Figure 2. A Diversity of siRNA Sources.
Several different categories of transcripts can adopt dsRNA structures that can be processed by Dicer into siRNAs. These duplexes can be intraor intermolecular, and although most are perfectly base paired, some (e.g., hairpin RNAs and gene/pseudogene duplexes) are not. An siRNA consists of a guide strand (red), which assembles into functional siRISC, and a passenger strand (blue), which is ejected and degraded. All forms of siRISC contain the siRNA bound to an Ago protein, and many if not most forms of siRISC contain additional factors. Target RNAs are then recognized by base pairing, and silencing ensues through one of several mechanisms. In many species, the siRNA populations that engage a target can be amplified by the action of RNA-dependent RNA polymerase (RdRP) enzymes, strengthening and perpetuating the silencing response.