MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A patient is diagnosed with a genetic disorder caused by a single nucleotide substitution in a gene, leading to a premature stop codon within the mRNA sequence. Which of the following types of mutations best describes this scenario?
- ASilent mutation
- BNonsense mutation
- CMissense mutation
- DFrameshift mutation
Show answer & explanationAnswer & explanation
Correct answer: B. Nonsense mutation
A nonsense mutation is a point mutation that results in a premature stop codon, leading to a truncated and often non-functional protein. The scenario explicitly states a 'premature stop codon' due to a single nucleotide substitution.
Why the other options are wrong
- A. A silent mutation results in no change to the amino acid sequence, despite a nucleotide change, due to codon degeneracy.
- C. A missense mutation results in a change in the amino acid sequence, but not necessarily a stop codon.
- D. A frameshift mutation is caused by insertions or deletions of nucleotides not in multiples of three, leading to a complete change in the reading frame and subsequent amino acid sequence, or a premature stop codon, but the question specifies a 'single nucleotide substitution' leading to a stop codon, making nonsense more precise.
Nonsense Mutation
A type of point mutation where a single nucleotide change results in a premature stop codon, leading to a truncated polypeptide chain.
- Single nucleotide substitution.
- Introduces a stop codon (UAA, UAG, UGA).
- Results in a shorter, often non-functional protein.
- Can have severe phenotypic consequences.
Memory trick: Nonsense: 'N'o more protein, 'N'ew stop!