Because of the double-stranded nature of DNA, essentially all of the nucleotides take the form of Watson–Crick base pairs between nucleotides on the two complementary strands of the double helix.
In contrast, both RNA and proteins are normally single-stranded. Therefore, they are not constrained by the regular geometry of the DNA double helix, and so fold into complex three-dimensional shapes dependent on their sequence. These different shapes are responsible for many of the common properties of RNA and proteins, including the formation of specific binding pockets, and the ability to catalyse biochemical reactions.