What is the effect of a mutation that changes a codon to a stop codon?

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

Answer: (C)

Changing a codon to a stop codon introduces a premature termination signal during translation, so protein synthesis ends early. The ribosome stops at that codon, producing a truncated polypeptide that usually lacks important downstream regions and is often nonfunctional or unstable. This type of mutation is called a nonsense mutation.

To contrast, a missense mutation changes one amino acid in the protein, which can alter function in various ways but doesn’t necessarily truncate the protein. A silent mutation changes a codon to another that still encodes the same amino acid, so the protein sequence remains unchanged. A frameshift mutation results from insertions or deletions that shift the reading frame, typically altering many downstream amino acids and often creating a nonfunctional protein.