RNA interference is a regulatory mechanism of mRNA degradation induced by dsRNA in a sequence specific manner. This process has been seen in all eukaryotes from yeast to mammals. RNAi is also known by some other names like post transcriptional gene silencing (PTGS), Transgene silencing, and quelling. RNAi plays an important role in defending cells against parasitic nucleotide sequences – viruses and transposons – but also in directing development as well as gene expression in general. Three types of ribonucleic molecules are involved in RNAi process-miRNA, siRNA and piwi interacting RNA (piRNA) out of which miRNA and siRNA are central to RNAi.
RNAi mechanism includes processing of ds RNA (induced directly or via a transgene or virus) by an RNAIII like enzyme called Dicer. The dsRNA is processed to a 21-30 Nu long RNA called siRNA in an ATP dependent manner and loaded to RNA-induced silencing complex (RISC) by dicer. RISC is thought to provide target specificity through base pairing of the guide strand with the target mRNA. The guide strand directs the gene silencing while anti guide strand or passenger strand is degraded during RISC activation. The endonucleases called argonaute proteins, cleave the mRNA strand that is complementary to bounded siRNA.