Fig. 1

MicroRNA Processing Steps Pathways. The process begins in the nucleus of cells and continue in the cytoplasm of cells. Primary miRNA (pri-miRNA) transcription is done mainly by RNA polymerase II. Drosha and DGCR8 trim 100 nt to 120 nt long pri-miRNA to become a shorter 70 nt pre-miRNA and it is exported into cytoplasm by exportin 5 for further processes. Pre-miRNA in the cytoplasm is cleaved by RNase III, Dicer I and Argonaute 2 (AGO2) to become a 20 nt to 30 nt long duplex miRNA consists of a guide strand and a passenger strand. From the duplex miRNA, the strand, 3′ to 5′ miRNA known as passenger miRNA is degraded due to PIWI domain in RISC. The guide strand 5′ to 3′ miRNA which is engaged with PAZ domain in RISC binds to its target mRNAs and causes translational repression and cleavage of target mRNAs