Nucleosides and nucleotides When a heterocyclic base is covalently linked to a cyclic sugar molecule a nucleoside is said to be formed (Figure 2). There are two types of sugar involved in naturally occurring nucleosides. Both of these are ribose based and differ only in the presence or absence of an ‘OH’ group at the 2-position of the sugar ring. In deoxyribonucleic acids, the ‘OH’ is missing; hence ‘deoxy’, and in ribonucleic acids the ‘OH’ is present. Once a bond is made between the base and the sugar, the carbon centers on the sugar are labeled 1¢, 2¢, etc. to distinguish between them and positions on the base. The sugar-base bond is made between the 1¢ C of the sugar and the N3 of pyrimidines, or N9 of purines, using the numbering shown in Figure 1.