RNA interference (RNAi) is a relatively newly-discovered mechanism in which the presence of small fragments of dsRNA whose sequence matches a given gene interferes with the expression of that gene.
RNAi appears to be a highly potent and specific process which is actively carried out by special mechanisms in the cell. The complete details of how it works are still unknown. It is known to involve the targeted degradation of mRNA transcripts, preventing translation and thus abolishing gene function. dsRNAs directs the creation of small interfering RNAs (siRNAs) which target RNA-degrading enzymes (RNAses) to destroy transcripts complementary to the siRNAs. RNAi is thought to have evolved as a defense against the expression of retrovirus genes or transposons integrated into the host genome.
It was first recognized in plants, but has now been found in most other eukaryotes.
RNAi has recently been applied as an experimental technique to "knockout" genes in model organisms for experimental analysis. (Because RNAi may not totally abolish expression of a gene, using it against a gene is sometimes referred as a "knockdown", to distinguish it from procedures in which the DNA sequence encoding a gene is removed.) Most functional genomics applications of RNAi were made on Caenorhabditis elegans, a tiny worm used as a model organism in genetics research.
The dsRNA that trigger RNAi may be usable as drugs. For example, dsRNA could repress essential genes in eukaryotic human pathogens or viruses that are dissimilar from any human genes, which would be analogous to how existing drugs work. Such applications of RNAi are currently only speculative.
Compare antisense mRNA.
In 1998, A. Fire and C. Mello injected double stranded RNA into C. elegans and noticed a potent gene silencing effect.¹ They coined the term RNAi,
but other biologists had previously used single-stranded RNA as an agent to inhibit translation.