RNAi is mediated by the assembly of a nuclease complex that leads to homologous mRNA degradation. When cells detect dsRNA, Dicer, an RNase III-like enzyme, processes the long duplexes into 20-25 bp siRNAs in what is called the RNAi initiating step. In the effector step of RNAi, the siRNA enters an RNAi specific nuclease complex known as the RNA-Induced Silencing Complex (RISC). The siRNA is unwound, exposing the antisense strand. This strand binds to the complementary mRNA, and the mRNA is cut by RISC in the center region of the guiding siRNA strand, destroying the mRNA and preventing the creation of protein.

RNA interference (RNAi) has emerged as a powerful tool for studying and manipulating gene expression. Short double-stranded RNAs activate a sequence-specific RNA degradation process that leads to post transcriptional inhibition of gene expression. The discovery of RNAi has revolutionized studies of gene function and has proven to be an invaluable resource for biotechnology.

RNAi can be thought of as a "watchdog" for the genome. It is believed to be an evolutionary mechanism for protection against viral dsRNAs and transposons. In fact, many plant viruses encode suppressors of PTGS. However, in mammals, the role of RNAi in viral protection is not well understood.

The following link provides an animated tour through the process of RNAi. The animation was specially created for Nature Reviews by Arkitek Studios.

http://www.nature.com/focus/rnai/animations/animation/animation.htm