The chemical composition of nucleotides. (a) The bases are purines, with nine-membered rings, or pyrimidines, with six-membered rings, with numbering systems as shown. In DNA and RNA, the purines are adenine and guanine; in DNA, the pyrimidines are cytosine and thymine; in RNA, the pyrimidines are cytosine and uracil. The ring atoms in the bases that connect to the ribose (N-9 for purines and N-1 for pyrimidines) are indicated with bold red numbers. (b) Nucleotides consist of a phosphate group, a pentose sugar, and a heterocyclic base; carbons in the pentose rings are numbered as shown, with numbers followed by a prime (′) to distinguish them from the numbered atoms of the bases. In DNA, the pentose is 2′-deoxyribose, which has no hydroxyl group on the 2′ carbon (red); in RNA, the sugar is ribose, which has a 2′ hydroxyl (red). A glycosidic bond links the 1′ carbon of ribose or deoxyribose to the base; β indicates the configuration of the base relative to the pentose ring.