DNA is a genetic material of all organism except RNA viruses.

(i) DNA is made up of two polynucleotide chains. The unit of polycleotide is nucleotide. Nucleotide is made up of nitrogenoces bases, pentose sugar and phosphate.

(ii) The bases are of two types- pyrines and pyrimidines.

(a) Purines- Adenine (A) and Guanine (G)

(b) Pyrimidines- Cytosine (C) and Thymine (T)

(iii) Sugar is in the form of dexyribose

(iv) Phosphate is in the form of orthophosphoric acid.

Chemical linkage:-

The base is attached to the number carbon of the sugar. The phosphate group is attached to the number 5-carbon of one sugar and number 3-carbon of sugar of adjacent nucleotide with loss of one molecule of water. This bond is called phosphocliester bond which forms polynucleotide.

Double helix:-

(i) On the basis of works by chargaff and Wilkins, Watson and crick proposed double helix model of DNA. According to this model, DNA is made up of two helical polynucleotides. Both relices are right handed and coiled around the same axis to form a double helix.

(ii) The basis are on the inside of helix in pairs arranged in such a fashion that purine of one chain pairs with pyrimmidine of another chain (A -T) and (G - C).

(iii) The pair between bases is faciliated by hydrogen bonding. The hydrogen bonds between A = T is two and between G ≡ C is three. That is why DNA with high G - C content has mas melting point.

(iv) The distance between two nucleotides is 3.4 Å. One turn of the helix convers a distance of 34 Å, hence 10 base pairs are accommodated per turn of the helix. Since one turn of the helix involves 360° it is evident that one-base-pair at an angle of 36°. These two helices are so coiled as to generate successively a major and a minor groove on each side.

(v) The diameter of double helix is about 20 Å.

(vi) The two helices of DNA run anti-parallel i.e. one strand in 5'-3' direction and another in 3'-5' direction. This is necessary to present pair of bases in such position to each other as to make possible formation of H-bond.