The double helix structure is justified by the existence of numerous interactions within the molecule. An interaction exists, first, within one simple chain, causing the helix to turn. A second process exists between each helix as the bases can be paired and stabilise via hydrogen bonds.

The double helix structure of DNA was first modelled by Watson and Crick in 1953. The two researchers had the following information:

• The physical composition of DNA;


• X-ray diffraction images of crystallised DNA taken by Rosalind Franklin and Maurice Wilkins of King’s College. These images showed a cross shape, typical of helix structures;


• The work of Erwin Chargaff, who demonstrated that for all DNA molecules the number of guanine units equals the number of cytosine units and the number of adenine units equals the number of thymine units.


• Analyses performed with an electronic microscope had shown that the diameter of a DNA molecule is 20 Å, which suggested that the molecule had two deoxyribose-phosphate chains.

After creating several molecular models, Watson and Crick succeeded in proposing a structure corresponding to all the crystallographic and biochemical data available at the time: Two strands made up of phosphate and sugar groups entwine to form a double helix of two antiparrallel spirals. The nitrogenous bases are bonded to the sugar groups of each strand and each base in one strand is paired with a base in the other strand by hydrogen bonds. A cytosine base is always paired with a guanine base and an adenine base with a thymine base. The two strands of a DNA molecule are referred to as complimentary.

In 1962, Crick, Watson and Wilkins were awarded the Nobel Prize for their work. Rosalind Franklin would probably have shared the prize with them, but she died prematurely of cancer.