The most common purines are adenine (A) and guanine (G), and the major pyrimidines are cytosine (C), uracil (U), and thymine (T). The purines form bonds to a five-carbon sugar (a pentose) via their N9 atoms, whereas pyrimidines do so through their N1 atoms (Table 3-1). In ribonucleotides, the pentose is ribose, while in deoxyribonucleotides (or just deoxynucleotides), the sugar is 2ⴕ-deoxyribose (i.e., the carbon at position 2¿ lacks a hydroxyl group)