MicroRNAs in pancreatic ductal adenocarcinoma

Figure 1 Schematic representation of miRNA biogenesis. miRNA genes are transcribed by RNA polymerase II (RNA pol II) into long transcripts called primary miRNAs (pri-miRNA) that contain multiple stem-loop/hairpin structures as independent units in the nucleus. pri-miRNA is processed by the nuclear RNase III Drosha and its cofactor DGCR8/Pasha to generate precursor miRNA (pre-miRNA). The pre-miRNA is rapidly exported to the cytoplasm by exportin 5, where it is further processed by a second RNase III, Dicer, that cuts off the terminal loop and generates a mature about 22-nucleotide miRNA. The mature miRNA is an imperfect double-stranded RNA duplex called miRNA/miRNA*. The double-stranded RNA duplex binds to a protein (Argonaute 2) as a part of the RNA induced silencing complex (RISC), while one of the strands of the duplex, which is complementary miRNA*, is released. The RISC, which contains its miRNA, binds to the target mRNA and triggers either mRNA degradation or inhibition of translation, depending on the degree of complementary between miRNA and its target.