How does RNA interference (RNAi) work?

In various organisms, including animals, fungi and plants, RNAi is initiated by double-stranded RNA (dsRNA). During RNAi, the long dsRNA is sliced into small pieces about 21 nucleotides long by an enzyme called Dicer. These small pieces, known as small interfering RNAs (siRNAs), then attach to Argonaute proteins. Once attached, one strand of the dsRNA is removed, leaving the remaining strand to pair with complementary sequences in messenger RNA (mRNA) based on base pairing rules: A pairs with U, and G pairs with C. When the siRNA binds to its target mRNA, the Argonaute protein can either cut the mRNA to destroy it or bring in other factors to regulate the mRNA in different ways.