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vert bar The Discovery of the Double Helix

James Watson and Francis Crick Cambridge April 1953

Genes had been shown by Oswald Avery, Colin MacLeod and Maclyn McCarty at the Rockefeller Institute in 1944, to be made of a polymer, a type of molecule, called DNA, which is found in chromosomes. In the early 1950s Watson and Crick at the Cavendish Laboratory at Cambridge University, using evidence collected by many colleagues, including X-ray data from Maurice Wilkins and Rosalind Franklin at King’s College London, produced a model of the structure of DNA. The atomic model showed two strands wound around one another in a form which was called The Double Helix. The model quickly suggested to Watson and Crick how chemical units in the DNA, called bases and given the symbols A, C, T and G, might hold the genetic information. These units are organised in pairs AT and GC, and these pairs form what look like steps in a helical ladder. The pairs can follow one another in any order, and the actual order in the DNA of an organism is the genetic information. There are 3 billion steps in human DNA, a truly immense amount of information. The model also suggested how the DNA molecule could direct its own replication, and how mutations, essential for the process of evolution, might occur. Such mutations can also be the cause of genetic disorders, such as cystic fibrosis, and are the fundamental cause of all cancers.

Genetical and biochemical evidence which showed that the model was probably correct, was accumulated, notably by Matthew Meselson and Franklin Stahl at the California Institute of Technology in 1958, Arthur Kornberg at Stanford University in 1961. Refined X-ray evidence was collected by Maurice Wilkins on the quasi-crystalline DNA fibres. The final atomic level structural proof came from Richard Dickerson’s detailed X-ray analysis in 1980 of true crystals of DNA by which time the genetical evidence for the structure was overwhelming.

Rosalind Franklin, an experienced X-ray crystallographer came to King’s and at Wilkins’ suggestion she joined the DNA work there in 1951, leaving for Birkbeck in early 1953. The King’s and Cambridge groups “muddled” along, sometimes co-operating, sometimes competing. Wilkins wrote that the discovery of the Double Helix owed much to a scintillating, fizzing collaboration between Watson and Crick, one a geneticist, one a physicist, who had mined every shred of informative evidence, and combined them to make a series of imaginative leaps in interpreting the data. One key idea lay in the magical number of biology - two.

A cell divides into two, it does not split into three of four. The amount of DNA doubles before a cell divides. There are two sexes, not three or more. Erwin Chargaff had discovered that the amount of A in DNA equalled the amount of T, and G equalled C, but he did not make realise the significance. Much more convincing evidence from G. R. Wyatt showed Watson that the bases in DNA are found in pairs. Hey presto, DNA should have two strands, not three as was thought for a while at Cambridge, and for longer at King’s. Crick had the brilliant idea that the two strands run in opposite directions. Watson discovered that they are held together by hydrogen bonds between specific base pairs, A and T, G and C; the doubleness fitted with the X-ray data.

It took great courage to propose the structure and especially to suggest that the structure determines the replication of the sequence. These brilliant deductions were made by Watson and Crick and changed the world of biology. Wilkins declined the offer to be a co-author on the main paper. He shared the Nobel prize for medicine with Watson and Crick in 1962. Franklin died, tragically young, a few years before the prize was awarded.



The original papers from Nature:

A Structure for Deoxyribose Nucleic Acid
Watson J.D. and Crick F.H.C.
Nature 171, 737-738 (1953)
April 25, 1953: James Watson and Francis Crick's classic paper that first describes the double helical structure of DNA. With some understatement they note that the structure “suggests a possible copying mechanism for the genetic material"
Molecular Structure of Deoxypentose Nucleic Acids
Wilkins M.H.F., A.R. Stokes A.R. & Wilson, H.R.
Nature 171, 738-740 (1953)
April 25, 1953: From the same issue, Wilkins, Stokes and Wilson analyse the X-Ray crystallography evidence, and suggest evidence that the structure exists in biological systems.
Molecular Configuration in Sodium Thymonucleate
Franklin R. and Gosling R.G.
Nature 171, 740-741 (1953)
April 25, 1953: Rosalind Franklin and Ray Gosling provide further evidence of the helical nature of nucleic acids, and conclude that the phosphate backbone lies on the outside of the structure.


Genetical Implications of the structure of Deoxyribonucleic Acid
Watson J.D. and Crick F.H.C.
Nature 171, 964-967 (1953)
May 30, 1953: Watson and Crick follow up with largely accurate speculation on how base pairing in the double helix allows replication of DNA.