Figure 1.

Biogenesis and functions of microRNAs (miRNAs), long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs). (A) MiRNAs are transcribed from genomic DNA by RNA polymerase II as primary miRNAs (pri-miRNAs). The pri-miRNAs are processed in the nucleus by Drosha producing precursor miRNAs (pre-miRNAs) which are exported to the cytoplasm by exportin 5. The pre-miRNAs in the cytoplasm are processed by Dicer, which cleaves the hairpin structure and produces a miRNA duplex. The miRNA duplex is loaded to Argonaute (AGO), which binds the guide strand and forms the RNA-induced silencing complex (RISC). The mature miRNAs guide the RISC complex to bind their cognate sites in the 3′ untranslated regions (3′UTR) of the target messenger RNA (mRNA) through imperfect base pair complementarity and induce its translational repression or degradation, thus exerting post-transcriptional gene regulation. (B) LncRNAs are transcribed from DNA by RNA polymerase II, similarly to miRNAs. Some lncRNAs undergo splicing and are 5′ capped and 3′ polyadenylated. LncRNAs function in the nucleus inducing chromatin modification and regulating cis and trans gene expression interacting with transcription factors (TFs). In addition, lncRNAs can exit the nucleus and exert different roles in the cytoplasm functioning as miRNA sponges and interacting with proteins and mRNAs to induce or repress their translation. (C) CircRNAs are transcribed from DNA by RNA polymerase II and are generated by backsplicing of exon or intronic regions. CircRNAs exit the nucleus and may function as miRNA and protein sponges or as scaffolds for protein interaction.