The double helical model of DNA structure was given by Watson and Crick.
DNA molecule is helical with two periodicities along its long axis, a primary one of 0.34 nm and a secondary one of 3.4 nm. The most predominant form of DNA present in the cell is B-form. It is a right-handed double helix and runs in the clockwise direction. The two strands of DNA run in antiparallel direction. The two strands in a DNA double helix are joined by hydrogen bonding (H-bonding) between bases. Adenine pairs with thyminne, and guanine pairs with cytosine. Adenine pairs with thymine with two H-bonds, whereas guanine forms 3 H-bonds with cytosine. This is known as complementary base pairing or Watson-Crick base pairing. The base pairs contribute to the thermodynamic stability of the double helix in two ways:
a) H-bonding between bases releases energy.
b) Hydrophobic base-stacking. The interactions between electron systems of stacked base pairs contribute to the stability of the helix.