Which statement best describes a nonsense mutation's effect on protein length?

Which statement best describes a nonsense mutation's effect on protein length?

• A. It extends the protein.
• B. It shortens the protein by introducing a stop codon.
• C. It changes the start codon.
• D. It has no effect on length.

Answer: (B)

A nonsense mutation introduces a premature stop codon, which causes translation to halt early and yields a shorter protein. Since the ribosome stops at the first stop codon it encounters, any premature one shortens the polypeptide compared with the normal, full-length protein. How much shorter depends on how far into the coding sequence the new stop codon appears. This is why the correct description is that the mutation shortens the protein by introducing a stop codon. It does not extend the protein, and it does not simply change the start codon; those would involve different mutation effects.

A nonsense mutation introduces a premature stop codon, which causes translation to halt early and yields a shorter protein. Since the ribosome stops at the first stop codon it encounters, any premature one shortens the polypeptide compared with the normal, full-length protein. How much shorter depends on how far into the coding sequence the new stop codon appears. This is why the correct description is that the mutation shortens the protein by introducing a stop codon. It does not extend the protein, and it does not simply change the start codon; those would involve different mutation effects.