Model for miRNA biogenesis and functional mechanisms. miRNA genes are transcribed by RNA polymerase II (pol II) into primary transcripts (pri-miRNAs) that are cleaved by the Drosha-DGCR8 complex to 60–70 nt pre-miRNAs. Pre-miRNAs are then transported to the cytoplasm by exportin-5 and there processed by the endonuclease Dicer to generate a double-stranded mature miRNA of about 21–23 nt. After a strand selection/separation process, the mature miRNA is incorporated into the RISC complex while the other strand is degraded. RISC complex will recognize and mediate cleavage or repression of specific mRNAs.
