A molecule of phosphate and a molecule of nucleoside combine to form a nucleotide. That is, one molecule of pentose sugar, one molecule of phosphate and one molecule of nitrogenous base is called nucleotide. Nucleotide is the structural unit of nucleic acid. A nitrogenous base is attached to the 1st carbon of a pentose sugar and a phosphate is attached to the 3rd or 5th carbon to form a nucleotide. It is of two types. Purine nucleotides and pyrimidine nucleotides.
(i) Purine Nucleotide: If the nitrogenous base forming the nucleotide is purine, it is called purine nucleotide. It is of two types. Adenosine nucleotide and guanosine nucleotide. Pentose sugars combine with adenine to form adenosine nucleotides and pentose sugars combine with guanine to form guanosine nucleotides.
(ii) Pyrimidine Nucleotide: If the nitrogenous base forming the nucleotide is pyrimidine, it is called pyrimidine nucleotide. It is of three types. Thymidine nucleotide, cytidine nucleotide and uridine nucleotide. Pentose sugars and thymine combine to form thymidine nucleotides, pentose sugars and cytosine combine to form cytidine nucleotides, and pentose sugars and uracil combine to form uridine nucleotides.
Different types of nucleotides are AMP, GMP, CMP, UMP, TMP, dAMP, dGMP, dCMP, dUMP, dTMP etc.