RNA Interference

In RNA interference, or RNAi, small non-coding RNAs bind to complementary messenger RNAs, mRNAs, to prevent their translation to proteins.

Three classes of small RNAs can accomplish RNAi— microRNAs or miRNAs are encoded by a cell's genome; small interfering or siRNAs are derived from exogenous viral double-stranded RNAs; and PIWI-interacting or piRNAs, are specific to the germline.

In general, the RNAi pathway begins when the enzyme Dicer cleaves double-stranded RNA to an approximately 20-25 nucleotide-long siRNA. siRNA then binds to a group of proteins called the RNA-induced silencing complex, RISC.

In RISC, the RNA guide strand remains in the complex while its complementary strand is removed.

The guide strand then binds to its complementary sequence in mRNA. Argonaute, an enzyme that is part of RISC, then cleaves the mRNA, thereby silencing the targeted gene.