**10. Mode of action of miRNAs**

After the production of mature miRNA by either canonical or noncanonical pathway, the resulting mature miRNA duplex is loaded into miRNA inducing silencing complex (miRISC) having Ago as core component [81]. After degradation of one strand, other strands of duplex, representing mature miRNA, then guide miRISC complex to target mRNA to determine its fate [82]. The stability of the base pairs at the ends of the strands of miRNA duplex usually determines which strand will be degraded. Usually, less stable strand avoids degradation [82]. There are different modes of action of miRNAs that influence the expression of target gene ranging from mRNA degradation, translational repression to activation of transcription [16, 83, 84]. Whatever the outcome is, the general way of miRNA-mRNA interaction relies on the sufficient sequence complementarity between mRNA and 5′ seed region (usually 2–8 nucleotides) of miRNA [85]. Perfect complementarity between mRNA and miRNA is likely to downregulate the expression of target gene through its mRNA degradation by endonuclease activity of miRISC complex. This type of miRNA-mediated gene regulation is mostly evident in plants [86]. However, in animals, there is mostly imperfect complementarity between mRNA and miRNA, which usually leads to translational repression of target gene [86]. Interestingly, there are solid experimental evidences that miRNAs are also involved in upregulation of target genes as well. One of its well-characterized examples is host miR-122-mediated upregulation of HCV RNA replication by stabilizing HCV RNA and preventing its decay [87].
