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 sugar and thymine combine to form thymidine nucleoside, pentose sugar and cytosine combine to form cytidine nucleoside and pentose sugar and uracil combine to form uridine nucleoside.
Nucleotides
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.
Dinucleotide
Two molecules of nucleotide together are called dinucleotide. The 5th carbon of the pentose sugar of one nucleotide and the 3rd carbon of the pentose sugar of the other nucleotide are joined by a phospho-diester bond to form a dinucleotide.
Polynucleotide
Several nucleotides are linked together by phospho-diester bonds to form a long chain. It is called polynucleotide. In polynucleotides, the phosphate molecule is attached to the 5th carbon of the pentose sugar on one side and to the 3rd carbon of the pentose sugar on the other side. Nucleotides in polynucleotides are oriented towards each other R5 → R3 carbons. Each single helix of a DNA molecule is a polynucleotide chain.