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What is RNAi

What is RNAi

RNA interference (RNAi) is a highly evolutionarily conserved mechanism of gene regulation. The classic RNAi occurs at the post-transcriptional level and is triggered by short double-stranded RNA (dsRNA) also known as short interfering RNA (siRNA) which is processed from long dsRNA by the RNase III enzyme Dicer or introduced into a cell exogenously. After being loaded into the so-called RNA-inducing silencing complex (RISC) in the cytoplasm, the siRNA would cause sequence-specific degradation of its homologous mRNA sequences. RNAi was first discovered in 1998 by Andrew Fire and Craig Mello in the nematode worm Caenorhabditis elegans [1]and later found in a wide variety of other organisms, including mammals.

Mechanism of RNA interference

A. On entering the cell, endogenously transcribed or exogenously introduced long dsRNA acts as a trigger of the RNAi process.

B. It is first processed by the RNase III enzyme Dicer in an ATP-dependent reaction.

C. Dicer processes long dsRNA into 21-23 nt siRNA with 2-nt 3' overhangs. siRNA can also be synthesized outside a cell and then be introduced into a cell through the process of transfection or electroporation.

D. The siRNA are incorporated into the RISC RNAi effector complex which consists of an Argonaute (Ago) protein as one of its main components. Ago cleaves and discards the passenger (sense) strand of the siRNA duplex leading to an active RISC.

E and F. The remaining (antisense) strand of the siRNA duplex serves as the guide strand and guides the RISC to its homologous mRNA, resulting in the endonucleolytic cleavage of the target mRNA. 

References

1. Fire A et al. Potent and specific genetic interference by double-stranded RNA in Caenorhabditis elegans. Nature. 1998 Feb 19;391(6669):806-11.

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