Structure of DNA 

Living organisms contains two types of nucleic acids. They are,

1. Deoxyribonucleic acid (DNA)

2. Ribonucleic acid (RNA)

They differ only in the nature of sugar molecule present. DNA contains deoxyribose sugar whereas RNA contains ribose sugar.

Components of nucleic acids

Nucleic acid is a long chain polymer. It is composed of a large number of monomeric units called nucleotides. Each nucleotide consists of a nucleoside and a phosphate group. A nucleoside is formed of a five 

carbon sugar (pentose) and nitrogenous base. There are two types of nitrogenous bases. They are the purines and pyrimidines. To Purines are bicyclic heterocyclic compounds. They have two rings in the structure. One is a five membered ring and other is a six membered ring.Exaples: Adinine and guanine.Pyrimidine are six membered heterocyclic compounds. Cytosine, 

thymine and uracil are the important pyrimidines.Purines are bicyclic heterocyclic compounds. They have two rings in the structure. One is a five membered ring and other is a six membered ring.Exaples: Adinine and guanine.Pyrimidine are six membered heterocyclic compounds. Cytosine, thymine and uracil are the important pyrimidines.

Sugar + Nitrogenous base =          Nucleoside

Nucleoside + Phosphoric acid =     Nucleotide

Polynucleotide =.                         Nucleic acid.

Structure of DNA

DNA is made up of three chemical components namely,

i. Sugar

ii nitrogenous base

iii phosphoric acid

Sugar:

The sugar present in DNA is deoxyribose.

Nitrogenous base:

In DNA, four different nitrogenous bases are present. They combine with the sugar molecule to form four types of nucleosides.

i. Deoxyadenosine Deoxyribose + Adenine

ii. Deoxyguanine Deoxyribose + Guanin

iii. Deoxycytidine Deoxyribose + Cytosine

iv. Deoxythymidine Deoxyribose + Thymine

Phosphoric acid:

The nucleosides combine with phosphoric acid to form four types of nucleotides.

i. Deoxyadenylic acid, ii. Deoxyguanylic acid

iii.Deoxycytidylic acid, iv. Deoxythymidylic acid

Watson and Crick model of DNA

1. The DNA molecule (Fig.5) has two polynuleotide chains.

2. The two chains are twisted around each other in opposite direction toform a right handed double helix.

3. The two chains are held together by hydrogen bonds between bases. A purine pairs with a pyrimidine. For example, adenine (purine) of one chain always pairs with thymine (pyrimidine) of another chain by tow hydrongen bonds,Similarly, guanine (purine) of one chain always pairs with cytosine (pyrimidine) of another chain by three hydrogen bonds.