Fig. 1.

Schematic representation of miRNA biogenesis. miRNA genes are transcribed by RNA pol II to generate long pri-miRNAs. The latter are cleaved by a Microprocessor complex, which includes DROSHA and DGCR8, and produce a 60–70-nucleotide pre-miRNAs. The pre-miRNAs are exported from the nucleus to the cytoplasm by XPO5 and further processed by DICER1 to produce mature miRNA duplexes. The mature guide strand is selected and loaded into RISC, which contains DICER1, TRBP, and AGO proteins. The guide strand directs RISC to the mRNA targets resulting in either mRNAs degradation or translational repression. Alterations at the level of miRNA gene itself or its promoter and the biogenesis core processing enzymes can occur resulting in a dysregulated miRNA expression profile. Abbreviations: miRNA; microRNA, RNA pol II; RNA polymerase II, pri-miRNAs; primary miRNAs, DGCR8; DiGeorge syndrome critical region 8, pre-miRNAs; precursor miRNAs, XPO5; exportin 5, RISC; miRNA-induced silencing complex, TRBP; transactivation-responsive RNA-binding protein, AGO; Argonaute, mRNA; messenger RNA.