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GATE 2022 BT Q.21: solution and answer

GATE 2022 · BTQ.21 · 1 mark · MCQ

In animal cells, the endogenously produced miRNAs silence gene expression by

  1. (A)

    base pairing with the 3'-untranslated region of specific mRNAs

  2. (B)

    blocking mRNA synthesis

  3. (C)

    binding to the operator site

  4. (D)

    base pairing with the 3' region of specific rRNAs

See the question in the paper
GATE 2022 BT question 21Tap to open full size

Answer and solution

ANSWER

Option A is correct: in animal cells, endogenous miRNAs silence gene expression by base pairing with the 3'-untranslated region of specific mRNAs.

Official answer: ASource: Official answer key, GATE 2022 BT (IIT Kharagpur)

  1. miRNA-mediated silencing (RNA interference / post-transcriptional gene silencing, PTGS) in animal cells acts after transcription, not by blocking it — the mature miRNA is loaded into the RNA-induced silencing complex (RISC).

  2. Option A: RISC-bound miRNA base-pairs, usually with imperfect complementarity, to the 3'-untranslated region (3'-UTR) of target mRNAs, leading to translational repression and/or mRNA degradation — this is the established mechanism of endogenous miRNA silencing, so A is correct.

  3. Option B: 'Blocking mRNA synthesis' describes inhibition of transcription. miRNAs act on already-synthesized (mature) mRNA, not on RNA polymerase or transcription itself — incorrect.

  4. Option C: 'Binding to the operator site' is a prokaryotic transcriptional repression mechanism (e.g., the lac repressor binding the operator), and has no role in eukaryotic miRNA-mediated gene silencing — incorrect.

  5. Option D: miRNAs base-pair with the 3'-UTR of target mRNAs, not with rRNAs; ribosomal RNA is not the functional target of gene-silencing miRNAs — incorrect.

  6. Hence only option A correctly describes the mechanism of endogenous miRNA-mediated gene silencing in animal cells.

KEY IDEA

Endogenous miRNAs in animal cells silence gene expression post-transcriptionally: mature miRNA loaded into the RNA-induced silencing complex (RISC) base-pairs, typically with imperfect complementarity, to the 3'-untranslated region (3'-UTR) of target mRNAs, causing translational repression and/or mRNA degradation — not transcriptional blocking.

WATCH OUT FOR

Confusing miRNA-mediated post-transcriptional silencing with transcriptional repression mechanisms (e.g., 'blocking mRNA synthesis' or 'binding to the operator site', the latter being a prokaryotic operon-repressor concept like the lac repressor, not applicable to eukaryotic miRNA action). Some students also mistakenly think miRNAs act on rRNA rather than mRNA.

From the GATE 2022 BT paper. Every question of this paper · Download the full paper · All 21 Gene expression questions, with notes

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