Fig. 1.

Biogenesis and mode of action of miRNAs in RNA silencing. A miRNA gene is expressed from the genome where it undergoes processing in the nucleus before exportation to the cytoplasm. It undergoes further processing to create a mature ~22 nt miRNA (in red). The miRNA binds to a multi-domain protein assembly forming a ribonucleoprotein complex known as the RNA-induced silencing complex (RISC). RISC uses the miRNA sequence to bind to complementary sequences in target mRNAs and physically obstruct translation. RISC also contains an Argonaute protein which is capable of cleaving the mRNA if there is perfect base pairing with the miRNA seed sequence. This type of post-transcriptional gene silencing takes place for up to two-thirds of genes in humans