Which mutation occurs when a nucleotide is added, shifting the reading frame?

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

Answer: (B)

Adding a nucleotide changes how the genetic code is read because nucleotides are read in groups of three (codons). Inserting one base shifts all downstream codons by one position, so the amino acids produced after the insertion are different. This specific change is called an insertion mutation, and it causes a frameshift in the reading frame. A deletion would remove a base and could also cause a frameshift if the number removed isn’t a multiple of three. A frame-shift mutation describes the result of such shifts, while the reading frame itself is just the way the sequence is read, not a mutation.