Figure 1.

Key steps in microRNA (miRNA) biogenesis and activity. miRNAs originate from the nucleus as pri-miRNA precursor molecules, organized as single transcriptional units or as a cluster of miRNAs, co-transcribed as a polycistronic transcript. They are processed by the RNAse III-type enzyme, Drosha, in association with the RNA-binding protein, DGCR8, into smaller precursor miRNAs (pre-miRNAs), then exported to cytoplasm, where they are cleaved by Dicer to their mature form of 22 nt double stranded miRNA. The guide strand of the mature miRNA is incorporated into the miRNA-induced silencing complex (miRISC), where it binds to target mRNA by partial complementarity with its 3′UTR. This results in translational inhibition, mRNA degradation or mRNA deadenylation of the recognized miRNA target. Ago, Argonaute.