Which mutation results in a premature stop codon, truncating the protein?

Which mutation results in a premature stop codon, truncating the protein?

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

Answer: (A)

The key idea is that a premature stop codon is created when a mutation changes a codon that normally codes for an amino acid into a stop codon, so translation ends early and the protein is shortened. This is exactly what a nonsense mutation does: it converts a sense codon into one of the stop codons (UAA, UAG, or UGA), producing a truncated protein because ribosomes stop translating sooner than intended. Silent mutations, in contrast, swap a codon for another that still encodes the same amino acid, so the protein sequence remains unchanged. Missense mutations change one amino acid to another, which can alter function but doesn’t necessarily shorten the protein. Frameshift mutations shift the entire reading frame by insertions or deletions, often leading to a downstream premature stop or extensive changes; the defining feature is the reading-frame shift itself, not just the creation of a stop codon.

The key idea is that a premature stop codon is created when a mutation changes a codon that normally codes for an amino acid into a stop codon, so translation ends early and the protein is shortened. This is exactly what a nonsense mutation does: it converts a sense codon into one of the stop codons (UAA, UAG, or UGA), producing a truncated protein because ribosomes stop translating sooner than intended.

Silent mutations, in contrast, swap a codon for another that still encodes the same amino acid, so the protein sequence remains unchanged. Missense mutations change one amino acid to another, which can alter function but doesn’t necessarily shorten the protein. Frameshift mutations shift the entire reading frame by insertions or deletions, often leading to a downstream premature stop or extensive changes; the defining feature is the reading-frame shift itself, not just the creation of a stop codon.