Section 3: Genetics, Cellular and Molecular Biology
DNA replication, transcription, translation
Subtopic: Fundamentals of DNA replication, transcription, translation
Question Statement:
A mutation in a gene that codes for a polypeptide results in a variant polypeptide that lacks
the last three amino acids. What type of mutation is this?
Options:
(A) Synonymous mutation
(B) Nonsense mutation
(C) Missense mutation
(D) Silent mutation
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2019 Q.21)**
**1. Governing Concept**:
- **Topic**: Molecular Genetics & Types of Point Mutations
- Point mutations in coding sequences:
- **Missense mutation**: Single base substitution resulting in an amino acid replacement.
- **Silent (synonymous) mutation**: Base substitution that does not change the encoded amino acid due to codon degeneracy.
- **Nonsense mutation**: A point mutation that converts a sense amino acid codon into a premature translation termination (stop) codon (**UAA, UAG, or UGA**).
- When a premature stop codon is introduced 3 codons before the normal terminus, translation terminates prematurely, generating a truncated variant polypeptide that lacks the last three amino acids.
- Hence, this is a **nonsense mutation**.
**2. Option Analysis**:
- **Option (A) [Synonymous mutation] is INCORRECT**: Produces full-length identical polypeptide.
- **Option (B) [Nonsense mutation] is CORRECT**: Introduces a premature stop codon, causing premature termination and loss of terminal amino acids.
- **Option (C) [Missense mutation] is INCORRECT**: Alters a single amino acid rather than truncating the chain.
- **Option (D) [Silent mutation] is INCORRECT**: Does not change protein length or sequence.
**3. Final Answer**:
- **Correct Choice**: **B**
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