Figure 1.

miRNA structure and biogenesis. miRNA genes are mainly transcribed by RNA polymerase II in the nucleus to produce pri-miRNA transcripts. The microprocessor complex, composed of the RNase III enzyme Drosha and the dimeric RNA-binding protein DGCR8, produces the pre-miRNA precursor product by cleavage of pri-miRNA. The pre-miRNA is transferred to the cytoplasm by exportin 5 (XPO5). A complex composed of the Rnase III enzyme Dicer and the transactivation response element RNA-binding protein (TRBP) cleaves the pre-miRNA into a mature double-stranded miRNA. A mature miRNA is then incorporated into the miRNA-associated multiprotein RNA-induced silencing complex (mi-RISC). The mature miRNA then binds to complementary regions in the target mRNA and acts as a guide through base pairing with the mRNA to modulate its expression. In most cases, the mature miRNA binds to the 3′-untranslated sequences (3′-UTR) of specific mRNAs via partially complementary sequences and inhibits the translation of the mRNAs into protein. If there is high complementarity between the miRNA and the mRNA, this leads to cleavage of the target mRNA.