RNA Interference (RNAi)

Every observable trait of an organism ultimately comes down to which genes get expressed, and expression itself is a two-stage process - transcription (copying genetic information from one, 'sense', strand of DNA into RNA) followed by translation (turning that RNA into protein). Newly made RNA is not immediately ready for translation, however; it first has to be processed and edited, which includes removing sections called introns, and this processing is also where certain regions of the genome get deliberately silenced before translation ever happens. RNA interference (RNAi) is a distinct, additional biological mechanism for controlling gene expression at the RNA level: it is a process in which RNA molecules actively inhibit gene expression or translation, specifically by neutralising a targeted messenger RNA molecule so that it is degraded before it can be translated into protein. The simplified model of how this works involves two sequential steps, each carried out by a ribonuclease enzyme. First, a 'trigger' RNA - either double-stranded RNA or the primary transcript of a microRNA - is processed by the enzymes Dicer and Drosha into short interfering RNA (siRNA), a much smaller RNA fragment. Second, these siRNA molecules are loaded into a protein complex called RISC (the RNA-induced silencing complex); during this loading process the siRNA unwinds from double- to single-stranded form, and that single strand then hybridises (base-pairs) directly with its complementary target mRNA, marking it for cleavage and silencing …