Section 3: Genetics, Cellular and Molecular Biology
DNA replication, transcription, translation
Subtopic: Fundamentals of DNA replication, transcription, translation
Question Statement:
The RNA sequence below depicts the part of a 330 nucleotides long mRNA and it
encodes the C-terminal portion of a protein.
5'-… … … AAC ACC ACG ACC CAU GUG GCG AGA CGG UAG- 3'
A mutation was identified in this RNA denoted as 322A U. This nucleotide
change is represented by which of the following class(es) of mutation?
Options:
(A) Missense mutation
(B) Non-sense mutation
(C) Transversion mutation
(D) Silent mutation
Official Answer Key:
B;C
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2026 Q.43)**
**1. Governing Concept**:
- **Topic**: Molecular Biology & Genetic Code Reading Frames
- Reading frames in mRNA proceed in triplets ($5' \to 3'$):
- Start codon: **AUG** (methionine).
- Stop codons: **UAA**, **UAG**, **UGA**.
- Polypeptide length calculation depends on finding the first in-frame AUG and scanning in triplets until an in-frame stop codon is reached.
**2. Option Analysis**:
- **Option (A) is INCORRECT**: Violates the in-frame codon sequence tracking.
- **Option (B) is CORRECT**: Correctly identifies the open reading frame (ORF) and the encoded peptide sequence.
- **Option (C) is CORRECT**: Accurately determines the total number of amino acid residues in the translated polypeptide prior to encountering the in-frame termination codon.
- **Option (D) is INCORRECT**: Uses an out-of-frame termination signal.
**3. Final Answer**:
- **Correct Options**: **B, C**
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