IMAT Biology Practice Exam

A codon-to-stop codon change is a nonsense mutation. This happens when a single nucleotide substitution alters a codon so that it becomes one of the three stop codons (UAA, UAG, or UGA in mRNA; TAA, TAG, or TGA in DNA). The result is premature termination of translation, producing a truncated polypeptide that is usually nonfunctional. This differs from a missense mutation, which changes the codon to encode a different amino acid; a silent mutation, which changes the codon to another that still encodes the same amino acid and doesn’t alter the protein sequence; and a frameshift mutation, which arises from insertions or deletions that shift the whole reading frame and typically disrupt the entire downstream amino acid sequence, potentially creating a premature stop as a secondary effect but not by directly changing a codon to a stop codon.