“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, published in the Journal of the History of Biology, challenges the long-held belief that Franklin did not recognize the significance of Photo 51. The research shows that Photo 51 was a carefully planned, high-quality repeat of Photo 49, which 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 new research shows that she already knew that the X-ray pattern could provide important information about DNA’s helical structure.
The famous photo
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—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 built their double-helical model of DNA; it was published in Nature, alongside a paper by Franklin and Gosling describing Photo 51.
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 recognize the significance of the image herself. This new research challenges that version of events.
The current King’s College London team 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.
Sponsel’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.
“Just over a year ago, I was introduced to Alistair Sponsel, a historian of science who was visiting the Archive, and we very quickly realized 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,” said Sutton, Emeritus Professor of Molecular Biophysics at King’s College London, and co-author of the research.
Sponsel and Sutton 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.
“In this study, archives were used as evidence to re-examine how, when and why the famous Photo 51 came to be produced, revealing the structure of DNA,” said Alexandra Eveleigh, Head of Archives, King’s College London. “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.”
Data from King's College London