The double helix of DNA

1953 Cambridge and London, England

In short

DNA is the molecule that holds the instructions for building and running every living thing. In 1953, in Cambridge, in England, James Watson and Francis Crick worked out that it is shaped like a spiral staircase, the double helix. A key step was an X-ray photograph taken in London in the laboratory of Rosalind Franklin, whose contribution was only fully recognised much later. The two strands can be separated and copied: that is how information passes from one cell to another.

Why it matters Understanding DNA opened the way to modern genetics, medical tests and new treatments.

Source: Encyclopaedia Britannica, “DNA”

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Illustration: The double helix of DNA (Cambridge and London, England) — Illustration created with AI for Storiascopio

Where: Cambridge and London, England

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The story

The instruction manual of life

Every living thing, from a bacterium to a whale, begins as a tiny instruction manual written inside its cells. At the start of the 20th century scientists knew this manual existed: they called it “heredity”, because it explained why children look like their parents. But they did not know what it was made of, or how it could be copied from one generation to the next.

An important clue came in 1944, when a group of researchers in New York led by Oswald Avery showed that the thing passing on characteristics was a molecule called DNA, short for deoxyribonucleic acid. It had been known since 1869, when a Swiss doctor, Friedrich Miescher, found it in the pus on used hospital bandages. Many scientists did not believe it, though: a molecule with only four kinds of building blocks seemed far too simple to hold all the instructions of life. In 1950 the chemist Erwin Chargaff noticed a curious rule: in any DNA, the amount of block A was always equal to the amount of T, and the amount of G equal to that of C.

A race between London and Cambridge

In the early 1950s, finding the shape of DNA had become a real race. In London, at King's College, Maurice Wilkins worked alongside Rosalind Franklin, a chemist who was an expert on crystals. They used X-rays: you shine a beam through a fibre of DNA, and from the pattern of dots on a photographic plate you can work out how the atoms are arranged. In May 1952 Franklin and her student Raymond Gosling took a very clear picture, known as “Photograph 51”. It showed a big “X” of dots: the typical sign of a spiral.

In Cambridge, at the Cavendish Laboratory, two other researchers were chasing the same answer with a different method: building models. They were the American James Watson, only 25 years old, and the Briton Francis Crick, 36 and still without a doctorate. They did not do X-ray experiments: they tried to fit together other people's data using pieces of metal and cut-out cardboard until everything matched. Their first model, in 1951, was wrong, and Franklin smiled at it. Even the famous American chemist Linus Pauling, who would later win a Nobel Prize (in 1954), suggested a three-stranded structure: that was wrong too.

At the start of 1953 Watson and Crick were able to see Franklin's results: Wilkins showed them Photograph 51, and a Medical Research Council report reached them through a colleague. Franklin did not know, and only many years later did people realise how decisive that data had been. With this information, and with Chargaff's rule, the two built the right model.

The spiral staircase

DNA is made of two very long strands twisted around each other, like a spiral staircase. The sides of the staircase are chains of sugars and phosphates; the steps are pairs of building blocks, which today we call bases: A (adenine) always pairs with T (thymine), and G (guanine) always with C (cytosine). That is why the amounts of A and T are equal, as Chargaff had seen. Above all, this shape explained how the manual is copied: you just open the staircase down the middle, like a zip, and each half becomes the mould for rebuilding the other. Two identical copies, from just one.

On 25 April 1953 the journal Nature published Watson and Crick's article, little more than one page long. The same issue also carried studies by Wilkins and by Franklin with Gosling, which confirmed the structure. In 1958 two American researchers, Matthew Meselson and Franklin Stahl, showed with an experiment that DNA really does copy itself in that way.

The Nobel Prize and a forgotten name

In 1962 the Nobel Prize in Medicine went to Watson, Crick and Wilkins. Rosalind Franklin had died in 1958, aged only 37, of cancer: Nobel Prizes are not awarded to people who have died. For many years her contribution was little known; today she is remembered as a scientist who was essential to this discovery, although historians still argue about how fairly her data was used.

Why it happened

DNA was discovered long before 1953, but nobody knew its shape. Three things made the solution possible: the 1944 experiments pointing to DNA as the molecule of heredity, the X-ray technique applied to crystals, already used to study molecules such as insulin, and Chargaff's rule about the amounts of the bases.

The mood of the time mattered too. After the Second World War, biology attracted physicists and chemists who brought new ideas: Crick himself was a physicist who had moved into biology. Several groups, in England and the United States, were also working on the same problem, and the competition made everyone faster.

What changed

The double-helix structure opened the way to molecular biology. In the 1960s scientists cracked the genetic code: three bases in a row, like three letters, say which building block to add to proteins, the molecules that make the body work. People finally understood how the instruction manual is read and turned into a living organism.

From 1990 to 2003 a huge international project, the Human Genome Project, read all of the roughly 3 billion base pairs in human DNA. Today we can compare the DNA of people, animals and plants, discover inherited diseases, identify a culprit from a tiny trace of blood, or work out how species are related.

In recent years new techniques, such as CRISPR, even make it possible to “edit” DNA in the laboratory: Emmanuelle Charpentier and Jennifer Doudna won the Nobel Prize in Chemistry in 2020 for this. These are extraordinary possibilities, and they raise ethical questions too: how far is it right to change the manual of life? It is a question that must now be answered not only by scientists, but by all citizens.

Key people and places

People

James Watson: American biologist (born 1928): he was 25 when he built the DNA model with Crick; Nobel Prize in 1962.
Francis Crick: British physicist and biologist (1916–2004): with Watson he proposed the double-helix structure; Nobel Prize in 1962.
Rosalind Franklin: English chemist (1920–1958): her “Photograph 51” of DNA was a decisive piece of evidence for the model.
Maurice Wilkins: New Zealand-born physicist and biologist (1916–2004): he studied DNA with X-rays at King's College London; Nobel Prize in 1962.
Erwin Chargaff: Austrian biochemist (1905–2002): he found that in DNA, A and T, and G and C, are present in equal amounts.

Places

Cavendish Laboratory, Cambridge: The University of Cambridge physics laboratory where Watson and Crick built their model.
King's College London: Where Franklin and Wilkins studied DNA with X-rays and where Photograph 51 was taken.

Good to know

Watson and Crick's famous DNA article was little more than a page long and appeared in the journal Nature in 1953.
Watson and Crick's first model was wrong; when they showed it to Rosalind Franklin, she spotted the mistakes at once.
If you unrolled all the DNA in a single human cell, it would be about two metres long.
Any two people have almost the same DNA: the difference is about one letter in every thousand.
DNA was already discovered in 1869 by Friedrich Miescher, who found it in the cells of pus.