**2. Biogenesis of miRNAs**

## **2.1 Canonical pathway for miRNA biosynthesis**

The biological synthesis of miRNAs may be either from intragenic or intergenic sequences. Most of the intragenetically synthesized miRNAs are from introns whereas some are from exons of the protein coding genes. miRNAs are also synthesized from intergenic sequences which are independent miRNA genes and have their own, specific promoters. There are canonical as well as non-canonical pathways for miRNA genesis [14]. The canonical pathway for miRNA biogenesis marks the transcription of the primary miRNA (pri-miRNA) from the miRNA genes by RNA Polymerase II (RNA Pol II) in the nucleus. After the transcription the pri-miRNA, which can be as long as 1000 nucleotides in length, they are processed by a microprocessor complex consisting of an RNA-binding protein DGCR8 and an RNase III Drosha. Pri-miRNA methylation by methyltransferase-like 3 (METTL3) marks it for recognition by DGCR8 of the microprocessor complex [15]. DGCR8 recognizes the intersection of the flanking single-stranded (ss) RNA and the stem loop in the pri-miRNA hairpin-structure after which, the Drosha is recruited and involves in a cleavage process [16]. This processing forms the precursor miRNAs or pre-miRNAs (~80 nucleotides in length) having 2 nucleotide 3′ overhangs, which are transported from the nucleus to the cytosol by Expotin-5 (XPO-5)/ Ran-GTP

## *Herpesviridae and microRNAs DOI: http://dx.doi.org/10.5772/intechopen.100370*

complex. In the cytosol, the Ran GTPase- activating protein brings about the hydrolysis of GTP, changing the Ran conformation, thereby, releasing the premiRNA bound to the XPO5 [17]. After the release, the RNAse III endonuclease Dicer removes ~22 base pairs (bp) of the pre-miRNA terminal loop to form the mature miRNA duplex (**Figure 1**). This processing step of the miRNA allows them to be eligible for loading onto the Argonaute (Ago) complex of proteins which are the essential components of the RNA-induced Silencing Complex (RISC), therefore, the miRNAs mediate their action (**Figure 1**). The decision for the specific loading of a strand of miRNA duplex on the RISC complex is made on the basis of the thermodynamic stability of the two strands and is accompanied by ATP hydrolysis [18]. The strand with a lower 5′ stability or a 5' Uracil is named as the *guide strand* (~22 nucleotides in length) and is loaded onto the Ago protein, while the strand not loaded onto Ago, named as the *passenger strand*, is cleaved by the slicer activity of the Ago and degraded by the ribonucleases [14, 19].
