Photo 51 is an X-ray diffraction image of deoxyribonucleic acid (DNA), the molecule carrying genetic instructions for all living organisms. This image was crucial in revealing DNA’s three-dimensional structure, a major scientific advance. Its clarity provided key evidence, significantly aiding the understanding of how genetic information is stored and transmitted. The photo’s creation and interpretation marked a significant moment in molecular biology.
The Photographer and Her Work
The scientist behind Photo 51 was Rosalind Franklin, a British chemist and X-ray crystallographer. She joined King’s College London in 1951, bringing expertise in X-ray diffraction, a technique that uses X-rays to reveal atomic arrangements in substances.
Franklin’s meticulous approach involved preparing high-quality DNA fibers and precisely controlling sample humidity. This allowed her to capture a particularly clear image of the “B-form” of DNA, a more hydrated and biologically relevant state. Photo 51 was captured on May 6, 1952, after a 62-hour exposure, marking it as the 51st in a series.
Unveiling the Double Helix
Photo 51 provided crucial insights into DNA’s structure. Its most striking feature was a distinct “X” shape in the center of the diffraction pattern, indicating DNA’s helical arrangement. This “X” pattern results from X-rays scattering off repeating units in a helix.
The precise dimensions and angles of the “X” and other reflections offered quantifiable data. Scientists deduced the DNA helix had a pitch of approximately 3.4 nanometers (nm) and a radius of about 1 nm. The image also showed ten repeating units, or base pairs, per turn. A “missing” fourth layer line suggested the two DNA strands were offset, indicating an asymmetrical structure with major and minor grooves.
The Story Behind the Discovery
The early 1950s saw a competitive pursuit to determine DNA’s structure. Rosalind Franklin worked with Maurice Wilkins at King’s College London, while James Watson and Francis Crick researched at the University of Cambridge. The information exchange, or lack thereof, between these groups became a subject of historical discussion.
In January 1953, Maurice Wilkins showed Photo 51 to James Watson without Franklin’s knowledge or permission. Watson immediately recognized the image’s significance, as it visually confirmed DNA’s helical nature and offered crucial measurements. This visual evidence, combined with Franklin’s detailed, but unpublished, crystallographic data, provided Watson and Crick with key information to refine their DNA double helix model. Watson and Crick published their proposed structure in April 1953, alongside papers by Franklin and Wilkins detailing their X-ray diffraction data. In 1962, Watson, Crick, and Wilkins received the Nobel Prize in Physiology or Medicine for their discovery; Rosalind Franklin had passed away in 1958 and was ineligible for the posthumous award.