The helical structure of DNA and RNA. In these representations, the sugar–phosphate backbone is shown as a solid bar with the bases extending away from the backbone and available for base pairing. (a) Ribbon model of a DNA double helix, consisting of two strands of DNA. Base pairs in the helix twist around the central axis. (b) Ribbon models of three RNA molecules, each consisting of a single strand of RNA: phenylalanine-tRNA from yeast, a self-cleaving RNA from the hepatitis delta virus (HDV), and a self-splicing intron from Tetrahymena. Each RNA includes short stretches of helical structure that fold into a three-dimensional shape. As discussed in the text, the differences in chemical structure between DNA and RNA are the basis for the differences in the three-dimensional structures that they form.