In May 1952, in a basement laboratory at King’s College London, 31-year-old physical chemist Rosalind Franklin and her doctoral student Raymond Gosling placed a carefully drawn fibre of highly hydrated DNA inside an X-ray camera. According to King’s College London’s account of the experiment, the exposure ran for a total of 62 hours. When the film was developed, the image catalogued as Photograph 51 carried a dark cross through its centre, the characteristic diffraction pattern of a helix.
Months before James Watson and Francis Crick announced their double-helix model, the basic geometry of DNA was already visible on Franklin and Gosling’s film. Photograph 51 was not a conventional photograph of a molecule, and it did not provide every part of the final structure by itself. What it supplied was a remarkably clear set of physical constraints that any successful model had to obey.
What Photograph 51 actually shows
An X-ray diffraction image is not a photograph in the ordinary sense. The DNA fibres used in the experiment contained hundreds of thousands of similarly oriented molecules, and X-rays were directed through them at right angles to their long axes. As the rays scattered from atoms in the sample, their interference created dark spots on the photographic film.
When a helix is viewed from the side, its repeating structure produces a distinctive cross-shaped diffraction pattern. That cross is the most immediately recognisable feature of Photograph 51. Its spacing also carried quantitative information, including evidence that the helix contained ten stacked bases in each complete turn.
The absent fourth spot in each arm of the cross supplied another clue. Franklin calculated that the two molecular chains were offset from one another by three-eighths of the helix’s pitch. By early 1953, she was describing a probable double helix with its bases on the inside and phosphate groups on the outside.
The A form and the B form
Franklin’s work also established that DNA fibres could adopt two distinct physical states depending on their water content. Under drier conditions, DNA formed the compact A form, which produced a complex and tightly packed diffraction pattern. At higher humidity, it shifted into the more extended B form, whose diffraction pattern was clearer and more regular.
Photograph 51 showed the wet B form. Franklin nevertheless devoted much of her attention to the more difficult A-form data because she believed a rigorous mathematical analysis of that pattern could produce a fuller structural answer. Her caution has sometimes been mistaken for an inability to recognise a helix, but her surviving notes and draft manuscripts show a more complicated picture.
Franklin was reluctant to settle on a model before the evidence justified it. That method made her slower to declare a final structure, but it also enabled her to reject arrangements that conflicted with the water content, molecular dimensions and phosphate placement revealed by her experiments.
How the image reached Cambridge
The strained working relationship between Franklin and Maurice Wilkins began with poorly communicated expectations at King’s. When Franklin joined the biophysics unit in 1951, director John Randall placed her in charge of the DNA diffraction work and reassigned Gosling to work under her supervision. Wilkins had already been studying DNA and believed Franklin would be joining his effort rather than taking charge of a separate one, according to an archival account from the Science History Institute.
The misunderstanding left Franklin and Wilkins approaching closely related work with conflicting ideas about authority, collaboration and ownership. Randall eventually divided their responsibilities, directing Franklin toward A-form DNA while Wilkins continued working on the B form.
By early 1953, Franklin was preparing to leave King’s for Birkbeck College. Wilkins had a copy of Photograph 51 because he was expected to take over the remaining work, and in January he showed it to Watson during a visit from Cambridge. Franklin did not know that the image had been shown.
What Watson and Crick did with the evidence
Watson and Crick were primarily model builders. They assembled representations of bases, sugars and phosphate groups, testing possible structures against experimental information produced elsewhere. Their first serious DNA model placed three strands at the centre and positioned the phosphate groups incorrectly, errors Franklin identified when she was invited to inspect it.
Photograph 51 made the helical geometry far harder to dispute. Other measurements from the King’s group supplied dimensions and chemical restrictions that helped narrow the remaining possibilities. Watson and Crick then integrated those constraints with chemical knowledge, helical diffraction theory and Erwin Chargaff’s work on the relative quantities of DNA’s bases.
Their complementary base-pairing arrangement was a genuine conceptual breakthrough. Pairing adenine with thymine and guanine with cytosine explained how two DNA strands could fit together and immediately suggested how genetic information might be copied.
On 25 April 1953, Nature published three papers together. Watson and Crick presented the model first, followed by papers from Wilkins and his colleagues and from Franklin and Gosling. Franklin and Gosling’s paper appeared to confirm a structure that Watson and Crick had already announced, although the experimental evidence had played a crucial role in constructing it.
Why Franklin did not share the Nobel Prize
Franklin left King’s in 1953 and established a research team at Birkbeck College. There she turned to the structures of viruses, producing work on tobacco mosaic virus and beginning research connected with poliovirus. Her studies helped lay foundations for structural virology, according to King’s College London’s biography of Franklin.
She was diagnosed with ovarian cancer in 1956 but continued conducting research and publishing. Franklin died in April 1958 at the age of 37.
The Nobel Prize in Physiology or Medicine was awarded to Watson, Crick and Wilkins in 1962. Franklin could not be considered because Nobel committees do not award prizes posthumously. Whether she would have been included had she lived cannot be known, and claims that a particular body of her later work would certainly have earned a Nobel remain speculation.
Did she know DNA was helical?
Franklin’s notebooks and a manuscript drafted in March 1953 show that she had moved well beyond merely noticing an X-shaped pattern. She considered a helical structure highly probable, identified ten bases per turn, placed the phosphate backbone on the outside and the bases on the inside, and recognised that two chains were likely involved.
What she had not yet discovered was the complementary pairing of the bases. That distinction matters because it avoids replacing one misleading version of the story with another. Franklin did not overlook the helix, but Watson and Crick did more than copy a completed structure from her photograph.
The most accurate account is that Franklin and Gosling produced essential experimental evidence and worked out many of the molecule’s structural requirements. Watson and Crick combined those requirements with information from several sources and found a model that explained both the double helix and the mechanism of genetic copying.
The image itself, and where its name travelled
Photograph 51 remains a small rectangle of grey and black, dominated by four blurred arms radiating from its centre. Its scientific importance lies not in its appearance alone but in how much structural information a trained crystallographer could extract from the placement and spacing of its marks.
In 2013, Google commemorated what would have been Franklin’s 93rd birthday with an official homepage Doodle. The illustration placed Franklin beside the double helix and a version of the diffraction cross that had become inseparable from her name.
The European Space Agency later named its ExoMars rover Rosalind Franklin. The mission currently targets a 2028 launch, and the rover is designed to drill as deep as two metres into Martian soil while looking for evidence that life may once have existed there.
The long correction
Recognition of Franklin’s role arrived in stages. Anne Sayre’s 1975 biography challenged the portrait left by Watson’s memoir. Brenda Maddox’s 2002 biography drew on family material, while Anna Ziegler’s play Photograph 51 brought the King’s laboratory conflict to the stage.
In 2023, historians Matthew Cobb and Nathaniel Comfort used a previously overlooked letter and an unpublished contemporary news article to offer a more nuanced interpretation. Their reassessment in Nature described Franklin as an equal participant in the discovery rather than merely a passive victim whose work was taken without contribution or agency.
That does not erase the unequal treatment she encountered or the fact that Photograph 51 was shown to Watson without her knowledge. It does, however, place Franklin back inside the scientific process as an active researcher developing her own interpretation of DNA, not simply as the owner of an image other people understood better.
Watson’s 1968 memoir The Double Helix helped establish a very different popular image. As Mental Floss notes in its account of the controversy, the book criticised Franklin’s appearance and minimised her role in the research. Watson could shape that story for decades after Franklin’s death, while she had no opportunity to answer it.
Watson himself died in 2025 at the age of 97. By then, the historical account had changed substantially, with Franklin’s diffraction work recognised as one of the central experimental foundations of the double-helix model.
Sixty-two hours in a basement
Return to the exposure itself. The sample was not a single visible molecule but a thin fibre containing hundreds of thousands of DNA molecules aligned in the same direction. X-rays passed through that fibre for 62 hours while film on the other side slowly recorded the rays scattered by its repeating atomic structure.
Franklin and Gosling developed the film on 6 May 1952. The cross was already there, sharp enough to reveal a helix and orderly enough to impose measurements on any model that followed.
Photograph 51 did not solve every part of DNA’s structure on its own. It did something equally important: it turned an invisible molecule into a set of geometric facts that could no longer be ignored. The image survived almost unchanged. The distribution of credit took far longer to resolve.
