Which mutation creates a stop codon where an amino acid should be?

Which mutation creates a stop codon where an amino acid should be?

• A. Nonsense mutation
• B. Missense mutation
• C. Silent mutation
• D. Frameshift mutation

Answer: (A)

The idea being tested is how a single nucleotide change can introduce a stop signal in the coding sequence. A nonsense mutation changes a codon that normally encodes an amino acid into a stop codon (for RNA: UAA, UAG, or UGA; for DNA equivalents: TAA, TAG, or TGA). This causes translation to terminate prematurely, producing a truncated protein that is usually nonfunctional. In contrast, a missense mutation swaps one amino acid for another, a silent mutation alters the codon without changing the amino acid due to codon redundancy, and a frameshift mutation shifts the reading frame by insertion or deletion, often changing many downstream codons and frequently creating a premature stop but not by turning a single codon into a stop signal itself.

The idea being tested is how a single nucleotide change can introduce a stop signal in the coding sequence. A nonsense mutation changes a codon that normally encodes an amino acid into a stop codon (for RNA: UAA, UAG, or UGA; for DNA equivalents: TAA, TAG, or TGA). This causes translation to terminate prematurely, producing a truncated protein that is usually nonfunctional.

In contrast, a missense mutation swaps one amino acid for another, a silent mutation alters the codon without changing the amino acid due to codon redundancy, and a frameshift mutation shifts the reading frame by insertion or deletion, often changing many downstream codons and frequently creating a premature stop but not by turning a single codon into a stop signal itself.