Figure 1.

The biogenesis of miRNAs. miRNA genes are usually transcribed by RNA polymerase II, which first produces large transcripts, referred to as pri-miRNAs. The pri-miRNAs are then cleaved by a complex, which is composed of RNA-binding protein DGCR8 and type III RNase, termed Drosha. The resultant ∼85-nucleotide stem-loop structure is pre-miRNA. Following the transportation from nucleus to cytoplasm by Ran/GTP/exportin 5 complex, the pre-miRNAs are cleaved by another RNase III enzyme, which is called Dicer. The miRNA duplexes are then unwound, and the mature miRNA is incorporated into an RISC complex. According to the complementarity between the miRNA and the targeted mRNA, the miRNA-loaded RISC complex mediates gene silencing by cleavage or by a translational repression.