‘Photo 51’ was a cleaner version of an earlier X-ray image, ‘Photo 49’, that Rosalind Franklin had already used to calculate key features of DNA, challenging long-held assumptions about her role in the discovery of the double helix.
The new research from King’s College London and the Science History Institute, and published in the Journal of the History of Biology , challenges the long-held belief that Franklin did not recognise the significance of ‘Photo 51’. The research shows that ‘Photo 51’ was a carefully planned, high-quality repeat of an earlier image, ‘Photo 49’, that Franklin had already used to calculate key features of DNA’s helical structure.
‘Photo 49’ contained a blemish caused by a component inside the X-ray camera slipping across the film and obscuring part of the image. Despite this, Franklin was able to extract useful information from the photograph and use it to determine key features of the structure of the DNA molecule.
At this point, Franklin had not established exactly how many strands made up the DNA molecule, whether it was a single, double or triple helix. But the research shows that she already knew that the X-ray pattern could provide important information about DNA’s helical structure.
Rosalind Franklin and her graduate student Raymond Gosling took ‘Photo 51’ at King’s College London in 1952. The image was produced by firing X-rays at DNA and recording the pattern created when the X-rays scattered from the DNA molecules. The resulting X-shaped pattern provided evidence that DNA had a helical structure.
The image became famous after it was shown in early 1953, and without Franklin’s knowledge, to James Watson, who was working with Francis Crick, at the University of Cambridge. Less than two months later, Watson and Crick had built their double-helical model of DNA; it was published in Nature , alongside a paper by Franklin and Gosling describing ‘Photo 51’.
James Watson later wrote that seeing the photograph had helped him and Crick develop their model of DNA. However, subsequent accounts have often portrayed Franklin as having failed to recognise the significance of the image herself. The new research challenges this version of events.
The researchers reached their conclusions by examining laboratory notebooks, manuscripts, photographs and other historical records held in collections in Philadelphia, Cambridge and London. They also examined Franklin’s original X-ray camera and photographic slides held in the archives of King’s College London.
Co-author of the study, Alistair Sponsel, began re-examining Franklin’s DNA research in 2025 after the Science History Institute acquired the History of Molecular Biology Collection from the J. Craig Venter Institute. The collection includes Franklin’s own copy of ‘Photo 51’, alongside notebooks, correspondence, photographs and other material documenting the history of molecular biology.
Alistair’s research led him to archives in the UK, including those of King’s College London, where Franklin and Gosling carried out their DNA research. This led to a collaboration with Brian Sutton, who had long been interested in the X-ray crystallographic studies of DNA carried out by his predecessors at King’s.
Professor Brian Sutton, Emeritus Professor of Molecular Biophysics at King’s College London, and co-author of the research, said: “I have been accompanying visitors to the King’s College Archives over many years, but the highlight and focus of attention is always the original glass negative of Photograph 51. Just over a year ago, I was introduced to Alistair Sponsel, a historian of science who was visiting the Archive, and we very quickly realised that we looked at Photograph 51 in different ways, and that together we might better understand exactly why she and Gosling took Photograph 51, and in particular the importance of Photograph 49. It has been exciting for me to see Photograph 51 in a different light”.
The researchers say their findings do not diminish the importance of Watson and Crick’s work in developing the double-helix model. However, they provide a fuller picture of how Franklin’s own experimental work contributed to understanding DNA’s structure, and show that she fully appreciated the significance of her work, in particular Photo 51.
The study also highlights the importance of understanding the technical details of how scientific experiments are carried out when reconstructing the history of major discoveries.
Alistair Sponsel, oral historian and curator of life sciences at the Science History Institute, and co-author of the research, said:
“Brian and I were each intrigued by the same features of Photo 51, and we wanted to gain a deeper understanding of why the image looked the way it did. After studying so many incredible documents and artifacts together, we were excited to realize that we shared a new understanding, not just of how the photograph was made, but of why Franklin had been motivated to take Photo 51 in the first place. We’ve had such fun working together and learning to see these incredible original sources through each other's eyes.”
Dr Alexandra Eveleigh, Head of Archives, King’s College London, said:
“King's Archives preserve photographs, laboratory records, correspondence, and other original materials that provide a direct connection to the pioneering scientific research carried out at the University. In this study, archives are used as evidence to re-examine how, when and why the famous 'Photo 51' came to be produced, revealing the structure of DNA. This illustrates the vital role of archives in the history of science: by preserving the original record, they allow established accounts to be revisited and refined, leading to a fuller and more nuanced understanding of how major scientific discoveries are made and the contributions of the people behind them.”
Social copy:
What if ‘Photo 51’ wasn’t the starting point for Rosalind Franklin’s understanding of DNA’s structure?
A new study from King’s College London [TAG FOLSM] and the Science History Institute [TAG] sheds new light on one of the most famous images in the history of science, and Rosalind Franklin’s role in understanding DNA’s structure.
The researchers found that ‘Photo 51’ was a carefully planned, high-quality repeat of an earlier image, ‘Photo 49’, which Franklin had already used to work out key features of DNA’s helical structure.
That challenges the long-held idea that Franklin did not recognise the significance of ‘Photo 51’.
By examining laboratory notebooks, manuscripts, photographs, correspondence and Franklin’s original X-ray equipment, the researchers have reconstructed why ‘Photo 51’ was made, and what Franklin already understood about the information it contained.
The findings offer a fuller picture of Franklin’s contribution to one of the most important discoveries in modern science, while highlighting how archives and original scientific records can help us revisit established stories.
Journal of the History of Biology