), a nonsense mutation (a stop codon appears early, truncating the protein), or a frameshift mutation (insertions or deletions that alter the reading frame and change downstream amino acids). The key idea is that only when the amino acid sequence remains unchanged is the mutation considered silent.