Nucleic acids are biopolymers, polymers of biological monomeric unit. The monomeric unit of nucleic acid is Nucleotides. Therefore, they are also called as polynucleotides. There are two types of nucleic acids. They are Ribonucleic acid (RNA) and Deoxyribonucleic acid (DNA).

Structure of Nucleic acid:

Nucleic acids are formed of long chain of nucleotide monomeric unit joined by 3ʹ→5ʹ phosphate linkage (as shown in figure:1). Nucleotides contains a nitrogenous base, a pentose sugar and a phosphate. Nucleoside contains a nitrogenous base and a pentose sugar. In nucleotides, nitrogenous bases are aromatic heterocyclic compounds. The nitrogenous bases are purines and pyrimidines. Purines found in RNA and DNA are Adenine and Guanine. Cytosine is found both in RNA and DNA. RNA contains Uracil and DNA contains Thymine. RNA contains D-ribose as pentose, the five carbon sugar unit whereas DNA has D-Deoxyribose as pentose. The pentose is attached to nitrogenous base by β-N-glycosidic linkage. In the purine nucleoside, the N9 of a purine ring forms a covalent bond to C1(1ʹ) of a pentose sugar. The N1 of a pyrimidine binds to C1(1ʹ) of a pentose in pyrimidine nucleosides. The phosphate forms ester bond to hydroxyl groups of adenosine for producing 5′- or 3′-monophosphates. Nucleoside monophosphates have only one phosphate moiety (AMP, TMP). Addition of second or third phosphates to nucleoside leads to production of nucleoside diphosphate (e.g. ADP) or triphosphate (e.g. ATP), respectively. The presence of negatively charges phosphate groups makes the nucleotides and nucleic acids anionic by nature.

Nucleic Acids - an overview | ScienceDirect Topics

Pharmacological application of purine, pyrimidine and nucleotide analogs

It is possible to alter heterocyclic ring or sugar moiety are modified for producing synthetic analogs of purines, pyrimidines, nucleosides annucleotides. These synthetic analogs are highly significant clinically (as given in details in table 1).

Table 1: Analogs of purine, pyrimidine and nucleotides with their applications

1.AllopurinolTreating hyperuricemia, gout
2.6-mercaptopurine, 5-Fluorouracil, 8-azaguanine, 5- or 6-azauridine3-deoxyuridine,5- or 6-azacytidine, AzathioprineTreatment of cancers
3.ArabinosyladenineViral encephalophitis, Neurological disease
4.Zidovudine or AZT (3-azido2′,3′ -dideoxythymidine)AIDS