FIGURE 1.

A diagrammatic illustration of the biogenesis and functions of circular RNA. circRNA are synthesized from either pre-mRNA or pre-tRNA. The loop structure in the cis/trans element derived biogenesis of circRNA is mediated by base pairing between the complementary sequences which flank the RBPs or the circularised exons. Further, EcRNA, or EIciRNAs are produced by either removing or retaining the intron sequences in the loop structure respectively. circRNA are also produced via lariat derived circularisation with an exon-skipping event. The generation of ciRNA depends on the 7-nt GU sequence and 11-nt C rich sequences present near the 5′ splice site and the branch point site respectively which form a lariat intron. The synthesis of tric RNA involves the presence of an intron-containing pre-tRNA which is cleaved at the BHB motif into an exon half and an intron part. The resulting exons halves are combined to form a mature tRNA and the intron termini form the tricRNA. CircRNAs function mainly as regulatory ncRNAs either by directly regulating the transcription of the gene, by serving as miRNA sponges, protein decoys/dynamic scaffolds, or in the translation of regulatory peptides and regulation of epigenetic modification.