Photo 51: the key discovery behind the structure of DNA

A photo taken by chemist and crystallographer Rosalind Franklin held the key to unravelling the structure of DNA.

Rosalind Franklin’s key experiment was a series of painstaking X-ray crystallography experiments with DNA samples containing different amounts of water. The most famous outcome of this is May 1952’s ‘Photo 51’, which revealed key details about the structure of DNA.

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The more a feature is repeated within a structure, the more the film will be bombarded with X-rays diffracted in the same way, and the darker the corresponding patch in the image.

The large dark patches at the top and bottom of the picture represent DNA’s bases, and the X-shaped blobs indicate a helix. The arms of the cross represent the planes of symmetry in a helix viewed from the side; the ‘zig’ and the ‘zag’ of its turns.

Photo 51 © KCL

There are 10 spots on each arm of the cross before you reach the large black patch at the top, which corresponds with 10 bases stacked one on top of the other in each turn of the helix. The fourth blob from the centre is missing, which indicates that one strand of DNA is slightly offset against the other.

Rosalind Franklin turned her attention to Photo 51 in early 1953.  Her notebooks suggest that she had gleaned all its key information and may, in time, have reached the same conclusions as James Watson and Francis Crick.

Rosalind Franklin (1920-1958) was a British chemist and crystallographer who is best known for her role in the discovery of the structure of DNA. 1955 © Getty Images

Franklin’s discovery was immortalised in the critically acclaimed 2015 play Photograph 51 by Anna Ziegler, which starred Nicole Kidman in an award-winning turn as the often-overlooked chemist.

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