RNA interference (RNAi) is a biological process in which RNA molecules inhibit gene expression or translation by neutralizing targeted mRNA molecules. RNAi is an RNA-dependent gene silencing process that is controlled by RISC and is initiated by short double-stranded RNA molecules in a cells cytoplasm, where they interact with the catalytic RISC component Argonaute. RNAi can be triggered by endogenous triggers such as foreign DNA or double-stranded RNA (dsRNA) of viral origin, aberrant transcripts from repetitive sequences in the genome such as transposons, and pre-microRNA (miRNA). RNAi can also be triggered experimentally by small pieces of RNA that enable RNA interference, such as small interfering RNA (siRNA) and microRNA (miRNA).
The RNAi pathway involves two steps, each involving ribonuclease enzyme. In the first step, the trigger RNA (either dsRNA or miRNA primary transcript) is processed into a short, interfering RNA (siRNA) by the RNase II enzymes Dicer and Drosha. In the second step, siRNAs are loaded into the effector complex RNA-induced silencing complex (RISC). The siRNA is unwound during RISC assembly, and the single-stranded RNA hybridizes with mRNA target. Gene silencing is a result of nucleolytic degradation of the targeted mRNA by the RNase H enzyme Argonaute (Slicer). If the siRNA/mRNA duplex contains mismatches, the mRNA is not cleaved. Rather, gene silencing is a result of translational inhibition.
RNAi has shown promise as a tool in molecular biology. It enables scientists to observe what happens in the absence of a protein whose function is still mysterious, and from there, learn what it does. Furthermore, RNAi enables "knockdown" experiments, in which the synthesis of a protein is reduced but not eliminated, as it would be if its gene were simply cut out or turned off. RNAi may become an important weapon against increasingly antibiotic-resistant bacterial infections, inhibiting bacteria in an entirely different way than antibiotics do. It is another possibility to explore in the fight against cancer.
