Q.3 Describe structure of DNA.
Answer:
James Watson and Francis Crick
In 1953, James Watson and Francis Crick proposed structure for DNA.
Double Helix
According to Watson-Crick model, a DNA molecule consists of two polynucleotide strands which are coiled around each other in the form of double helix.
Sugar-phosphate Backbone
There is phosphate-sugar backbone on the outside of double helix.
Nitrogenous Bases
Nitrogenous Bases are present on the inside of double helix.
Base Pairing
In double helix, the nitrogenous bases of opposite nucleotides form pairs through hydrogen bonds.

Specificity in Base pairing
Pairing is very specific. The nitrogenous base Adenine of one nucleotide forms pair with Thymine of opposing nucleotide while Cytosine forms pair with Guanine.
Hydrogen Bonding
There are two hydrogen bonds between Adenine and Thymine while there are three hydrogen bonds between Cytocine and Guanine.