RNA interference (RNAi) is a natural cellular process in which small RNA molecules, such as small interfering RNA (siRNA) and microRNA (miRNA), regulate gene expression by specifically targeting and silencing specific messenger RNA (mRNA) molecules. RNAi is highly conserved across many eukaryotic organisms and plays a crucial role in various biological processes, including development, immune response, and the maintenance of genome integrity.

RNAi works through the following steps:

1. Biogenesis of small RNA molecules: The process begins with the transcription of double-stranded RNA (dsRNA) molecules from endogenous or exogenous sources. Endogenous sources include the transcription of miRNA genes, while exogenous sources include the introduction of synthetic siRNA or the uptake of viral dsRNA.
2. Processing of dsRNA: The enzyme Dicer, an RNase III-type endonuclease, cleaves the long dsRNA molecules into shorter fragments, generating siRNA or miRNA duplexes of around 21-23 nucleotides in length.
3. Formation of the RNA-induced silencing complex (RISC): The siRNA or miRNA duplexes are incorporated into a multi-protein complex called the RNA-induced silencing complex (RISC). Within the RISC, the siRNA or miRNA duplex is unwound, and the guide strand (antisense strand) is retained, while the passenger strand (sense strand) is degraded.
4. Target recognition and silencing: The guide strand of siRNA or miRNA within the RISC complex specifically binds to complementary sequences in the target mRNA molecules. This binding leads to either the cleavage and degradation of the target mRNA (in the case of siRNA) or the repression of translation and subsequent mRNA destabilization (in the case of miRNA).

RNAi has become a powerful tool in molecular biology and has been widely used for studying gene function, validating drug targets, and developing gene-specific therapeutics. RNAi technology has the potential to revolutionize the treatment of various diseases, including cancer, viral infections, and genetic disorders.