Current Models of the RNAi
Both biochemical and genetic approaches (see "The Genes and Enzymes Involved in PTGS and RNAi" below for a discussion of genetic approaches used to undersand RNAi) have led to the current models of the RNAi mechanism. In these models, RNAi includes both initiation and effector steps (27, see also a Flash animation of "How Does RNAi Work?", from reference 3).
In the initiation step, input dsRNA is digested into
21-23 nucleotide small interfering RNAs (siRNAs), which have also been
called "guide RNAs" (reviewed in 3,
Evidence indicates that siRNAs are produced when the enzyme Dicer, a
member of the RNase III family of dsRNA-specific ribonucleases,
processively cleaves dsRNA (introduced directly or via a transgene or
virus) in an ATP-dependent, processive manner. Successive cleavage events
degrade the RNA to 19-21 bp duplexes (siRNAs), each with 2-nucleotide 3'