How does siRNA work in RNA interference (RNAi)?

The siRNA is a short double-stranded RNA that is derived from foreign RNA molecules uptaken by cells. During RNAi, the ribonuclease protein “Dicer” is first activated, cutting these exogenous long dsRNA into small fragments of 20-25 base pairs, i.e., siRNA. Then, these siRNAs integrate into a multi-subunit protein called the RNA-induced silencing complex (RISC) and are separated into single strands. One of the two single strands is degraded, while the remaining one is available to base-pair to its target mRNA. Once mRNA is bound to siRNA, mRNA will be cleaved and destroyed. However, the siRNAs remain unharmed throughout the process, which can bind to and destroy other newly-synthesized matching mRNA molecules. In this way, no mRNA is available for translation; thus, protein production from the target gene is silenced.