RNA interference (RNAi) - Post-transcriptional gene silencing (PTGS) induced by the direct introduction of dsRNA. The term "RNA interference" was first used by researchers studying C. elegans.
siRNAs - Small interfering RNAs. Current models of PTGS indicate that these 21-23 nucleotide dsRNAs mediate PTGS. Introduction of siRNAs can induce PTGS in mammalian cells. siRNAs are apparently produced in vivo by cleavage of dsRNA introduced directly or via a transgene or virus. Amplification by an RNA-dependent RNA polymerase (RdRP) may occur in some organisms. siRNAs are incorporated into the RNA-induced silencing complex (RISC), guiding the complex to the homologous endogenous mRNA where the complex cleaves the transcript.
The Biochemical Mechanism of RNAi
So how does injection of dsRNA lead to gene silencing? Many research groups have diligently worked over the last few years to answer this important question. A key finding by Baulcombe and Hamilton provided the first clue. They identified RNAs of ~25 nucleotides in plants undergoing cosuppression that were absent in non-silenced plants. These RNAs were complementary to both the sense and antisense strands of the gene being silenced (24).
Further work in Drosophila using embryo lysates
and an in vitro system derived from S2 cells shed more ligh