What is a frameshift mutation and what is its typical consequence?

• A. An insertion or deletion not in multiples of three shifts the reading frame, often producing a nonfunctional protein.
• B. A single base pair substitution shifts the reading frame but usually preserves the amino acid sequence.
• C. A four-nucleotide insertion preserves the reading frame and leaves the protein functional.
• D. A silent mutation changes codons without changing the amino acid sequence.

Answer: (A)

Frameshift mutations happen when nucleotides are inserted or deleted in a number not divisible by three, so the reading frame of codons is altered. Since codons are read in groups of three, shifting the frame changes every downstream codon, producing a radically different amino acid sequence from the mutation point onward. This usually wrecks the protein’s structure and function and often introduces a premature stop codon, resulting in a truncated, nonfunctional protein. In contrast, insertions or deletions in multiples of three add or remove whole amino acids without changing the reading frame, which tends to be less disruptive, though it can still affect function. A single base substitution changes one codon but doesn’t shift the frame, so it isn’t a frameshift. A four-nucleotide insertion would also shift the frame, not preserve it, and typically cause a severe disruption.