Double Helix RNA Chains

Despite being single-stranded, RNA molecules often exhibit a great deal of double-helical character. This is because RNA chains frequently fold back on themselves to form base-paired segments between short stretches of complementary sequences. If the two stretches of complementary sequence are near each other, the RNA may adopt one of various stem-loop structures in which the intervening RNA is looped out from the end of the double-helical segment as in a hairpin, a bulge, or a simple loop.

Double Helical
Characteristics of RNA

       In an RNA molecule having regions of complementary sequences, the intervening stretches of RNA may become “looped out” to form one of the structures illustrated in the figure. (a) Hairpin (b) Bulge (c) Loop
       The stability of such stem-loop structures is in some instances enhanced by the special properties of the loop. For example, a stem-loop  with the “tetraloop” sequence UUCG is unexpectedly stable due to special base-stacking interactions in the loop. Base pairing can also take place between sequences that are not contiguous to form complex structures aptly named pseudoknots. The regions of base pairing in RNA can be a regular double helix or they can contain discontinuities, such as noncomplementary nucleotides that bulge out from the helix.