RNAi
RNA interference (RNAi) is a naturally occurring, highly specific mode of gene regulation mediated by short double-stranded RNA (dsRNA) of approximately 20-22 base pairs. One strand of the dsRNA, called the guide strand, shuttles into a cytoplasmic multiprotein complex called the RNA-induced silencing complex (RISC). The guide strand bound to RISC searches the resident population of messenger RNAs (mRNAs) for complementary sequences. Base pairing between the guide and the target mRNA sequence leads to the down-regulation of the expression of the targeted mRNA by any of several pathways. When the complementarity between the guide and the target RNAs is high, the mechanism of down regulation involves the cleavage of the mRNA by an Argonaute protein within the RISC. These short dsRNAs that are capable of inducing RNAi-mediated down regulation of gene expression can be either produced by the cell or introduced externally into the cell using chemically synthesized or in vitro transcribed RNA sequences. These exogenous RNAi molecules can be either siRNAs or shRNAs.
The mechanism of RNA interference