Discovery of the DNA Double Helix (1953): Watson and Crick

The discovery of the DNA double helix in 1953 revealed the structure of the molecule that carries genetic information in living things. At the Cavendish Laboratory in Cambridge, James Watson and Francis Crick built a model of two intertwined strands joined by paired bases, drawing crucially on X-ray data obtained at King’s College London by Rosalind Franklin and Maurice Wilkins. Published in Nature on 25 April 1953, the model showed how hereditary information could be stored and copied, and it founded modern molecular biology.

Background: the search for the gene

By the mid-20th century biologists knew that heredity followed rules, first described by Gregor Mendel in his pea plant experiments, and that genes were carried on chromosomes. What genes were made of, and how they worked, remained unknown.

DNA, deoxyribonucleic acid, had been isolated by the Swiss chemist Friedrich Miescher in 1869, but it was long thought too simple to carry information. It is made of a sugar, a phosphate group and four bases: adenine (A), thymine (T), guanine (G) and cytosine (C). Many scientists believed that proteins, with their 20 different amino acids, were the more likely material of heredity.

That view began to change in 1944, when Oswald Avery, Colin MacLeod and Maclyn McCarty in New York showed that DNA could transfer inherited traits between bacteria. In 1952 Alfred Hershey and Martha Chase reached a similar conclusion with viruses. Meanwhile the biochemist Erwin Chargaff had found that in DNA the amount of A always equals the amount of T, and G equals C, a pattern known as Chargaff’s rules. Nobody yet understood why.

Two laboratories, two approaches

At King’s College London, in the Medical Research Council biophysics unit led by John Randall, Maurice Wilkins had begun studying DNA by X-ray diffraction. In this technique X-rays are scattered by a crystal or fibre, and the resulting pattern of spots reveals how its atoms are arranged.

In January 1951 Randall recruited Rosalind Franklin, an expert X-ray crystallographer who had worked in Paris. Franklin and her PhD student Raymond Gosling improved the equipment and discovered that DNA exists in two forms, a drier crystalline “A” form and a wetter “B” form. A misunderstanding over who was in charge of the DNA work created lasting friction between Franklin and Wilkins.

In Cambridge, the 23-year-old American biologist James Watson arrived at the Cavendish Laboratory in autumn 1951 and began working with Francis Crick, a 35-year-old British physicist still completing his PhD. Their laboratory, headed by Sir Lawrence Bragg, was officially studying protein structure. Watson and Crick chose to approach DNA by building physical models, inspired by the American chemist Linus Pauling, who had used models to discover the alpha helix in proteins in 1951.

Their first attempt was a failure. In November 1951 Watson attended a seminar at King’s where Franklin presented her results. Working partly from his imperfect memory of the talk, he and Crick built a three-stranded model with the phosphate backbones on the inside. When Franklin, Wilkins and colleagues came to Cambridge to see it, Franklin pointed out that it could not be right: her data showed that the phosphates were on the outside and that the molecule held much more water.

Embarrassed, Bragg told Watson and Crick to stop work on DNA and leave it to King’s. They largely did so for about a year.

The race of early 1953

In December 1952 news arrived that Linus Pauling was proposing a structure for DNA. When his paper arrived in Cambridge in late January 1953, Watson and Crick saw that Pauling’s three-stranded model contained a basic chemical error. They feared he would soon correct it, and Bragg allowed them to resume.

On 30 January 1953 Watson visited King’s. Wilkins showed him an X-ray image of B-form DNA, now known as Photo 51, which Gosling had taken under Franklin’s supervision in May 1952. Its striking X-shaped pattern was a clear sign of a helix. Wilkins showed it without Franklin’s knowledge; she was preparing to leave King’s for Birkbeck College.

Soon afterwards Crick saw a Medical Research Council report from December 1952 that summarised the King’s group’s work, including Franklin’s measurements. It was passed on by Max Perutz, a member of the committee that received it. The data indicated the dimensions of the helix and suggested that it had two strands running in opposite directions.

Building the double helix

The last puzzle was how the bases fitted inside the backbones. In late February 1953 Watson experimented with cardboard cut-outs of the bases. The American chemist Jerry Donohue, who shared their office, told him that textbooks showed the wrong chemical forms for some bases. Using the correct forms, Watson found that adenine paired with thymine, and guanine with cytosine, producing pairs of identical shape.

This pairing explained Chargaff’s rules at a stroke. On 28 February, according to Watson’s memoir, Crick announced at lunch in the Eagle pub that they had discovered the secret of life. Over the following week they built a full-scale model of metal plates and rods, completed around 7 March 1953, and checked that it fitted the X-ray data.

In the model, two sugar-phosphate backbones wind around each other in a right-handed helix about 2 nanometres wide, with the base pairs stacked like the rungs of a twisted ladder, about ten pairs per turn. The two strands run in opposite directions.

Publication

Watson and Crick’s one-page paper appeared in Nature on 25 April 1953, alongside two papers from King’s: one by Wilkins, Alexander Stokes and Herbert Wilson, and one by Franklin and Gosling that included Photo 51. Readers were not told how much the Cambridge model depended on the King’s data.

The paper contained one famous understated sentence. The authors noted that the specific base pairing “immediately suggests a possible copying mechanism for the genetic material”. A second paper, published on 30 May 1953, spelled out how each strand could act as a template for a new partner strand.

Key people

• James Watson: American biologist who worked out the base pairing; author of the bestselling memoir The Double Helix (1968).
• Francis Crick (1916–2004): British physicist and biologist who led much of the theoretical work, later proposing the “central dogma” of molecular biology.
• Rosalind Franklin (1920–1958): chemist and crystallographer whose precise X-ray data and measurements were essential to the model. She later did important work on virus structure.
• Maurice Wilkins (1916–2004): New Zealand-born British physicist who began the King’s DNA work and later refined the structure with further X-ray studies.
• Raymond Gosling (1926–2015): Franklin’s doctoral student who took Photo 51.

Consequences and legacy

The double helix was accepted gradually, as further evidence accumulated. In 1958 Matthew Meselson and Franklin Stahl showed experimentally that DNA copies itself in the way the model predicted, with each new molecule keeping one old strand. Through the 1960s scientists, including Crick, cracked the genetic code, showing how sequences of three bases specify amino acids in proteins.

In 1962 Watson, Crick and Wilkins shared the Nobel Prize in Physiology or Medicine. Franklin had died of ovarian cancer on 16 April 1958, aged 37, and the prize is not awarded posthumously. Watson’s 1968 memoir portrayed her in a dismissive way, which provoked lasting criticism. Later biographies and historical research have restored her central role, and many historians now describe the discovery as the product of two groups whose work was closely intertwined.

The structure opened the way to genetic engineering in the 1970s, DNA sequencing, DNA fingerprinting in forensic science and medical genetics. It culminated in efforts such as the Human Genome Project, which read the full human DNA sequence. The double helix itself became one of the most recognisable images in science. Within weeks of its publication, the Miller-Urey experiment showed that life’s simpler building blocks could form by ordinary chemistry, and together the two results set the agenda for research into how life works and how it began.

Myths and facts

• Myth: Rosalind Franklin was simply robbed of the discovery. Her data were used without her knowledge, and her contribution was long underplayed. But she had not proposed base pairing, and the decisive insight came from combining both groups’ work. Franklin herself remained on friendly terms with Watson and Crick and collaborated with Crick in later years.
• Myth: Watson and Crick discovered DNA. DNA had been known since 1869. They discovered its three-dimensional structure.
• Myth: The double helix immediately proved DNA was the genetic material. It made the case far stronger, but full acceptance came through later experiments such as Meselson and Stahl’s in 1958.

Timeline

• 1951, January: Rosalind Franklin joins King’s College London.
• 1951, November: Watson and Crick’s first, incorrect, three-chain model is rejected.
• 1952, May: Raymond Gosling, working with Franklin, takes Photo 51.
• 1953, 30 January: Wilkins shows Photo 51 to Watson in London.
• 1953, 28 February: Watson finds the A–T and G–C base pairing.
• 1953, c. 7 March: The full double-helix model is completed in Cambridge.
• 1953, 25 April: Watson and Crick’s paper and the King’s papers appear in Nature.
• 1958, 16 April: Rosalind Franklin dies, aged 37.
• 1962, December: Watson, Crick and Wilkins receive the Nobel Prize in Physiology or Medicine.

Why it matters

The double helix explained, in a single elegant structure, how life stores and passes on its instructions. It turned genetics into a molecular science and laid the foundation for biotechnology, modern medicine and forensic science, while the story of its discovery remains a lasting lesson about collaboration and credit in research.