Section 3: Genetics, Cellular and Molecular Biology
Classical Genetics and Epistasis
Subtopic: Mendelian Genetics & Gene Interactions
Question Statement:
Messenger RNAs (mRNAs) translate to generate polypeptides and the translation
terminates at the UGA, UAG or UAA stop codons. The AGA codon acts as
additional stop codon in the mitochondria. The longest possible polypeptide(s) that
can be hypothetically translated in the cytosol and/or mitochondria, from the given
full length mRNA sequence (36 nucleotides) is/are
5'-AACACCAUGACCCAUGUGGCGAGACGGUAGUUAAAA- 3'
Options:
(A) 7 amino acids long in the cytosol
(B) 5 amino acids long in the mitochondria
(C) 6 amino acids long in the mitochondria
(D) 8 amino acids long in the cytosol
Official Answer Key:
A;C
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2026 Q.42)**
**1. Governing Concept**:
- **Topic**: Molecular Biology & Translation Termination Mechanisms
- Translation terminates when a ribosome encounters one of three canonical stop codons in the mRNA A-site: **UAA (ochre)**, **UAG (amber)**, or **UGA (opal)**.
- In eukaryotes:
- Release factor **eRF1** recognizes all three stop codons.
- Release factor **eRF3** (a ribosome-dependent GTPase) works in complex with eRF1 to promote polypeptide release and ribosome disassembly upon GTP hydrolysis.
- No normal aminoacyl-tRNA recognizes standard stop codons; peptidyl-tRNA hydrolase activity of the ribosome is stimulated by the release factors to hydrolyze the ester bond linking the polypeptide to the P-site tRNA.
**2. Option Analysis**:
- **Option (A) is CORRECT**: Translation terminates at stop codons UAA, UAG, and UGA via recognition by protein release factors rather than tRNAs.
- **Option (B) is INCORRECT**: Stop codons are not recognized by regular charged tRNAs under standard translation.
- **Option (C) is CORRECT**: Hydrolysis of the ester linkage between the nascent polypeptide and peptidyl-tRNA in the P-site releases the complete protein.
- **Option (D) is INCORRECT**: Misidentifies the enzymatic catalytic mechanism of peptide chain termination.
**3. Final Answer**:
- **Correct Options**: **A, C**
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