Pencil sketch of the DNA double helix by Francis Crick

The Crick papers: 1953 The double helix

1/3/03. By Giles Newton

James Watson and Francis Crick's discovery of the double-helical structure of DNA marks the beginning of a branch of science that we now know as molecular biology.

By the late 1940s, DNA was largely accepted as the molecule that carried genetic information. Yet it was not at all clear what the structure of DNA might be, or how DNA 'worked'.

When Crick and Watson began their quest in 1951 to uncover the structure of DNA, the basic units of DNA had been identified – nucleotides, each containing a phosphate, a sugar, and one of the four nitrogen bases – adenine, cytosine, guanine and thymine. An important clue to the relationship of these ingredients had also been discovered: in 1950, biochemist Erwin Chargaff found that the arrangement of nitrogen bases in DNA varied widely, but adenine:thymine and cytosine:guanine always occurred in a one-to-one ratio.

Another clue had come from Linus Pauling's discovery in 1948 that many proteins take the shape of an alpha helix, spiralled like a spring coil. This suggested that other helical structures might be discovered. As Crick wrote: "Helices were in the air, and you would have to be either obtuse or very obstinate not to think along helical lines."

The double helix
Sketch of the double helix, loose sheet, early 1953.

In April 1953, Watson and Crick published the correct structure of DNA. They had pulled together the evidence from Chargaff's biochemistry and from X-ray diffraction images of DNA and built physical models to work out how it fitted together. The structure has two sugar-phosphate strands twisted into a double helix; the two strands being linked by bonds between complementary bases (adenine and thymine, cytosine and guanine).

The structure of DNA has become an icon of modern biology. At the time, however, Watson and Crick modestly noted that: "This structure has novel features which are of considerable biological interest."

"The first Nature paper was both brief and restrained. Apart from the double helix itself, the only feature that has excited comment was the short sentence: 'It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.' This has been described as 'coy', a word that few would normally associate with either of the authors, at least in their scientific work. In fact it was a compromise, reflecting a difference of opinion. I was keen that the paper should discuss genetic implications. Jim was against it. He suffered from periodic fears that the structure might be wrong and that he had made an ass of himself. I yielded to his point of view but insisted that something be put in the paper, otherwise someone else would certainly write to make the suggestion, assuming we had been too blind to see it. In short, it was a claim to priority."

Crick, 'What Mad Pursuit' (1988), 66.

The Crick papers: 1953 The double helix

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