Which statement describes a frameshift mutation near the start of a gene?

• A. The mutation affects only noncoding regions.
• B. Only the first amino acid is changed.
• C. The amino acid sequence remains unchanged.
• D. Many downstream amino acids are changed and a premature stop codon may occur.

Answer: (D)

Frameshift mutations shift the reading frame of codons, so inserting or deleting a nucleotide near the start changes how every downstream codon is read. Because translation proceeds codon by codon from the start, this shift alters many amino acids along the length of the protein and often creates a premature stop codon, truncating the protein. That’s why the statement describing numerous changes in downstream amino acids and a possible early stop codon is the best fit. Noncoding regions aren’t the target of this kind of mutation, and a frameshift in the coding region does not spare the downstream sequence nor limit change to just the first amino acid.