Fig. 1

Canonical miRNA biogenesis. miRNAs are transcribed by RNA polymerase II from miR genes into a pri-miRNA transcript (> 70 nt), which is metabolized by a microprocessor complex (Drosha and DGCR8), forming the pre-miRNA (~65 nt). The pre-miRNA is then exported from the nucleus by exportin-5 in a Ran-GTP dependent manner. Once in the cytoplasm, it is processed into a dsRNA of ~22 nt by a complex composed of Dicer and a dsRNA-binding protein (TRBP or PACT). Then, this dsRNA is incorporated into the pre-RISC, where strand selection takes place: one strand is degraded (passenger strand, miRNA*) and the other strand (guide strand, miRNA) remains incorporated in the RISC. Through base pairing, miRNAs can form dsRNA with cellular mRNA, usually at the 3′ UTR region, and may either suppress its translation or induce its degradation. Figure by Wagner Nagib de Souza Birbeire