Sugar: The 5 membered cyclic reducing sugar is present in nucleic acid. These are two different varients, the sugar molecule which contains hydroxyl group at 3´ and known as ribose otherwise it is known as deoxyribose sugar. Based on sugar, nucleic acid is classified as RNA or DNA; Ribose sugar is present in RNA whereas deoxyribose is present in DNA. The purpose of sugar in the nucleic acid is to provide the attachment site for nitrogenous bases.

Nitrogenous Bases: There are two varients; These are 9 membered, conjugated double bond system, Purine such as Adenine (A) and Guanine (G). The 6 membered single ring system, pyrimidine such as Thymine (T), Uracil (U) and Cytosine (C). The presence of nitrogenous bases in DNA/RNA is pre-determined. DNA has Adenine, Guanine, Thymine and Cytosine (and strictly no Uracil) whereas RNA has Adenine, Guanine, Uracil and Cytosine (and strictly no Thymine). The exact reasoning behind this distribution we will discuss in the later part of the lecture.

Over all Structure of Nucleic acid: The DNA is doubled stranded whereas RNA is single stranded (in most of the cases). The individual monomer responsible for making DNA or RNA is nucleotide and as a result, DNA or RNA can be considered as polynucleotide. Individual nucleotide is nucleoside attached to one or more phosphate group and can be termed as (1 phosphate group) monophosphate nucleoside, (2 phosphate group) di-phosphate nucleoside and (3 phosphate group) Tri-phosphate nucleoside. Each nucleoside is composed of nitroengous base attached to the sugar through glycosidic bonds (Figure 26.2).         

Figure 26.2: Nucleotide and its variants.