**2. miRNA biochemical synthesis**

miRNAs are encoded in the genomes (inter or intragenic) and are transcribed from genes located in nuclear DNA; however, such genes are not eventually translated into protein [3]. These transcribed genes are typically longer than the eventual gene product miRNA and undergo much post-transcriptional modification between initial transcription and the functional miRNA end-product. After initial transcription of the DNA sequence, the miRNA sequence contains a reverse-complement base pair segment that forms a double stranded RNA hairpin loop. The entire DNA transcript, including the double stranded RNA loop, constitute the primary miRNA structure (called pri-miRNA). Pri-miRNA is usually several kilobases long and has local stem loop structures. The primary transcripts undergo further processing in the nucleus. The ribonucleases Drosha and DiGeorge syndrome critical region gene 8 (DGCR8) complex are mainly involved in the pri-miRNA processing, which is cleaved at the stem of the hairpin structure and generates a hairpin intermediate of about 70–100 nucleotides, called pre-miRNA.

The pre-miRNA is then transported out of the nucleus to the cytoplasm for further processing to become mature miRNA. There are nuclear pore complexes in the nuclear membrane where the pre-miRNA can be transported out of the nucleus by means of the RanGTP-dependent nuclear transport receptor exportin 5. In the cytoplasm, the pre-miRNA is processed by another ribonuclease, Dicer to create a mature miRNA. The mature miRNA is a double-stranded miRNA of variable length (~18–25 nucleotides). After the generation of mature miRNA duplex by Dicer, the miRNA duplex is incorporated into an Ago family protein complex, which generates an effector complex. Then one strand of the miRNA is degraded, whereas the other strand remains bound to Ago as mature miRNA (guide strand). After strand separation, the guide strand or mature miRNA is incorporated into an RNA-induced silencing complex (RISC). After loading, the miRNA promotes the RISC to its target mRNA and induces mRNA degradation or translational repression (see **Figure 1**).
