Watson and Crick deduced the structure of DNA in 1953 at the MRC unit in Cambridge. The seed that generated this was Watson’s presence at a conference in Naples in 1951, where an x-ray diffraction picture from DNA was shown by Maurice Wilkins from King’s College in London. This made a strong impression on Watson – the first indication that genes might have a regular structure. After his PhD work in the US on bacteriophage genetics, Watson had come to Europe to learn some nucleic acid biochemistry – by 1950, more and more experiments were pointing towards DNA as the essential genetic material – but seeing the DNA x-ray pattern fired him into getting into the x-ray diffraction field. And , via John Kendrew, he found his way to the MRC unit.

The MRC Unit had been established in 1947 and comprised only Max Perutz and John Kendrew studying haemoglobin crystals by x-ray diffraction. The unit was housed in the Cavendish Laboratory, Cambridge at the head of which was Sir Laurence Bragg. Francis Crick joined the unit in 1949 and became involved in the haemoglobin project. However, he became involved in helical diffraction when in 1951, Pauling’s paper proposing the a-helical structure in proteins was published.

Later that year, Jim Watson joined the unit (its first biologist) and began by trying to crystallize myoglobin for Kendrew. The unsuccess of this left much time for discussion with Crick, whose office he was sharing, and the topic of DNA structure naturally arose - particularly how to determine it. They were inclined to follow the method of Pauling who had deduced the a-helical structure by building a model consistent with the x-ray patterns from fibrous proteins. Like proteins, DNA was built from similar units – the bases adenine (A) thymine (T) guanine (G) and cytosine (C), and so it seemed likely that DNA too had a helical structure. The published x-ray patterns of DNA were not very clear, and so contact was made with King’s. Watson attended a DNA colloquium there in November 1951, at which Rosalind Franklin described her results.

Watson brought back a less-than-accurate account to Cambridge, but with Crick produced a three-strand model structure only a week later. Invited to view this, Franklin pointed out that it was inconsistent with her results – it had the phosphate groups on the inside whereas her results showed they were on the outside, and the water content was too low. The work at Cambridge stopped abruptly for a bit.

Watson & Crick building the original model of DNA
at the MRC Unit in 1953.

Courtesy of Mr. Anthony Barrington Brown

In July 1952, Erwin Chargaff visited the unit and told of his 1947 findings that the ratios of A/T and G/C were unity for a wide variety of DNAs. Crick became convinced that base pairing was the key to the structure. Prompted by receiving a flawed manuscript on DNA structure from Pauling, Watson again visited King’s and Wilkins showed him a DNA x-ray pattern taken by Franklin of the pure B-form showing clear helical characteristics, plus the intense 10th layer line at 3.4A and a 20A equatorial reflection indicating the molecular diameter. Perutz also showed them a report on the work of the King’s group which gave the space group of the crystalline A-form as C2, from which Crick deduced that there were two chains running in opposite directions.

Watson began pursuing the idea of hydrogen bonding using cardboard cutouts of the four bases. He found that (A+T) and (G+C) could be bonded together to form pairs with very similar shapes. On this basis a model was built consistent with the symmetry and with Chargaff’s results, and a paper was published in April 1953 in Nature accompanied by ones from the Wilkins and Franklin groups at King’s. Watson and Crick’s paper ends with the oft-quoted line “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material”.

Watson and Crick, with Wilkins, were awarded the Nobel Prize for Medicine in 1962 “for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material”. Rosalind Franklin had died in 1958.

This page is archived from the DNA: 50 years of the Double Helix Conference in 2003.

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