Which mutation is a base-pair substitution that creates an early stop codon?

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

Answer: (C)

A base-pair substitution that creates an early stop codon stops translation sooner than normal, producing a shortened protein. This is called a nonsense mutation because the altered codon becomes one of the stop signals, halting synthesis at that point. Stop codons (in RNA: UAA, UAG, UGA) terminate the polypeptide chain, so an early termination often yields a nonfunctional protein.

Understand that other outcomes from substitutions can occur: a silent mutation changes a codon but still codes for the same amino acid, leaving the protein unchanged; a missense mutation changes one amino acid to another, which may or may not affect function depending on the role of that residue. Frameshift mutations arise from insertions or deletions that shift the reading frame, altering many downstream amino acids and frequently introducing premature stop codons, but they are not base-pair substitutions.