Double-helical characteristics of RNA. (a) Some of the diversity in the secondary structure of RNA is shown in these examples of G–U base pairs, bulges, internal loops, and hairpin loop structures. (b) An RNA helix in the form of a hairpin structure similar to the one shown in (a); notice how the unpaired bases at the hairpin loop are incorporated into the structure while maintaining the overall helical geometry of an A-form duplex.