A molecule of pentose sugar and a molecule of nitrogenous base is called a nucleoside. It is of two types. Purine nucleosides and pyrimidine nucleosides.
(i) Purine Nucleosides: If the nitrogenous base forming the nucleoside is purine, it is called purine nucleoside. Carbon 1 of pentose sugar is linked to nitrogen 9 of base by glycosidic bond to form purine nucleoside. It is of two types. Adenosine nucleoside and guanosine nucleoside. Pentose sugars combine with adenine to form adenosine nucleosides and pentose sugars combine with guanine to form guanosine nucleosides.
(ii) Pyrimidine Nucleosides: When the nitrogenous base forming the nucleoside is pyrimidine, it is called a pyrimidine nucleoside. The 1st carbon of the pentose sugar is joined to the 1st nitrogen of the base by a glycosidic bond to form a pyrimidine nucleoside. It is of three types. Thymidine nucleoside, cytidine nucleoside and uridine nucleoside. Pentose sugars and thymine combine to form thymidine nucleosides, pentose sugars and cytosine combine to form cytidine nucleosides and pentose sugars and uracil combine to form uridine nucleosides.