Nucleosides consist of a purine or pyrimidi base attached a sugar through an energy-poor B-glycosidic bond. Naturally occurring pentose sugars include ribose and 2-deoxyribose (Figure 6- 13). When the sugar is attached to a base, the numbered carbon atom the sugar are given a designation to distinguish them from positions of atoms in the -deoxyriboso refers to C2 of the sugar. Ribonucleosides contain ribose, and 2'-deoxyribonucleosides contain deoxyribose.