Rosalind Franklin, a chemist and X-ray crystallographer, was instrumental in revealing the molecular structure of DNA. Joining King’s College London in 1951, her rigorous experimentation and expertise in X-ray diffraction provided critical data. Her discoveries laid foundational groundwork for one of the most significant biological insights of the 20th century, crucial for understanding how genetic information is stored and passed down.
The X-ray Diffraction Technique
X-ray diffraction is a scientific method used to determine the atomic and molecular structure of crystalline materials. This technique directs a beam of X-rays at a crystallized sample. When X-rays encounter arranged atoms, they scatter, creating a unique pattern of diffracted rays.
Scattering occurs because the X-ray wavelength is comparable to atomic spacing. Scattered X-rays interfere constructively, creating stronger signals captured on a detector. By analyzing the angles and intensities, scientists deduce the three-dimensional arrangement of atoms and molecules.
Rosalind Franklin was highly skilled in this technique, having used it to study coal and viruses. Her expertise produced exceptionally clear DNA diffraction patterns, superior to others. This precision was essential for discerning DNA’s subtle structural details.
The Forms of DNA: A and B
Rosalind Franklin’s meticulous X-ray diffraction work revealed DNA could exist in two distinct forms, A and B. She discovered these by carefully controlling humidity levels, observing how the molecule’s structure changed with hydration.
The A-form of DNA is a drier, crystalline structure appearing at lower humidity. Its X-ray diffraction pattern was sharper and more detailed than the B-form. While helical, its compact nature made it less straightforward to interpret for the overall double helix.
The B-form of DNA is a wetter, hydrated structure forming under higher humidity. This is the form predominantly found in living cells. The X-ray diffraction pattern for B-DNA, famously captured in “Photo 51” by Franklin and Raymond Gosling, displayed a distinct X-shape characteristic of a helical molecule. This image provided crucial evidence for a double-helical structure with specific dimensions.
The Significance of Franklin’s Work
Franklin’s identification of the A and B forms of DNA and her high-resolution X-ray diffraction images, particularly Photo 51, were significant for understanding DNA’s structure. Photo 51 provided unambiguous evidence of DNA’s helical nature, showing the characteristic X-pattern and revealing key measurements like base pair spacing and helix diameter.
This precise data was instrumental to James Watson and Francis Crick in their development of the double helix model. Franklin’s experimental data provided the empirical basis that confirmed and refined their ideas. Her measurements indicated the phosphate backbone was on the outside of the helix and suggested a two-stranded structure, both crucial insights.
Despite the foundational nature of her contributions, Franklin’s work was not always fully recognized during her lifetime. Her paper, co-authored with Raymond Gosling, was published in Nature alongside Watson and Crick’s groundbreaking announcement in 1953. Her rigorous scientific methodology and the clarity of her X-ray images were indispensable in solving the puzzle of DNA’s structure.