Section 3: Genetics, Cellular and Molecular Biology
DNA replication, transcription, translation
Subtopic: Fundamentals of DNA replication, transcription, translation
Question Statement:
In animal cells, the endogenously produced miRNAs silence gene expression by
Options:
(A) base pairing with the 3'-untranslated region of specific mRNAs
(B) blocking mRNA synthesis
(C) binding to the operator site
(D) base pairing with the 3' region of specific rRNAs
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2022 Q.21)**
**1. Governing Concept**:
- **Topic**: Molecular Biology & Post-Transcriptional Gene Regulation (miRNA Biogenesis)
- In animal cells, endogenously expressed microRNAs (miRNAs) are transcribed by RNA Polymerase II as long primary transcripts (pri-miRNAs).
- In the nucleus, the microprocessor complex (Drosha + DGCR8/Pasha) cleaves pri-miRNA into a $\sim 70$-nt hairpin precursor (pre-miRNA).
- After Exportin-5 transport to the cytoplasm, the RNase III enzyme **Dicer** cleaves the hairpin loop to generate a mature $\sim 21-23$-nt miRNA duplex.
- The guide strand is loaded into the **RNA-Induced Silencing Complex (RISC)** containing **Argonaute (Ago)** proteins to mediate translational repression or mRNA target cleavage.
**2. Option Analysis**:
- **Option (A) is CORRECT**: Formally matches the official biogenesis and silencing mechanism.
- **Option (B) is INCORRECT**: Misidentifies cytoplasmic processing machinery.
- **Option (C) is INCORRECT**: Erroneous nuclear transport pathway.
- **Option (D) is INCORRECT**: Erroneous endonuclease cleavage specificity.
**3. Final Answer**:
- **Correct Choice**: **A**
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