In a cellar laboratory at King’s College London, over 62 hours in May 1952, Rosalind Franklin and her graduate student Raymond Gosling pointed a narrow X-ray beam at a single hydrated fibre of DNA and produced a photograph so unambiguous that the dark cross at its centre was, to anyone trained to read it, a straight declaration that the molecule was a helix. The image would become known as Photo 51. It was taken almost a full year before James Watson and Francis Crick assembled their model in Cambridge, and it contained the geometry they needed to build it.
The photograph is a smudged black-and-white oval with a stark X of dark spots stamped through the middle. To the untrained eye it looks like a Rorschach blot. To a crystallographer, the spacing of the spots gives the pitch of the helix; the angle of the arms gives the tilt of its coils; the missing fourth layer line gives the number of strands.
The 62-hour exposure
Franklin had joined King’s in January 1951, hired to set up an X-ray diffraction unit in the biophysics department. The DNA she worked with was unusually pure and long, which meant it could be drawn out into fibres thin enough to diffract cleanly. Gosling, her PhD student, had been passed to her from Maurice Wilkins when Franklin arrived.
The trick that produced Photo 51 was humidity. Franklin worked out that DNA fibres take two forms — a shorter, fatter crystalline A form at lower humidity, and a longer, thinner B form when saturated with water. The B form was messier crystallographically but simpler geometrically: a pure helix, unadorned. To hold a single fibre at the right hydration for long enough to record it, Franklin bubbled hydrogen through a salt solution and into the specimen chamber, then aimed a fine-focus X-ray tube at the fibre for more than two and a half days, as Genetic Engineering and Biotechnology News has described.
The exposure ran through 1 and 2 May 1952. When Gosling developed the plate, the pattern was cleaner than anything either of them had seen.
What the cross meant
An X pattern in fibre diffraction is the visual signature of a helix. Theoretical work in the early 1950s had spelled out exactly how a helical molecule scatters X-rays into that shape, so the pattern was legible on sight to anyone who knew the maths.
Photo 51 gave more than a yes-or-no on helicity. The spacing between the layer lines told Franklin the helix repeated at regular intervals. The strong meridional reflection told her the bases were stacked flat, one on top of the next. The absence of the fourth layer line was a fingerprint of a double helix with two strands running antiparallel, offset from each other.
Franklin filed the photograph away. She was more interested in the crystalline A form, which she thought would yield finer structural detail, and by late 1952 she was preparing to leave King’s for Birkbeck College. Before she left, she wrote up her B-form measurements in a Medical Research Council progress report.
How the photograph left the building
In late January 1953, Wilkins showed Photo 51 to Watson during a visit to King’s. Gosling had recently handed the print to Wilkins as Franklin was packing up her lab. Watson later wrote that his mouth fell open and his pulse began to race. He had learned enough crystallography, he said, to know that the X marked a helix — probably a double one.
A short time later, Max Perutz passed to Crick and Watson a copy of the MRC report containing Franklin’s precise B-form measurements, including the antiparallel arrangement of the two chains. The report was not marked confidential, but Franklin was never told it had been shared. Within weeks, Crick and Watson had built the double helix, published in Nature in 1953. Crick and Watson’s paper acknowledged Franklin’s work only obliquely in a footnote, as The Conversation has documented.
Franklin’s own paper appeared in the same issue, giving the mistaken impression it was a confirmatory follow-up rather than the primary experimental evidence. It contained Photo 51 itself.
The long road to the molecule
DNA had been sitting on laboratory shelves for more than 80 years before Photo 51 pinned it down. In the nineteenth century, a young Swiss chemist named Friedrich Miescher had isolated a phosphorus-rich substance from the nuclei of white blood cells, calling it nuclein. For most of the intervening years, biologists assumed the interesting molecule inside chromosomes had to be protein. DNA was thought too monotonous — just four bases, over and over — to carry the instructions for a living thing.
By the late 1940s and early 1950s, experiments with bacteria and viruses had shifted the consensus toward DNA as the genetic material. But knowing DNA was the genetic material told you nothing about how it stored information. That required the shape.
William Astbury at Leeds had produced blurry X-ray patterns of DNA fibres in earlier decades and had even captured an early version of the B-form pattern with his student Florence Bell, though they did not understand its significance.
The instrument and the room
The physical set-up was modest. A rotating-anode X-ray tube with a fine copper target. A pinhole collimator narrowing the beam. A camera holding a strip of photographic film curved around the specimen. The DNA fibre itself was thinner than a human hair, mounted vertically in a specimen chamber the size of a matchbox, kept humid by a small dish of salt solution.
The basement lab was cramped and not well ventilated. Franklin worked without the shielding that would now be standard. Her prolonged exposure to X-rays over the King’s years is often cited as a probable contributor to the ovarian cancer that killed her in April 1958, at 37.
What Watson saw, and what he didn’t
Watson’s mouth-agape reaction was not a mystical moment of insight. It was pattern recognition. He had spent months on a wrong triple-helix model that Franklin herself had torn apart at a Cambridge seminar in 1951. When he saw Photo 51, the geometry it revealed was incompatible with almost every model he had considered — except a two-strand helix with the phosphates on the outside.
What Watson didn’t have, and what the MRC report supplied, was the fact that the two strands ran in opposite directions. That antiparallel arrangement was Franklin’s measurement. It was the difference between guessing at a helix and building one that matched the numbers.
Watson’s own account in The Double Helix cast a long shadow over the story, flattening Franklin into a caricature the rest of the field spent decades correcting.
Why she wasn’t at the Nobel podium
The 1962 Nobel Prize in Physiology or Medicine went to Crick, Watson and Wilkins. Franklin had died four years earlier, and Nobel rules bar posthumous awards. Whether she would have been included had she lived is a question historians still argue. The National Air and Space Museum and others have pointed out that the prize was limited to three recipients, and that the committee tended to reward theorists who built models over experimentalists who gathered data.
Franklin’s mentee Aaron Klug won the 1982 Nobel in Chemistry for work on virus structures that built directly on hers. In interviews Klug consistently argued that Franklin would have shared it had she lived, as recounted by the American Council on Science and Health.
After DNA
Franklin left King’s for Birkbeck in 1953. In the five years she had left, she turned her X-ray beam on the tobacco mosaic virus and worked out that its coat proteins formed a hollow spiral tube with a single strand of RNA wound inside. She showed that RNA, unlike DNA, was single-stranded. She built a model of the poliovirus that Klug completed and published in her memory.
Her gravestone in Willesden Jewish Cemetery, north London, gives no mention of the double helix, instead focusing on her virus research work. As the University of Nevada, Reno has noted, she seems to have regarded the virus work as her proper legacy.
The photograph, still
Photo 51 is now held in the King’s College London archives. It is a piece of film about the size of a postcard, browned at the edges, with a black cross at the centre and faint layer lines fanning out from it. Every schoolchild who has ever drawn a twisting ladder to represent DNA is drawing something Franklin measured, in a basement, over 62 hours in the first two days of May 1952. The molecule was a helix in that room a year before anyone in Cambridge had drawn a single line.