RNA interference (RNAi)

RNAi is a mechanism of gene silencing originally described in plants and invertebrates and more recently in mammalian cells. RNAi is used as a form of host defense against invading genetic elements such as viruses, transgenes or transposable elements in some species. RNAi operates by cleaving long double stranded RNAs into 21-25mers. These 21mer RNAs (also known as small interfering RNAs; siRNAs) are incorporated into an endonuclease complex called RISC (RNA-induced silencing complex) and act as a guide to cleave homologous target mRNA. siRNAs can be derived in a number of ways: 

    - microRNAs (miRNAs) encoded in the genome form hairpin RNA structures that are cleaved by an RNase-III-like enzyme Dicer into ~21mers

    - long double stranded RNAs often formed as intermediates of viral replication are recognized by Dicer and cleaved into 21mers

    - introduction of chemically synthesized siRNA duplexes into cells e.g. by transfection

    - RNA transcribed from DNA vectors form hairpin structures similar to those seen with miRNA that are recognized and cleaved to 21mers by Dicer

Due to the specificity of mRNA knockdown and the potential to silence any gene, RNAi has become an attractive method with which to target genes of interest either as a means to assign function or as potential therapeutic agents.