Fig. 1.

The processes of siRNA and miRNA-mediated gene silencing. In the miRNA pathway, primary RNA transcripts (pri-miR) are processed by the enzyme Drosha into 60 base pair precursors (pre-miR) in the nucleus. Following export into the cytoplasm, the pre-miR is cleaved into single-stranded 22 nucleotide sequences by the enzyme Dicer and then incorporated into the RISC. The mature miRNA guides the RISC to target mRNA sequences, where it induces either degradation or translational inhibition. The siRNA pathway is similar to the miRNA pathway, with the exception that long, exogenous double-stranded RNA is the initial agent that is processed into a silencing complex