The Image That Changed Biology
Photo 51 is a landmark image in molecular biology, significant for understanding life. This X-ray diffraction photograph was instrumental in revealing the structure of deoxyribonucleic acid (DNA). Its clarity and information were instrumental in advancing scientific knowledge.
X-ray crystallography, the method behind Photo 51, determines the three-dimensional molecular structure of a crystal. This technique directs X-rays at a crystallized molecule, which then diffracts as it passes through the arrangement of atoms. The scattered X-rays, captured on film, provide data scientists interpret to map the molecule’s structure.
Photo 51 showed a clear diffraction pattern for the “B-form” of DNA, the more hydrated and biologically active form. The distinct pattern on the film, characterized by its “X” shape, immediately suggested a helical arrangement of DNA. This evidence offered insights into DNA’s architecture.
The Scientist Behind Photo 51
Photo 51 was produced by Rosalind Franklin, a British chemist and X-ray crystallographer, with her PhD student Raymond Gosling. Franklin joined King’s College London in 1951 to apply her X-ray diffraction expertise to DNA. An accomplished crystallographer, she developed her skills during postdoctoral research in Paris.
Franklin’s work involved obtaining high-quality X-ray diffraction images of DNA fibers. She controlled DNA sample hydration, discovering the molecule existed in two forms (A and B) based on water content. Photo 51 was taken in May 1952, after exposing DNA to X-rays for reportedly 62 hours at high humidity.
The image, labeled “Photo 51” as the 51st in a series, was the clearest B-form DNA diffraction pattern at the time. This achievement required scientific rigor and technical skill. Franklin and Gosling became experts in preparing pure DNA strands suitable for analysis.
Photo 51’s Role in Discovering DNA’s Structure
Photo 51 provided tangible evidence instrumental in elucidating DNA’s double helix structure. The image’s prominent X-shape clearly indicated a helical arrangement for DNA strands. Specific angles and intensities of diffraction spots contained detailed information about the molecule’s dimensions and repeating units.
Analyzing the spacing between the X shape’s dashes, Franklin determined the distance between nucleotides, DNA’s smallest repeating units. Dark patches at the top and bottom indicated regular stacking of DNA’s bases. Measuring the distance between these patches allowed calculation of the distance between bases within the structure. The image also revealed ten bases stacked per turn of the helix.
The distinct pattern suggested phosphate groups faced outside the helix, while bases were positioned horizontally inward. When James Watson saw Photo 51, it spurred him and Francis Crick to build their DNA model. They utilized information from Photo 51, including its helical nature and specific dimensions, to construct their accurate double-helical model.
Acknowledging the Contribution
Photo 51 and Rosalind Franklin’s role were not fully recognized during her lifetime. Maurice Wilkins, a King’s College London colleague, showed Photo 51 to James Watson without Franklin’s knowledge or permission. Watson and Crick published their model of the DNA double helix in 1953. A paper by Franklin and Gosling appeared in the same Nature issue, seeming to confirm their model rather than being acknowledged as foundational.
In 1962, James Watson, Francis Crick, and Maurice Wilkins received the Nobel Prize for their DNA findings. Rosalind Franklin passed away in 1958, making her ineligible for the award, as Nobel Prizes are not awarded posthumously. This led to debate and a re-evaluation of her contributions.
Over time, Franklin’s instrumental work and Photo 51’s direct impact on DNA’s structure discovery gained increasing recognition. Efforts ensure she receives appropriate credit for her research and the clear evidence provided. Her work is now widely acknowledged as fundamental to understanding the molecule of heredity.