In the standard Watson-Crick model of DNA, all of the sugar groups have the C2ʹ endo conformation. Th is results in close contact between one of the oxygen atoms of the 3ʹ phosphate group and the hydrogen atom at the 2ʹ position of the ribose ring in DNA (Figure 2.8). Th e distance between the phos- phate oxygen and the hydrogen atom is 1.9 Å. If the 2ʹ hydrogen were replaced by a hydroxyl group, as in the ribose ring of RNA, the two oxygen atoms would repel each other strongly. Instead, in RNA, the sugar adopts a C3ʹ endo conformation (Figure 2.9), which causes a change in phosphate separation and a general modi- fi cation of the double-helical structure