Which term refers to a mutation that shifts the reading frame?

• A. Reading frame
• B. Insertion mutation
• C. Frame-shift mutation
• D. Deletion mutation

Answer: (C)

A frame-shift mutation happens when nucleotides are inserted or deleted in a way that changes how the sequence is read in groups of three. The reading frame is the way the cell groups nucleotides into codons to make amino acids. If the number of nucleotides added or lost isn’t a multiple of three, every downstream codon shifts, so the amino acid sequence changes and a premature stop codon is often introduced. That disruption is what makes it a frame-shift mutation. If the insertion or deletion were in multiples of three, the reading frame would stay the same, and only the length of the protein would change, not the downstream grouping.