Cobra Effect · Genetics
The shape that explained heredity
How an X-ray photograph and a chemical rule revealed the double helix of DNA.
7 cards, read aloud in 3:25, with a test and sources.
By the early 1950s, several teams were racing to find the shape of DNA.
Once DNA was known to carry genes, its structure became one of the biggest puzzles in biology. At King’s College London, Maurice Wilkins and Rosalind Franklin studied it with X-rays. At the Cavendish Laboratory in Cambridge, James Watson and Francis Crick tried to build models of it. In the United States, Linus Pauling proposed a structure early in 1953, but it was wrong.
Rosalind Franklin was an expert at photographing molecules with X-rays.
Shining X-rays through a fibre of DNA leaves a pattern of spots on film. The pattern holds clues to the shape of the molecule, for anyone who can read it. In May 1952, working with her student Raymond Gosling, she took an image that became known as Photo 51. It was among the clearest pictures of DNA that anyone had made.
Photo 51 showed a cross of spots, the mark of a helix.
The spots formed a clear X shape. To a trained eye, that cross meant the molecule was a spiral, a helix. The spacing of the spots gave measurements of its size and its regular repeats. Franklin was working carefully towards her own analysis of the structure.
In January 1953, Wilkins showed Photo 51 to Watson, without Franklin knowing.
Wilkins and Franklin worked in the same laboratory, but did not get on well. When Watson visited King’s College, Wilkins showed him the photograph. Watson later wrote that the pattern pointed straight to a helix. Franklin did not know that her image had been shown to him.
Another clue came from the chemistry of Erwin Chargaff.
DNA is built from four kinds of base, known by their letters: A, T, G and C. Chargaff had found that DNA always contains as much A as T, and as much G as C. The amounts differed from one species to another, but the pairs always matched. Nobody yet knew why.
In April 1953, Watson and Crick published the double helix.
Their model had two strands twisted around each other, with the bases pairing in the middle: A with T, and G with C. That explained Chargaff’s rule at a stroke. Their short paper appeared in Nature, in the same issue as papers from Wilkins, and from Franklin and Gosling. They noted that the pairing suggested how DNA might be copied.
So when a discovery carries famous names, ask whose work it rests on.
Franklin’s images and measurements helped make the model possible, yet she did not know Watson had seen Photo 51. She died of cancer in 1958, aged thirty seven. In 1962 Watson, Crick and Wilkins shared the Nobel Prize, which is not awarded after death. Today Franklin is widely recognised as central to one of the great discoveries in biology.
Sources
- Photo 51 and the discovery of DNA’s structure, King’s College London. How Franklin and Gosling made Photo 51, and what it showed.
- Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid, James Watson and Francis Crick, Nature, 1953. The short paper that set out the double helix.
- What was Rosalind Franklin’s true role in the discovery of DNA’s double helix?, Science News. A careful look at how Franklin’s work was used, and what she knew.
Nearby ideas
- Genes are made of DNA. How dead bacteria, a purified extract and a kitchen blender showed genes are made of DNA.
- Peas that counted. How a friar counted thousands of peas and found the rules of inheritance.
- Reading the whole book of a human. How reading all three billion letters of human DNA changed what we know about ourselves.
- Scissors that find their own place. How a bacterial defence against viruses became a tool for editing genes.
- The pattern only you carry. How patterns of DNA bands reunited a family and solved a murder.
- The fly with white eyes. How one white-eyed fly showed that genes sit on chromosomes.