Deoxyribonucleic acid, commonly known as DNA, holds the instructions for life, serving as the fundamental blueprint for all known living organisms. Scientists sought to unravel its precise architecture to understand how it could store and transmit genetic information.
Understanding X-ray Diffraction
To decipher the intricate structures of molecules too small to be seen directly, scientists employ X-ray diffraction. This method aims a concentrated beam of X-rays at a crystallized or highly ordered sample. X-rays are suitable because their wavelengths are comparable to atomic distances within a molecule.
When the X-ray beam interacts with atoms in the sample, the X-rays scatter. In materials with a regular, repeating atomic arrangement, such as DNA fibers, these scattered X-rays can interfere. Some reinforce each other, creating a distinct pattern of spots or lines on a detector. Analyzing these patterns allows researchers to deduce the three-dimensional arrangement of atoms within the molecule.
Photo 51: Unlocking DNA’s Secrets
Photo 51, a significant X-ray diffraction image, was captured by chemist Rosalind Franklin and her graduate student Raymond Gosling in May 1952. This photograph, of the “B form” of DNA, a wetter, more crystalline state, yielded a clear diffraction pattern. The image provided direct visual evidence of DNA’s structural characteristics, offering clues to its overall shape and dimensions.
Photo 51’s prominent “X” shape indicated DNA had a helical, or spiral, configuration. The specific angles and spacing within this pattern allowed precise calculations of the helix’s dimensions. Researchers determined one complete turn, or pitch, measured approximately 3.4 nanometers, and the helix radius was about 1 nanometer.
Further analysis of the pattern revealed the dense phosphate backbone was on the outside of the helix. This insight placed the sugar-phosphate components on the exterior, with the less dense nitrogenous bases positioned on the interior. Photo 51’s clarity and detailed measurements provided an empirical foundation for understanding DNA’s architecture.
How Franklin’s Data Shaped the Double Helix
Rosalind Franklin’s X-ray diffraction work, particularly Photo 51 and her accompanying data, provided the empirical evidence that guided the construction of the double helix model. Her precise measurements and structural insights were important for James Watson and Francis Crick in building an accurate DNA representation. Watson, upon seeing Photo 51, recognized its implications for a helical structure.
Beyond the photograph, Franklin’s unpublished research reports, containing detailed dimensions and her conclusion about the external placement of the phosphate backbone, were also accessible to Watson and Crick. Francis Crick deduced the antiparallel orientation of the two DNA strands from Franklin’s calculations. Her data served as a check and confirmation for their model-building attempts, ensuring the accuracy of the proposed double helical structure.
Recognizing Rosalind Franklin’s Contribution
Rosalind Franklin’s contributions to understanding DNA’s molecular structure were fundamental to a major discovery in biology. She was a rigorous experimentalist, whose insistence on carefully collected data yielded important results. Despite her work’s impact, her specific contributions remained largely unrecognized during her lifetime.
Franklin died in 1958, before the 1962 Nobel Prize in Physiology or Medicine was awarded to James Watson, Francis Crick, and Maurice Wilkins for their work on DNA. Nobel Prizes are not awarded posthumously, making her ineligible for the recognition. However, in recent years, Rosalind Franklin’s scientific legacy and her role in unraveling DNA’s structure have gained deserved recognition.