Figure 2.

miRNA biogenesis and function. miRNAs are transcribed from the genome to create primary (pri-)miRNAs. The first round of cleavage occurs in the nucleus, carried out by the RNase III enzyme Drosha and associated factor Pasha/DGCR8, resulting in the shorter precursor (pre-)miRNA. Nuclear export is mediated by Exportin 5 (XPO5) and Ran-GTP. The second cleavage is carried out in the cytoplasm by the RNase III enzyme Dicer as part of a protein complex that produces the mature miRNA. Strand selection results in release of one of the two miRNA strands, which is degraded. The retained strand is loaded onto Argonaute protein (e.g. Ago2) and forms RISC along with other proteins such as GW182. Silencing can occur by promoting deadenylation and mRNA degradation or translational repression.