Fig. 1

The biogenesis of several kinds of ncRNAs. a Most of miRNA genes are transcribed by Pol II and produce greater than 200-nt pre-miRNAs, which contain at least one hairpin structure harboring the miRNA sequence. In the nucleus, the pre-miRNAs are cleaved into approximately 70-nucleotide pre-miRNAs with a stem-loop structure by Drosha, an RNAse III enzyme. The pre-miRNAs are subsequently exported to the cytoplasm and then cleaved by another RNAse III enzyme, Dicer. Finally, the ~ 22 miRNA duplex was loaded into RISC and the mature single-stranded miRNA guides RISC to recognize mRNA targets. b The endogenous siRNA can be derived from shRNA. The transcription of shRNA gene is driven by a U6 or H1 promoter. ShRNA are then cleaved by Dicer to form mature ~ 21 siRNAs that subsequently are loaded into RISC. c LncRNAs are pervasively transcribed in the genome. According to the origins of transcription sites, lncRNAs can be summarized into different types, including enhancer-associated lncRNA, promoter-associated lncRNA, exonic and intronic lncRNA, long intergenic lncRNA, and antisense lncRNA. d Schematic representation of circRNA generation. Most of circRNAs are derived from pre-mRNAs and characteristic of spliceosome-dependent. CircRNA can be classified into various types, including exon circRNA, intron circRNA, and extron-intron circRNA. A novel type circRNA, called read-through circRNA (rt-circRNA), has been identified (marked in dotted line). The rt-circRNA is circularized from read-through transcripts