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

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

Answer: (A)

Shifting the reading frame happens when a nucleotide is inserted or deleted, so the grouping of nucleotides into codons changes for all downstream amino acids. This is a frameshift mutation. Because codons are read in triplets, adding or removing a single base alters every codon after the mutation, often producing a nonfunctional protein or an early stop signal.

The other options don’t necessarily change the reading frame. A point mutation is a single base change that can alter one amino acid but keeps the triplet grouping the same. A chromosomal mutation involves large-scale changes to chromosome structure or number, not just a single-nucleotide frame shift. A silent mutation changes a base but still codes for the same amino acid due to codon redundancy, leaving the protein sequence unchanged.