Silent mutation is defined as a mutation that changes a single nucleotide, but does not change the amino acid created. Which option best represents this definition?

Silent mutation is defined as a mutation that changes a single nucleotide, but does not change the amino acid created. Which option best represents this definition?

• A. A mutation that changes a single nucleotide, but does not change the amino acid created
• B. A mutation that changes an amino acid
• C. A mutation that inserts a stop codon
• D. A mutation that deletes a base

Answer: (A)

The key idea is that some single-nucleotide changes don’t alter the resulting protein because the genetic code is degenerate: many amino acids are encoded by more than one codon. If a mutation changes a codon to another codon that still specifies the same amino acid, the amino acid sequence remains unchanged. This is a silent (synonymous) mutation, so the protein’s primary structure is preserved even though the DNA sequence has changed. This fits the definition because it explicitly describes a nucleotide change that does not change the amino acid produced. In contrast, mutations that change an amino acid, introduce a stop codon, or delete a base will typically alter the protein—missense changes the amino acid, nonsense adds a premature stop, and a base deletion can cause a frameshift that shifts the entire downstream amino acid sequence.

The key idea is that some single-nucleotide changes don’t alter the resulting protein because the genetic code is degenerate: many amino acids are encoded by more than one codon. If a mutation changes a codon to another codon that still specifies the same amino acid, the amino acid sequence remains unchanged. This is a silent (synonymous) mutation, so the protein’s primary structure is preserved even though the DNA sequence has changed.

This fits the definition because it explicitly describes a nucleotide change that does not change the amino acid produced. In contrast, mutations that change an amino acid, introduce a stop codon, or delete a base will typically alter the protein—missense changes the amino acid, nonsense adds a premature stop, and a base deletion can cause a frameshift that shifts the entire downstream amino acid sequence.