RNAi is an RNA-dependent gene silencing process that is mediated by the RNA-induced silencing complex (RISC). The process is initiated by the RNase-III-like enzyme Dicer, which binds and cleaves long dsRNA to produce short double-stranded fragments with two nucleotide 3’ overhangs on either end, called small interfering RNAs (siRNAs). siRNAs are coupled to RISC complex then binds to a target mRNA in a sequence specific manner by base-paring and induce mRNA target cleavage. The cleaved RNA can be recognized by the cell as being aberrant and then destroyed. Therefore without mRNA there is not any translation to get proteins afterwards. This technique is also referred sometimes as gene “knockdown” as RNAi may not totally destroys the expression of the gene but just somehow turns the genes off , unlike the classic processes of gene “knockout” in which expression of a gene is completely eliminated by removing its DNA sequence.
The great power of RNAi in silencing the gene expressions with high specificity has made this technology an interesting subject for many recent studies, trying to use RNAi as a treatment for HIV, viral hepatitis, cancer and genetic diseases. However the therapeutic applications of this method has not yet been approved and still there is considerable difficulties, including difficulties in efficient delivery and uncertainty about potential toxicity of this method , which are left to be overcome in order to apply in vivo. Also potential side effect of using cellular machinery for directing sequence silencing is the big question that is not answered yet.
The following animation shows the RNAi process. It is from the Nature Reviews RNAi collection:
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