RNA interference (RNAi).

1. Formation of small interfering RNAs (siRNAs)
2. Activation of RNase III like enzyme Dicer
3. Inactivation of target RNA
4. Formation of RISC complex

Choose the correct answer from the options given below:

1. (A), (B), (C), (D)
2. (A), (C), (B), (D)
3. (B), (A), (D), (C)
4. (C), (B), (D), (A)

   RNA interference (RNAi) follows a precise sequence where Dicer first processes dsRNA into siRNAs, followed by RISC formation and target degradation. The correct order is (B), (A), (D), (C).

   RNAi Pathway Overview

   RNA interference silences genes via double-stranded RNA (dsRNA) processing in eukaryotes.
   Key steps involve Dicer cleaving dsRNA into siRNAs (~21-25 nt), siRNA loading into RISC, and Argonaute-mediated mRNA cleavage.
   This post-transcriptional process regulates gene expression, defends against viruses, and aids research.

   Correct Answer: (B), (A), (D), (C)

   The sequence is: Activation of Dicer (B), siRNA formation (A), RISC complex formation (D), target RNA inactivation (C).
   Dicer (RNase III enzyme) first processes long dsRNA into siRNAs; these duplexes then assemble into RISC, where the guide strand directs Ago2 to cleave complementary mRNA.
   This order ensures precise silencing, as confirmed in standard RNAi models across organisms.

   Option Breakdown

   Option	Order (Labels)	Explanation	Correct?
   (A), (B), (C), (D)	siRNA → Dicer → Inactivation → RISC	Wrong; siRNAs form after Dicer activation, and RISC precedes inactivation.​	No
   (A), (C), (B), (D)	siRNA → Inactivation → Dicer → RISC	Wrong; skips logical flow—Dicer must precede siRNA and inactivation.​	No
   (B), (A), (D), (C)	Dicer → siRNA → RISC → Inactivation	Correct; matches canonical pathway: processing, loading, targeting, cleavage.	Yes
   (C), (B), (D), (A)	Inactivation → Dicer → RISC → siRNA	Wrong; inactivation is final; siRNAs can’t form after it.	No

   Why This Order Matters

   Disrupting sequence (e.g., no Dicer) halts siRNA production, blocking RISC activation.​
   RISC unwinds siRNA, discards passenger strand, and uses guide strand for mRNA binding/degradation via Ago slicer activity.
   Applications include gene knockdown in therapy and functional genomics.​