Which mutation inserts a premature stop codon, typically resulting in a nonfunctional protein?

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

Answer: (B)

The key idea is that a premature stop codon ends protein synthesis early, producing a truncated protein that is usually nonfunctional. In protein production, codons tell the ribosome which amino acids to add, and stop codons (UAA, UAG, UGA) signal the end of the chain. A nonsense mutation is a single-nucleotide change that converts a codon for an amino acid into a stop codon. Translation halts at that point, yielding a shortened polypeptide that often lacks essential regions needed for proper function.

The other mutation types don’t fit this description by definition. A frameshift mutation shifts the entire reading frame by insertions or deletions, altering downstream amino acids and sometimes creating a stop, but its defining feature is the reading-frame shift. A missense mutation changes one amino acid to another, leaving the rest of the sequence intact. An inversion mutation rearranges a DNA segment, which can disrupt gene function in various ways but does not inherently introduce a premature stop codon.