Q.20 Which of the following is correct about RNA interference (CRNAi)?

1. RNAi take place in all prokaryotic organisms as a method of cellular defense.
2. RNAi involves silencing of a specific mRNA due to a complementary dsRNA molecule that binds to and prevents translation of the mRNA.
3. RNAi involves silencing of all types of RNA due to a complementary dsRNA molecule that binds to and prevents translation of the mRNA.
4. RNAi involves silencing of a specific mRNA by the host cell itself due to overproduction of mRNA by sense strand.

   RNA Interference Correctly Silences Specific mRNA via Complementary dsRNA

   RNAi silences specific mRNA through complementary dsRNA binding, preventing translation—a key defense against viruses and transposons in eukaryotes. The second statement accurately describes this mechanism.

   Introduction

   RNA interference (RNAi) precisely targets specific mRNA for silencing using complementary dsRNA molecules. This post-transcriptional gene regulation mechanism is essential for eukaryotic defense and development. This guide evaluates each statement for accurate exam preparation in molecular biology.

   Statement Analysis

   RNAi pathway: dsRNA → Dicer processing → siRNA/miRNA → RISC loading → target mRNA cleavage or translation inhibition.

   • RNAi take place in all prokaryotic organisms as a method of cellular defense: Incorrect. RNAi is eukaryotic-specific; prokaryotes use CRISPR-Cas for defense. No Dicer/RISC homologs in bacteria/archaea.​

   • RNAi involves silencing of a specific mRNA due to a complementary dsRNA molecule that binds to and prevents translation of the mRNA: Correct. siRNA guides RISC to complementary mRNA, causing cleavage (perfect match) or translation block (mismatches). Argonaute-2 (Slicer) executes.

   • RNAi involves silencing of all types of RNA due to a complementary dsRNA molecule that binds and prevents translation of the mRNA: Incorrect. RNAi targets specific complementary sequences, not all RNAs. Specificity via 21-23 nt siRNA/miRNA.​

   • RNAi involves silencing of a specific mRNA by the host cell itself due to overproduction of mRNA by sense strand: Incorrect. RNAi triggered by exogenous dsRNA (viruses) or endogenous miRNA precursors, not sense strand overproduction alone.​

   Mechanism Details

   Core Pathway:

   1. Long dsRNA → Dicer (endoribonuclease III) → 21-nt siRNA duplexes

   2. siRNA → RISC (Argonaute + accessories) → guide strand selection

   3. RISC + target mRNA → cleavage (Ago2) or translational repression​

   Trigger	Processing	Outcome
   Viral dsRNA	Dicer → siRNA	mRNA degradation
   miRNA genes	Drosha → pre-miRNA → Dicer	Translation inhibition
   Transposons	Endogenous siRNA	Genome stability​

   Biological Roles

   • Defense: Viral RNA degradation (perfect complementarity)

   • Development: miRNA temporal/spatial gene regulation

   • Epigenetics: chromatin silencing via nuclear RNAi

   Applications & Exam Relevance

   • Therapeutics: siRNA drugs (Patisiran FDA-approved)

   • Research: Gene knockdown (Drosophila, C. elegans pioneers)

   • GATE tip: Specificity + dsRNA trigger + RISC = RNAi signature​

   Mnemonic: “dsRNA Dicer Delivers Destruction to Designated mRNA.”