DNA: stuff of life
DNA structure
The function of DNA depends to a large extent on its structure. The discovery of the structure of DNA by James Watson and Francis Crick is one of the most famous scientific discoveries of all time. The two scientists used evidence collected by others, particularly Rosalind Franklin and Maurice Wilkins, to deduce the shape of DNA.
Franklin's experiments
One of the most important pieces of evidence came from Franklin's experiments of shining X-rays through crystals of the DNA molecule, and using photographic film to record where the scattered X-rays fall. The shadows on the film can be used to work out where the dense molecules lie. This technique is known as X-ray crystallography.
In 1953 Watson and Crick published their idea that DNA must be shaped like a double helix. A double helix resembles a twisted ladder. Each 'upright' pole of the ladder is formed from a backbone of alternating sugar and phosphate groups. Each DNA base (A, C, T or G) is attached to the backbone and the bases form the rungs. There are ten 'rungs' for each complete twist in the DNA helix.
Watson and Crick suggested that each 'rung' of the DNA helix was composed of a pair of bases, joined by hydrogen bonds. Thus A would always form hydrogen bonds with T, and C with G.
Working out the arrangement of bases in the DNA helix could also have been assisted by 'Chargaff's rules'.
