1. NUCLEIC ACID
Nucleic acid: Nucleic acid is the polymer of nucleotide. There are two types of
nucleic acid:
1. Deoxyribonucleic acid (DNA)
2. Ribonucleic acid (RNA)
DNA: DNA is a very long thread like macromolecule made up of a large number of
deoxy ribonucleotides, each composed of a base, sugar and a phosphate. These
bases of the DNA molecule carry genetic information, whereas their sugar part
and Phosphate groups perform a structural role.
The sugar molecule present in the DNA lacks an oxygen atom that is present in
ribose.
The bases present in the DNA are:
1. Purine: Adenine (A), Guanine (G)
2. Pyrimidines: Thymine(T), Cytosine (C)
Uracil is another pyrimidine bases present only in RNA instead
of Thymine.

2. Nucleoside: a nucleoside contains a purine or pyrimidine base & a ribose or
deoxyribose sugar. Four nucleoside units in the DNA are called deoxyadenosine,
deoxy guanosine, deoxythymidine & deoxycytidine. In a deoxyribonucleoside, N-9
of a purine or N-1 of a pyrimidine is attached to C-1 of deoxyribose.
Nucleotide: a nucleotide is a phosphate ester of a nucleoside. The site of
esterification in nucleotides is the hydroxyl group attached to C5 of the sugar. This
compound is called a nucleoside 5’-phosphate or 5’- nucleotide. For example
deoxyadenosine 5’- triphosphate (dATP) is the active precursor of DNA synthesis.

3. Adenine is always paired with thymine by making two hydrogen bonds.
Guanine is always paired with Cytosine by making three hydrogen bonds.
Structure of DNA:
The backbone of DNA which is invariant throughout the molecule consists of
deoxyribose linked by phosphate groups. The 3’-hydroxyl of the sugar moiety is
joined to the 5’-hydroxyl of the adjacent sugar by a phosphodiester bridge.
The Variable part of DNA is its sequence of four kinds of bases (A, G, C, and T). The
corresponding nucleotide units are called deoxyadenylate, deoxyguanylate,
deoxycytidylate and deoxythymidylate.