Which mutation arises from adding or deleting one or two bases, shifting the reading frame?

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

Answer: (A)

Frameshift mutations occur when one or two bases are added or deleted, shifting the reading frame of the mRNA. Because codons are read in groups of three nucleotides, changing the number of nucleotides by 1 or 2 alters every downstream codon, producing a cascade of incorrect amino acids and often a premature stop. This dramatic change explains why this option is the best fit. In contrast, missense mutations change one amino acid without shifting the frame, nonsense mutations create a stop codon without rebaselining the reading frame, and silent mutations do not alter the encoded amino acid. An insertion or deletion of three bases would add or remove a single amino acid but leave the reading frame intact, which is not a frameshift.