In the Watson-Crick base pairing, adenine (A) forms a base pair with thymine (T), and guanine (G) with cytosine (C) in DNA. In RNA, thymine is replaced by uracil (U). But there is also non-Watson-Crick base pairing with other hydrogen bonding patterns, especially in RNA. Each turn in DNA have 10 base pairs . The back bone is constituted by sugar and phosphate .