What mutation shifts the reading frame by inserting or deleting a nucleotide?

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

Answer: (D)

Reading frames are read in groups of three nucleotides to form codons. If you insert or delete a single nucleotide, the downstream grouping shifts by one, so every codon after the mutation changes, typically producing a string of different amino acids and usually an early stop. That big shift in the downstream sequence is what characterizes a frameshift mutation. The other mutation types don’t alter the reading frame: a silent mutation changes a codon to a synonymous one, leaving the same amino acid; a missense mutation changes one amino acid but keeps the rest of the frame intact; a nonsense mutation creates a premature stop codon without shifting the reading frame. So inserting or deleting a nucleotide that changes the downstream codon grouping is the frameshift mutation.