What is the primary role of siRNA in gene regulation?

• A. Catalyze transcription initiation
• B. Stabilize mRNA
• C. Mediate RNA interference to silence gene expression
• D. Catalyze protein folding

Answer: (C)

Small interfering RNA (siRNA) functions to silence gene expression through RNA interference. It starts as a long double-stranded RNA that Dicer processes into short siRNA duplexes. One strand, the guide strand, loads into the RNA-induced silencing complex (RISC), which uses the siRNA sequence to find a complementary messenger RNA. When there is strong base pairing, Argonaute within RISC cleaves the target mRNA, leading to its degradation and a decrease in the corresponding protein. Sometimes imperfect pairing can repress translation or promote mRNA decay, but the overall effect is reduced gene expression. This post-transcriptional silencing is the hallmark of siRNA’s role, unlike initiation of transcription, mRNA stabilization, or assisting in protein folding.