RNA interference (RNAi) refers to downregulation of gene expression through the use of small RNA molecules, which mediate gene expression by either inhibiting translation or causing premature degradation of the genes’ mRNAs.
Two types of small RNA molecules are involved in RNA interference: microRNA (miRNA) and small interfering RNA (siRNA).
The RNAi pathway occurs in many eukaryotes, including humans, and plays a central role in defending cells against viruses and transposons.
The RNAi process begins when an enzyme known as dicer cleaves long doublestranded RNA into small double-stranded RNA fragments (siRNA) approximately 20 nucleotides in length.
The double-stranded siRNA then unwinds into two single-stranded RNAs: the passenger strand (sense strand), which subsequently degrades, and the guide strand (antisense strand), which associates into the RNA-induced silencing complex (RISC).
Next, the guide strand pairs with a complementary sequence in a messenger RNA molecule and induces cleavage using argonaute, the catalytic component of the RISC complex.
Since the mRNA is degraded, its encoded protein is not produced. The result is posttranscriptional gene silencing. This effect is often referred to as “knockdown” because gene expression continues, though in greatly reduced extent. In “knockout,” by contrast, gene expression is entirely absent.