Figure 1.

Biogenesis of circRNA. The spliceosome mechanism that normally catalyzes the linear splicing of pre-mRNA s can also perform a back-splicing reaction between two exons, resulting in the formation of circRNA. Back-splicing uses the same canonical splicing machines and canonical splicing sites that are needed for linear splicing. Mechanically, backsplicing requires that the donor and acceptor sites of backspliced exons are in close proximity to each other. This can be achieved by direct pairing of RNA bases of inversely complementary sequences in the introns flanking the backspliced exons which bind to these flanking introns.