Which term describes a mutation that changes the reading frame?

• A. Frameshift mutation
• B. Point mutation
• C. Chromosomal mutation
• D. Silent mutation

Answer: (A)

Reading frame is determined by grouping nucleotides into codons of three. A mutation that changes the reading frame happens when nucleotides are inserted or deleted in a number not divisible by three, so every downstream codon is read in a new frame. This is called a frameshift mutation. Because the downstream triplets are shifted, the amino acid sequence changes dramatically and a premature stop codon is often introduced, producing a nonfunctional protein.

This is different from a single-nucleotide change that doesn’t shift the reading frame, which is a point mutation and may or may not alter the amino acid sequence. Larger-scale changes that affect chromosome structure are chromosomal mutations, not specifically about changing how codons are read. A silent mutation is a nucleotide change that does not alter the encoded amino acid, so the protein sequence remains the same.