**5. The pathway of miRNA biogenesis and gene silencing**

The process of miRNA biogenesis starts in the nucleus as depicted in the following Figure [12, 31, 32]. miRNAs are transcribed as hundreds or thousands of base long large primary miRNA species (pri-miRNA) by RNA polymerase II or RNA polymerase III. These pri-miRNA transcripts fold into a stem loop or hairpin structures with capped 5' end and polyadenylated (poly A) tail on 3' end [55]. Following transcription by RNA polymerase II/III, pri-miRNA transcripts are trimmed to about 60 to 100 nucleotide hairpin structures with ~2 nucleotide 3' overhang to form precursor miRNAs (pre-miRNAs) by the action of nuclear microprocessor complex. Microprocessor complexes are formed of Drosha (RNASEN), a nuclear ribonuclease RNase III enzyme and its partner DGCR8 (DiGeorge critical region 8) also called as Pasha (Partner of Drosha). The pre-miRNA transcripts are then shuttled to cytoplasm for further processing via Exportin5 and Ran-GTP6. Pre-miRNAs are processed further in the cytoplasm by the action of Dicer, another RNase III enzyme, with the assistance of double-stranded RNA binding proteins (dsRBPs) including TRBP (tar RNA binding protein), resulting in the cleavage of hairpin loop of pre-miRNAs leading to formation of ~22 nucleotide mature miRNA duplexes. Mature duplex is composed of a matured miRNA strand referred as guide strand and a complimentary strand referred as the passenger strand. The gene silencing capability depends on Dicer-mediated loading of one of the miRNA strands, usually guide strand, in the RNA-induced silencing complex (RISC) together with Argonaute (Ago) protein. The RISC guides the miRNA to bind to its complementary sequence within the 3'-UTR of its target mRNA. The degree of complementarity between the seed sequence of the miRNA and the 3'- UTR of its target mRNA determines whether to mediate mRNA degradation or to disrupt translation.
