Double helix

Watson and Crick deduced a structural model for DNA which is a double helix. The features of this Watson-Crick model are:

1. Two helical polynucleotide chains coil around a common axis and run in opposite directions to each other.

2. The outside of the double helix structure are the sugar-phosphate backbones whereas the inside are the bases (purine and pyrimidine).

3. The bases are almost perpendicular to the common axis that being coiled around. They are separated from their adjacent ones by 3.4 A and there is a rotation of 36 degrees per base.

4. The diameter of the double helix is 20 A.

The bases on one of the polynucleotide chains are held to the ones on the other polynucleotide chain on the same stack by specific hydrogen bonds. They are base pairs and the bases are complementary to each other. Although the bonding is weak, there are a large number of them in a DNA molecule, which stabilize the structure.