The basics of the biogenesis of miRNA. In most cases, RNA pol II transcribes long primary miRNA (pri-miRNA) transcripts which are cleaved by the RNase III endonuclease Drosha, producing a pre-miRNA. Exportin-5 mediates pre-miRNA exit from the nucleus to the cytoplasm where it is further processed by Dicer, another RNase III endonuclease, producing a 20–22 nucleotide mature miRNA duplex. One strand of the duplex is incorporated into the protein complex RISC (RNA-induced silencing complex) based on its complementarity to the 3′UTR of the mRNA. At this point, the mRNA is either posttranscriptionally silenced or degraded [11].
