FIGURE 1.

Biogenesis of canonical microRNAs. MiRNA genes are transcribed into a single-stranded primary miRNA by POL II or POL III, followed by the modification of the stem-loop structure of pri-MiRNAs by DROSHA, resulting in the formation of a 70–100 nt pre-miRNA with the assistance of the RNA-binding protein DGCR8. Subsequently, pre-miRNAs are exported to the cytoplasm by Ran GTPase and Exportin-5 and cleaved by Dicer to form mature miRNA double strands, one of which, known as the guide strand, is introduced into the RISC complex containing AGO. The other strand, called the passenger chain, is degraded. The miRNA-loaded RISC combines with the reverse complementary sequence within the 3′UTR of the target mRNA, suppressing the translation or degradation of the target mRNA.