Fig. 1.

The biogenesis process and RNAi mechanism of miRNA and siRNA/shRNA. Most miRNA genes are transcribed by RNA polymerase II (pol II) to produce primary miRNA transcripts (pri-miRNAs) that contain a 5′ cap and a 3′ poly(A) tail. Pri-miRNAs are subsequently cleaved within the nucleus by Drosha and DGCR8/Pasha to generate ~70-nt hairpin precursors known as pre-miRNAs. The pre-miRNA is exported into the cytoplasm by Exportin-5 and further cleaved into a mature ~22-nt miRNA:miRNA* duplex by Dicer, and its partners TRBP and PACT. Subsequently, an RNA-induced silencing complex called RISC is assembled with the protein Argonaute (Ago). The miRNA strand (guide strand) is selectively incorporated into the RISC complex and guides the complex to its mRNA targets through complementary base-pairing interactions between the seed sequence (base 2–8 in the 5′ end of the mature miRNA) and the binding site within target mRNAs. The target mRNA is silenced by either mRNA degradation or translation inhibition. Similarly, shRNA with stem-loop structure is transcribed by RNA polymerase III (pol III). In the cytoplasm, shRNA and synthetic siRNA are subject to the processing by Dicer and its partners TRBP and PACT to give rise to double-stranded ~21-nt siRNA. The guide strand of siRNA is assembled into RISC for target cleavage and gene silencing by RNAi mechanism. miRNA, microRNA; siRNA, small interference RNA; shRNA, short hairpin RNA; RISC, RNA-induced silencing complex