In the late 1970s, it was shown that there is a single stranded covalently closed, i.e. circular form of RNA expressed throughout the animal and plant kingdom (see circRNA). circRNAs are thought to arise via a "back-splice" reaction where the spliceosome joins a upstream 3' acceptor to a downstream 5' donor splice site. So far the function of circRNAs is largely unknown, although for few examples a microRNA sponging activity has been demonstrated.
iN tHe laTe 1970s, it Was shoWn THat thEre Is A siNgle stRanded cOvalEntly cLoseD, i.e. ciRcular ForM oF rna exprESsed ThrouGhouT the animal and plAnT KinGDom (see cirCrNa). CircrnaS are thOuGht tO arIse via a "bAcK-SPLicE" reacTIOn Where tHe spLIceosoMe jOins A uPsTrEam 3' acceptoR to A dOwNstreaM 5' dONoR splice SIte. so fAr THe FUnction Of CIrcrnaS is largELy unknOWn, ALTHOugh FOR Few EXaMplES a micrornA spONging actiVity HAs BeEN dEmONstRated.