Rosalind Franklin's X-ray Diffraction Photograph Demonstrated That Dna

Rosalind Franklin's X-ray diffraction photograph, known as Photo 51, irrevocably demonstrated that DNA is a helix, revolutionizing our understanding of the molecule of life and paving the way for the Watson-Crick model. This single image, born from meticulous scientific work and complex interpretation, provided crucial evidence about DNA's structure, forever changing the course of biology and genetics. To fully appreciate the impact of Photo 51, we must delve into the historical context, Franklin's scientific approach, the data revealed by the photograph, and the controversies surrounding its use.

The Road to Photo 51: Franklin's Early Life and Research

Rosalind Elsie Franklin (1920-1958) was a British chemist and X-ray crystallographer whose work was central to the understanding of the molecular structures of DNA, RNA, viruses, coal, and graphite. Her early life was marked by intellectual curiosity and a strong drive for academic achievement. Despite facing some societal barriers as a woman in science during the mid-20th century, Franklin excelled in her studies, earning a Ph.D. in physical chemistry from Cambridge University in 1945.

After her doctoral work, Franklin gained expertise in X-ray diffraction techniques while working at the Laboratoire Central des Services Chimiques de l'Etat in Paris. This experience proved invaluable when she joined the Medical Research Council (MRC) Unit at King's College London in 1951. There, she was tasked with using X-ray diffraction to study the structure of DNA.

At King's College, Franklin worked alongside Maurice Wilkins, a biophysicist who had also been investigating DNA using X-ray diffraction. Their collaboration was fraught with tension and misunderstandings, stemming partly from unclear lines of authority and differing scientific approaches. Franklin focused on meticulous data collection and rigorous analysis, while Wilkins was more inclined towards model building. This strained relationship would later become a significant aspect of the controversy surrounding the discovery of DNA's structure.

X-Ray Diffraction: A Primer

Before diving into the specifics of Photo 51, it is essential to understand the principles of X-ray diffraction. This technique relies on the wave-like properties of X-rays to probe the structure of crystalline materials. Here's a simplified explanation:

1. Crystallization: The substance to be studied (in this case, DNA) is prepared in a crystalline form. A crystal is a solid material whose constituents (atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a lattice that extends in all directions.

2. X-Ray Beam: A beam of X-rays is directed at the crystal.

3. Diffraction: When the X-rays encounter the atoms in the crystal, they are scattered or diffracted. The scattered X-rays interfere with each other, creating a diffraction pattern. This pattern consists of spots or rings of varying intensity, which are recorded on a detector (in Franklin's case, photographic film).

4. Analysis: The diffraction pattern is then analyzed using mathematical techniques to determine the arrangement of atoms within the crystal. The position and intensity of the spots provide information about the spacing and symmetry of the crystal lattice, which in turn reveals the structure of the molecules within the crystal.

X-ray diffraction is a powerful tool because the wavelength of X-rays is comparable to the spacing between atoms in a crystal. This allows the X-rays to "see" the atomic structure of the material.

The Creation of Photo 51

Franklin's meticulous approach to X-ray diffraction was crucial in obtaining high-quality images of DNA. She carefully controlled the hydration level of the DNA samples, recognizing that the amount of water present affected the molecule's structure. She identified two forms of DNA:

• Form A (dry): This form produced a less clear diffraction pattern.
• Form B (wet): This form, obtained at higher humidity levels, yielded a much sharper and more informative diffraction pattern.

Photo 51 was taken in May 1952 by Franklin's Ph.D. student, Raymond Gosling, using a sample of DNA in the B form. The process involved carefully aligning the DNA fibers and exposing them to X-rays for an extended period (around 100 hours). This long exposure time was necessary to capture a clear diffraction pattern, as DNA is a relatively weak scatterer of X-rays.

The resulting photograph was a marvel of scientific achievement. It showed a distinct X-shaped pattern with a dark, intense band at the top and bottom, and a series of less intense spots arranged in a characteristic pattern. This pattern was not immediately interpretable to the untrained eye, but Franklin recognized its significance.

What Photo 51 Revealed About DNA

Photo 51 provided several crucial pieces of evidence about the structure of DNA:

1. Helical Structure: The X-shaped pattern was a clear indication that DNA was a helix. This type of pattern is characteristic of helical structures, as the repeating units of the helix cause the X-rays to diffract in a specific way.

2. Regularity of the Structure: The well-defined spots in the diffraction pattern indicated that the DNA molecule had a highly regular and repeating structure. This suggested that the building blocks of DNA (the nucleotides) were arranged in a consistent manner.

3. Spacing of Repeating Units: The distance between the spots in the diffraction pattern provided information about the spacing of the repeating units along the DNA molecule. Franklin estimated that the distance between these units was 3.4 angstroms (0.34 nanometers).

4. Diameter of the Helix: The overall pattern of the diffraction pattern also provided information about the diameter of the DNA helix. Franklin estimated that the diameter was around 20 angstroms (2 nanometers).

Franklin meticulously analyzed the data from Photo 51 and other diffraction patterns, combining this with her knowledge of chemistry and crystallography. She concluded that DNA was likely a helix with a diameter of 20 angstroms, and that the phosphate groups were probably located on the outside of the molecule. She presented these findings in a seminar in November 1951 and in an internal progress report in December 1952.

The Watson-Crick Model and the Controversy

While Franklin was diligently working on her analysis, James Watson and Francis Crick at the University of Cambridge were also trying to determine the structure of DNA. They were taking a more model-building approach, relying on existing data and intuition to construct possible structures.

In January 1953, Wilkins showed Watson Photo 51 without Franklin's knowledge or permission. This was a pivotal moment. The clear evidence of a helical structure immediately spurred Watson and Crick to refine their model. They also gained access to Franklin's Medical Research Council report, containing a summary of her data and conclusions.

Using this information, Watson and Crick were able to construct a model of DNA that accurately accounted for all the available data. Their model proposed that DNA was a double helix, with two strands of nucleotides wound around each other. The sugar-phosphate backbone was on the outside of the helix, and the bases (adenine, guanine, cytosine, and thymine) were located on the inside, pairing specifically (A with T, and G with C). This model elegantly explained how DNA could carry genetic information and how it could be replicated.

Watson and Crick published their groundbreaking paper in Nature in April 1953. In the same issue, Franklin and Gosling published their paper presenting the X-ray diffraction data, including Photo 51, that supported the helical structure of DNA. Wilkins and his colleagues also published a paper presenting their own X-ray diffraction data.

In 1962, Watson, Crick, and Wilkins were awarded the Nobel Prize in Physiology or Medicine for their discovery of the structure of DNA. Franklin was not included in the award. This omission has been a source of considerable controversy and debate.

The Ethical Implications

The controversy surrounding Photo 51 and the Nobel Prize raises important ethical questions about the conduct of scientific research and the recognition of scientific contributions.

• Access to Data: The fact that Wilkins showed Photo 51 to Watson without Franklin's permission is considered by many to be a breach of scientific ethics. Franklin was actively working on the problem of DNA structure, and her data should have been treated as confidential until she was ready to publish it.

• Attribution of Credit: While Watson and Crick acknowledged Franklin's contribution in their paper, many feel that she did not receive sufficient credit for her crucial role in the discovery. Her meticulous experimental work and insightful analysis were essential for confirming the helical structure of DNA and for guiding Watson and Crick in their model building.

• Gender Bias: Some have argued that Franklin's gender played a role in her lack of recognition. In the 1950s, women in science often faced discrimination and were not always taken as seriously as their male colleagues. It is possible that Franklin's contributions were undervalued because she was a woman in a male-dominated field.

• The Nobel Prize: The Nobel Prize is only awarded to living individuals. Since Franklin died of ovarian cancer in 1958 at the age of 37, she was not eligible for the prize in 1962. However, some argue that the Nobel Committee could have recognized her contribution in other ways, such as through a special citation or memorial lecture.

Franklin's Legacy

Despite the controversies surrounding Photo 51 and the Nobel Prize, Rosalind Franklin's legacy as a pioneering scientist is secure. Her work on DNA, RNA, viruses, coal, and graphite has had a profound impact on science and technology.

• DNA Structure: Photo 51 and Franklin's analysis of X-ray diffraction data were essential for determining the structure of DNA. This discovery revolutionized biology and genetics, leading to a deeper understanding of heredity, disease, and evolution.

• RNA Structure: Franklin also made significant contributions to the understanding of RNA structure. She began working on RNA structure in 1954 and produced important data before her death in 1958.

• Virus Structure: Franklin was a pioneer in the use of X-ray diffraction to study the structure of viruses. Her work on the tobacco mosaic virus (TMV) provided insights into the organization and assembly of viral particles. She led the team that produced the first X-ray images of the polio virus.

• Coal and Graphite: Before her work on DNA, Franklin made important contributions to the study of coal and graphite. Her research helped to understand the physical properties of these materials and their behavior during combustion.

Conclusion: The Enduring Impact of Photo 51

Rosalind Franklin's Photo 51 stands as a testament to the power of scientific inquiry and the importance of meticulous experimental work. This single image, born from Franklin's expertise in X-ray diffraction, provided irrefutable evidence that DNA is a helix, forever changing our understanding of the molecule of life.

While the controversies surrounding the discovery of DNA's structure and the awarding of the Nobel Prize continue to be debated, Franklin's contributions are now widely recognized and celebrated. Her legacy as a pioneering scientist and a role model for women in science is secure. Photo 51 serves as a reminder of the importance of collaboration, ethical conduct, and the pursuit of knowledge in the scientific endeavor. It is a powerful symbol of how a single image can transform our understanding of the world and pave the way for groundbreaking discoveries.