Which mutation is defined as a base-pair substitution that results in a codon for a different amino acid?

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

Answer: (B)

A base-pair substitution that changes the codon to specify a different amino acid is a missense mutation. The genetic code links each codon to a particular amino acid, so altering that codon can alter the amino acid incorporated into the growing protein, changing its sequence at that spot. The code is redundant, so some substitutions don’t change the amino acid at all (silent mutations). If a substitution creates a stop codon, translation ends early (nonsense mutation). If the mutation insertions or deletions shift the reading frame, every downstream codon is read differently (frameshift mutation). In this case, the substitution alters the amino acid identity, which defines a missense mutation.