Question

Normally, a certain gene produces a short protein with the amino acid sequence methionine-lysine-serine-tyrosine. After experiencing a mutation, the gene now produces a protein with the amino acid sequence methionine-lysine-alanine-threonine. What type of point mutation would likely cause this change? Explain your answer.

Answer 1

A single-base insertion or deletion (a frameshift point mutation).

Reason: a substitution of one base changes at most one codon, but here the first two amino acids (Met, Lys) are unchanged while the 3rd and 4th both change (Ser → Ala and Tyr → Thr). A one-nucleotide insertion or deletion after the second codon shifts the reading frame so that all downstream triplets are read differently, producing different amino acids for positions 3 and 4.