Rosalind Franklin and Photo 51: What Really Happened

Rosalind Franklin and Photo 51 have become symbols of injustice in science. The familiar account is simple: Franklin photographed the double helix, James Watson saw the image without her permission, and he and Francis Crick claimed the discovery. That version reflects a genuine conflict, but it does not accurately describe how the structure was solved.

Photo 51 was not a conventional photograph in which two intertwined DNA strands could be seen directly. It was an X-ray diffraction pattern produced when X-rays scattered from aligned fibres of the molecule. Turning such a pattern into a molecular structure required calculations, chemical evidence and physical model-building.

Rosalind Franklin and her doctoral student Raymond Gosling produced the image at King’s College London. The exposure began on 2 May 1952 and lasted for a total of 62 hours. The resulting plate displayed a prominent cross-shaped arrangement of dark spots, which became one of the best-known scientific images of the twentieth century.

What Photo 51 actually showed

The X-shaped pattern was compelling evidence that DNA had a helical organization. The spacing of the reflections revealed repeating structural dimensions, including a helical repeat of approximately 34 ångströms, or 3.4 nanometres.

The photograph did not, however, contain a complete double-helix model. On its own, it could not establish the number of molecular chains, the directions in which they ran, the position of the phosphate groups or the way in which the bases paired.

Franklin obtained unusually informative diffraction patterns because she controlled the humidity of the DNA samples with exceptional care. She established that DNA existed in at least two forms. The relatively dry A form produced a complicated but information-rich pattern. The more hydrated B form, represented by Photo 51, was less sharply ordered but displayed its helical character much more clearly.

For much of the project, Franklin concentrated on the A form. This did not mean that she had failed to recognize the significance of Photo 51. From the perspective of an X-ray diffraction specialist, the more ordered A form appeared to offer a better opportunity for a rigorous structural solution rather than a preliminary visual interpretation.

Franklin’s caution is sometimes portrayed as an inability to see an obvious answer. In reality, she resisted committing to a final molecular model until the available data could test its details.

How the data reached Watson and Crick

In January 1953, Maurice Wilkins showed Photo 51 to James Watson. Franklin was not present, and her direct permission was not requested. The circumstances remain debated: Franklin was preparing to leave King’s, while Gosling was expected to continue his work under Wilkins.

Watson immediately recognized that the cross-shaped pattern strongly supported a helix. But Photo 51 did not reveal the complete structure to him. Watson and Crick were already considering helical models and primarily needed reliable dimensions and structural constraints.

They obtained even more specific information from an internal Medical Research Council report. Max Perutz passed the report to Crick because he did not regard it as confidential. It contained unpublished results from Franklin, including measurements of the unit cell, the water content of the molecule, the external position of the phosphate groups and information about structural symmetry.

These data helped Crick determine how the molecular chains had to be arranged. Franklin, however, was not told that her calculations had been given to the Cambridge group and were being used in model-building.

The history therefore cannot be reduced honestly to either of two extremes. Watson and Crick did not copy a finished discovery from one photograph. At the same time, access to Franklin’s unpublished data gave them a substantial advantage, and those results were used without a transparent discussion with their author.

The successful model combined several independent lines of evidence. Watson and Crick drew on the X-ray measurements from King’s, the known chemistry of nucleotides, Erwin Chargaff’s observations about base ratios and the construction of physical molecular models.

The decisive chemical step was the recognition that adenine pairs with thymine, whereas guanine pairs with cytosine. These pairs had matching dimensions, allowing two chains to fit inside the helix while the sugar–phosphate backbones remained on the outside. Complementary pairing also immediately suggested a possible mechanism for copying genetic information.

What Franklin had concluded herself

By early 1953, Franklin had returned to the B form. Her working notes considered a two-chain helix, although she had not yet established the final arrangement of those chains. She had also recognized that the sequence of bases could vary without disrupting DNA’s general framework, allowing the sequence itself to carry biological information.

She did not reach the complete solution. Franklin did not formulate complementary base pairing or construct the final model with two chains running in opposite directions. Watson and Crick completed that step first.

On 25 April 1953, Nature published three consecutive papers. Watson and Crick’s double-helix model appeared first, followed by the X-ray study from Wilkins, Alec Stokes and Herbert Wilson, and then Franklin and Gosling’s article containing Photo 51.

This order made the two experimental papers appear to confirm a model that had already been developed independently in Cambridge. In reality, the King’s data did more than confirm the double helix. They provided structural constraints that made the correct model much easier to identify.

Watson and Crick stated in their paper that they had been stimulated by general knowledge of the unpublished results of Franklin, Wilkins and their colleagues. The precise importance of those results, however, was not widely discussed until much later.

In 1962, the Nobel Prize in Physiology or Medicine was awarded to Watson, Crick and Wilkins. Franklin had died from ovarian cancer in 1958, at the age of 37, four years before the award.

Reducing her legacy to a missed Nobel Prize would also be misleading. After moving to Birkbeck College, Franklin established a research group and made major contributions to understanding the structures of tobacco mosaic virus and other viruses. Her work helped to establish the emerging field of structural virology.

Photo 51 was crucial evidence for the helical structure of DNA, but it did not contain a ready-made answer. Franklin’s contribution was much broader: she prepared highly controlled samples, distinguished the A and B forms of DNA, obtained precise measurements and developed structural conclusions that other researchers used.

This is not simply the story of a stolen revelation. It is the history of a collective discovery, unequal access to information and scientific credit that was distributed far less transparently than it should have been.