Chemical composition of DNA:-

1) DNA is macro biomolecule showing complete structure with high
            molecular weight.

2) DNA is a long chain polymer of nucleotides (Polynucleotide sequence).

3) DNA shows following chemical components.

A. Sugar:-

1) Sugar in DNA is deoxyribose.  (C5H10O4).

2) It is pentose sugar i.e. contains 5 carbon atoms. Its functional group is
            aldehyde (CHO) and thus, it is an aldopentose.

3) It shows pentagonal rings formed by 4 carbon atoms and one terminal
                       oxygen atom.

4) Four carbon atoms of ring are numbered as one, two, three and four
            while fifth carbon atom is outside the ring as part of CH2 OH.

5) Sugar in DNA contains one oxygen atom less than ribose sugar hence
            name deoxyribose. (Oxygen from ‘OH’ group at 2nd carbon is removed).

B) Phosphoric Acid:-

1) It is an inorganic molecule present in nucleotide.

2) It has 3 reactive OH groups of which 2 are involved in formation of sugar phosphate; backbone of each DNA stand.

3) It remains attached with 3rd. Carbon of one sugar molecule and 5th carbon of next sugar molecule.

C) Nitrogen Base:-

1) These are complex organic compounds containing Nitrogen in their ring hence name nitrogen base.

2) These are of 2 types.

a) Purines:-

These are larger bicyclic i.e. double ring compounds for adenine (A), Guanine (G).

b) Pyrimidines:-

These are smaller and monocyclic i.e. containing single ring ex: - cytosine(c), thymine (T) (In RNA thymine is replaced by Uracil (U).)

3. In DNA, Purine, pyrimidine ratio always equals to one.

‘Nucleotide’.

1) Nucleic acids are long chain polymers of nucleotides.

2) Each nucleotide is chemically composed of sugar molecule, N-base and phosphoric acid unit.

3) Phosphoric acid molecule attaches either to 3rd. position or 5th position of sugar molecule.

4) N- base remains attached to first position or 1st carbon atom of sugar molecule and forms nucleoside glycosidic bond Nucleoside = Sugar molecule + 
N- base.

5) The nucleotide of DNA is called as deoxyribonucleotide whereas in RNA it is called as ribonucleotide.

6) The different types of nucleotides are as follows.

a) Adenine Nucleotide

b) Guanine Nucleotide

c) Cytokine Nucleotide

d) Thymine Nucleotide (in DNA only)

e) Uracil Nucleotide (in RNA only)

7) Information for protein synthesis is obtained in form of nucleotides.