Both microRNAs (miRNA) and small interfering RNAs (siRNA) play integral roles in the RNAi. Both these molecules play similar roles but differ in their origin . siRNAs are derived from the cleavage of long dsRNA precursors, which could either be produced endogenously or introduced into the cell from outside. When produced endogenously, siRNAs are derived from the cleavage of long dsRNAs which are produced by RNA-dependent RNA polymerases or from transcription of genes or transposable elements. The cleavage of long precursor dsRNAs into siRNA is catalysed by the Dicer endonuclease. One of the two strands of siRNA is finally incorporated into the RISC.
miRNAs, in contrast, are endogenously derived from the cell. They are encoded within the host genome. The miRNA transcripts contain “near-complementary inverted repeats” which fold back on themselves to form hairpin or stem-loop structures. These are processed in the nucleus by the RNase III enzyme Drosha, and a protein called Pasha in Drosophila or DGCR8 in mammals. The processing reaction generates pre-miRNA, which is further cleaved by the cytoplasmic RNase III endonuclease Dicer complex to produce mature miRNA. This is then incorporated into the RISC. The miRNA could silence genes by two different modes: in plants, miRNAs cleave the cognate mRNAs; in animals, miRNAs predominantly inhibit translation by targeting partially complementary sequences in the 3´ UTR (untranslated region) of mRNAs.