Fig. 1.

microRNA biogenesis and target binding. (A) The primary miRNA transcript (pri-miRNA) is processed in the nucleus by Drosha ribonuclease. The resulting pre-miRNA (approximately 70 nucleotides in length) is exported to the cytoplasm where it is cleaved by Dicer to form the miRNA duplex. (B) The mature strand of the miRNA duplex is incorporated into the RNA-induced silencing complex (RISC) and directs binding to mRNA target sequences, often in the 3′UTR. Binding is specified by complementarity between the target mRNA and miRNA seed sequence (nucleotides 2–7). An example of sequence complementarity is shown in (B).