• A DNA is a macromolecule formed of several thousand units or monomers. These monomers are called nucleotides.
• A nucleotide is formed of one molecule of deoxyribose sugar, one molecule of phosphoric acid and one of the four nitrogenous bases.
• A DNA contains following characters:

1. DNA molecule consists of two parallel chains of polynucleotides forming a double helical structure.
2. Both the strands are spirally coiled.
3. The two strands are antiparallel.
4. These helical strands have sugar-phosphate chain on the outside and purines and pyrimidine bases on the inner side of the helix.
5. Both the strands are joined together by weak hydrogen bonds.
6. The distance between two strands is 20A0.
7. There are 10 pairs on a complete turn (34A0). The distance between two base pairs is 3.4 A⁰.
8. A purine base is attached to its specific base. i.e. Adenin(A) attaches to Thymine(T) by two hydrogen bonds , cytosine( C) to Guanine(G) by three hydrogen bonds.
9. A DNA molecule contains 28% adenine, 24% Guanine, 20% cytosine and 28% thymine.
10. Nucleotides in a helix are joined together by Phosphodiester bonds.

It was discovered by Friedrich Miescher in 1869.