Figure 1.

MicroRNA synthesis and function. MicroRNA genes are transcribed by RNA polymerase II to produce primary microRNAs (pri-microRNAs) and cleaved by Drosha/DGCR8 into pre-microRNAs. Pre-microRNAs are exported from the nucleus by exportin-5 and Ran-GTP, and then the hairpin is cleaved by Dicer/TRBP in the cytoplasm. The microRNA duplex is unwound and one strand is complexed with argonuate 2 (AGO2) to form the RNA-induced silencing complex (RISC), and the unused passenger strand (miRNA*) is degraded. The microRNA guides the RISC to a partially complementary target region in the 3′ untranslated regions of one or more genes, resulting in translational repression or target cleavage. Distinct proteins or protein complexes are depicted as colored ovals. Red and blue lines indicate RNA.