Correct Answer: (C) Both P and Q

RNA interference (RNAi) is a conserved post-transcriptional gene silencing mechanism that degrades target mRNAs after transcription. It operates through the RNA-induced silencing complex (RISC), which uses small interfering RNAs (siRNAs) to recognize and cleave complementary sequences.

Option Analysis

(A) P only: Incorrect. While RNAi is post-transcriptional gene silencing (P), it also requires RISC (Q) for effector function, making this incomplete.

(B) Q only: Incorrect. RISC mediates silencing (Q), but the process fundamentally constitutes post-transcriptional gene silencing (P), distinguishing it from transcriptional mechanisms.

(C) P and Q: Correct. RNAi silences genes post-transcriptionally via 21-25 nt siRNAs loaded into RISC, which then degrades homologous mRNAs sequence-specifically.

(D) neither P nor Q: Incorrect. Both statements accurately describe core RNAi features established since its discovery in C. elegans and plants.

RNA interference (RNAi) is an event of post transcriptional gene silencing that works through RNA induced silencing complex, regulating gene expression across eukaryotes. Discovered in the 1990s, RNAi uses double-stranded RNA triggers processed into siRNAs by Dicer enzymes.

RNAi Mechanism Steps

• Initiation: dsRNA → Dicer cleavage → 21-25 nt siRNAs.

• Effector Phase: siRNAs load into RISC; Argonaute unwinds and cleaves target mRNA.

• Amplification: RdRP generates secondary siRNAs in plants/fungi.

Biological Roles

• Defense against viruses/transposons.

• Developmental timing (miRNAs).

• Genome stability via heterochromatin formation.​

Applications

Therapeutic RNAi targets cancer genes; agricultural GM crops resist viruses. Nobel Prize 2006 (Fire & Mello) validated its significance.​