Photo 51 is the famous X-ray diffraction image of deoxyribonucleic acid (DNA), captured in May 1952 at King’s College London by scientist Rosalind Franklin and her graduate student, Raymond Gosling. The photograph provided the most precise visual evidence available at the time, confirming that DNA possessed a highly ordered, repeating structure. Its clarity was key evidence that led to the eventual determination of the double helix structure.
The Science Behind the Image
The technique used to capture Photo 51 is known as X-ray crystallography, a method for determining the atomic and molecular structure of a crystal. This process involves aiming a focused beam of X-rays at a crystallized sample.
When the X-rays strike the atoms, they scatter in a process called diffraction. If the atoms are arranged in a regular, repeating pattern, the scattered waves interfere with each other, creating a specific pattern of bright spots on a photographic film. This pattern is not a direct picture of the molecule, but rather a map of its repeating structural elements. Scientists then use complex mathematical calculations to translate the geometric arrangement of the spots back into the three-dimensional structure of the molecule.
Preparing the Perfect DNA Sample
Obtaining a clear diffraction pattern required an incredibly pure and well-ordered DNA sample, a task that fell to Franklin and Gosling. They used highly purified sodium thymonucleate, a form of DNA, and stretched it into thin fibers. This process aligned the countless DNA molecules parallel to one another within the fiber, which was necessary to get a coherent diffraction signal.
Control of the hydration level of the DNA fiber was essential. Franklin discovered that DNA existed in two forms: the drier A-form and the highly hydrated B-form. The B-form, which is the structure found in living cells, only appeared when the relative humidity was maintained above 75 percent. To achieve this, the sample was placed in a custom-built camera where the atmosphere was saturated with water vapor.
Capturing the Diffraction Pattern
The physical process of taking Photo 51 involved specialized apparatus to produce a focused X-ray beam. Franklin and Gosling employed a fine-focus X-ray tube and a camera with extremely narrow collimators to narrow the beam down to a pencil-thin stream. The prepared DNA fiber was then mounted precisely in the path of this beam.
The faintness of the diffraction signal meant that a single exposure had to be long to record the pattern onto the photographic film. Photo 51 was the result of exposing the DNA fiber to the X-rays for approximately 62 hours, beginning on May 2, 1952. The entire apparatus was sealed to maintain the critical humidity level and often flushed with hydrogen gas to prevent the X-rays from scattering off molecules in the air.
Decoding the X: Why Photo 51 Mattered
The visual appearance of Photo 51 was its most telling feature, displaying a symmetrical “X” shape in the center. This pattern, a characteristic signature known to crystallographers, provided evidence that the DNA molecule was a helix. The arms of the “X” represented the repeating structural features of the helical molecule.
By measuring the angles and spacing of the spots within the pattern, Franklin and others could immediately deduce the basic physical dimensions of the helix. The distance between the repeating horizontal bands, or layer lines, indicated the pitch of the helix, which was calculated to be 3.4 nanometers. The intensity and position of the spots suggested there were 10 repeating units, or base pairs, per turn of the helix. Subtle features, such as the missing data on the fourth layer line, provided a clue that the molecule was not a single helix but contained two interwound, offset chains.