Figure 1

The flowchart highlights the processes involved in the following miRNA biogenesis: (1) The process begins inside the nucleus, where RNA polymerase II or III initiates the transcription of miRNA-coding genes to produce ‘pri-microRNA’s[10, 11]. (2) A microprocessor complex comprising both Drosha, an RNase III class enzyme, and Pasha, identifies and cleaves pre-microRNAs generating pre-microRNAs [12–14]. (3) Pre-microRNA molecules are transferred to the cytoplasm through exportin-5-mediated transport, which uses GTP that is bound to the Ran protein. The function of exportin-5 is dependent upon the GTP-bound form of the Ran co-factor for specific binding to the corresponding substrates. Therefore, this process comprises the hydrolysis of Ran-GTP to Ran-GDP, via the Ran GTPase-activating protein in the cytoplasm [15, 16]. (4) In the cytoplasm, Dicer acts to cleave pre-miRNA molecules and, with the action of Argonaute 2 which is required for miRNA-induced silencing, forms an RNA-induced silencing complex (RISC) leading to the creation of a miRNA-induced silencing complex (miRISC). (5) Interaction between the miRNA and its target mRNA in the miRISC can cause either translational repression or miRNA degradation depending upon the degree of complementarity.