Figure 1.

MicroRNA biogenesis and function. MicroRNAs (miRNA) are generated via the mirtron or canonical pathway. Primary microRNA (pri-miRNA) from the canonical pathway is further processed by the Drosha-DGCR8 complex into precursor miRNA (pre-miRNA). Pre-miRNAs derived from both pathways are then exported from the nucleus by Exportin 5. In the cytoplasm, this pre-miRNA is further processed by the Dicer/TRBP/PACT complex into a duplex miRNA. One of the strands (guide strand or mature miRNA) is then loaded into the miRISC complex along with the target mRNA. Depending on the degree of sequence complementary, miRNAs lead to translational repression, deadenylation, or degradation of the target mRNAs, all reducing downstream protein expression. Adenosine deaminases acting on RNA (ADARs) may inhibit various steps of miRNA processing thus reducing or shifting miRNA function. See text for more detail.