GATE BT 2021 • Q12
MCQ • 1 Mark
Section 3: Genetics, Cellular and Molecular Biology
DNA replication, transcription, translation
Subtopic: Fundamentals of DNA replication, transcription, translation
Question Statement:
The enzyme that transcribes the eukaryotic genes encoding precursor ribosomal RNAs (pre-rRNAs) of 28S, 18S and 5.8S rRNAs is
Options:
-
(A) RNA polymerase I
-
(B) RNA polymerase Il
-
(C) RNA polymerase III
-
(D) RNA polymerase IV
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2021 Q.12)**
**1. Governing Concept**:
- **Topic**: Molecular Biology & Eukaryotic RNA Polymerases
- Eukaryotes utilize three specialized nuclear RNA polymerases:
- **RNA Polymerase I**: Transcribes a single polycistronic pre-rRNA ($45\text{S}$ in mammals, $35\text{S}$ in yeast) which is cleaved into the **28S, 18S, and 5.8S rRNAs** in the nucleolus.
- **RNA Polymerase II**: Transcribes pre-mRNAs, most snRNAs, and miRNAs.
- **RNA Polymerase III**: Transcribes 5S rRNA, tRNAs, U6 snRNA, and other small non-coding RNAs.
**2. Option Analysis**:
- **Option (A) [RNA polymerase I] is CORRECT**: The specific nucleolar polymerase transcribing pre-rRNA encoding 28S, 18S, and 5.8S rRNAs.
- **Option (B) [RNA polymerase II] is INCORRECT**: Transcribes protein-coding mRNAs.
- **Option (C) [RNA polymerase III] is INCORRECT**: Transcribes 5S rRNA and tRNAs.
- **Option (D) [RNA polymerase IV] is INCORRECT**: Found in plants for siRNA biogenesis.
**3. Final Answer**:
- **Correct Choice**: **A**
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