Watson and Crick’s Model of DNA

Watson and Crick’s Model of DNA
Introduction
James Watson and Francis Crick, in 1953 proposed structure of the DNA molecule.
Structure of DNA
Watson and Crick suggested a ladder like organization of DNA.
i. Double Helix
ii. Backbone of DNA
iii. Pairing of Bases
i. Double Helix
Each molecule of DNA is made up of two polynucleotide chains which twisted around each other and form a double helix.
ii. Backbone of DNA
The uprights of the ladder are made up of sugar and phosphate parts of nucleotide and the rungs are made up of a paired nitrogenous bases.
iii. Pairing of Bases
The pairs are always as follows

• Adenine always pairs with thymine and cytosine with Guanine.
• The two polynucleotide chains are complimentary to each other and held together by hydrogen bonds.

Hydrogen Bonding
There are two hydrogen bonds between Adenine and Thymine (A=T) and three between Cytosine and Guanine.
Distance

• Both polynucleotide strands remain separated by 20 A⁰ distance.
• The coiling of double helix is right handed and complete turn occurs after 34 A⁰. In each turn 10 nucleotide pairs are present, therefore the distance between two pairs is about 3.4 A⁰.