X-ray crystallography is a scientific method that reveals the atomic architecture of molecules. It has been instrumental in deciphering structures of countless biological molecules, from simple salts to complex proteins. Its application to deoxyribonucleic acid, or DNA, represents a significant breakthrough in biology, reshaping our understanding of heredity and life. The insights gained laid the groundwork for modern genetics and molecular biology.
Principles of X-ray Crystallography
X-ray crystallography operates on the principle of diffraction, where waves bend as they encounter an obstacle. A beam of X-rays is directed at a crystal, a solid material with an ordered arrangement of atoms. The X-rays, with wavelengths similar to atomic distances, interact with electrons, causing them to scatter in specific directions and create a unique pattern of diffracted beams.
To perform this, a pure crystal of the molecule is prepared. This crystal is mounted on a goniometer, which precisely positions and rotates it in the X-ray beam. As the crystal rotates, it generates a series of two-dimensional diffraction patterns, captured by a detector. These patterns appear as spots, or reflections, with varying intensities.
The collected diffraction patterns, containing information about the angles and intensities of scattered X-rays, are analyzed using computational methods. The arrangement and brightness of these spots provide clues about the electron density within the crystal. By piecing together thousands of these 2D patterns from different orientations, scientists construct a three-dimensional map of electron density, revealing the precise positions of atoms.
Unveiling DNA’s Structure
Rosalind Franklin and Maurice Wilkins at King’s College London primarily undertook the application of X-ray crystallography to DNA in the early 1950s. Franklin meticulously prepared DNA fibers and subjected them to X-ray beams. She discovered that DNA could exist in two forms, an “A” form and a “B” form, depending on the humidity of the sample.
Franklin captured several X-ray diffraction images, with “Photo 51” being particularly significant. Taken in May 1952 by Franklin and her PhD student Raymond Gosling, this image of the “B” form of DNA provided unprecedented detail. Photo 51 clearly displayed an “X” shape, a characteristic pattern indicating a helical arrangement.
The distinct cross pattern and spot spacing in Photo 51 offered measurements. The vertical separation indicated a repeating unit along the helix, and the overall pattern suggested intertwined DNA strands. Franklin’s analysis led her to conclude DNA was a helix, a double helix, with phosphate groups on the outside. Franklin initially focused more on the “A” form, and the full implications of Photo 51 were not immediately published by her.
The Double Helix Model
Building upon X-ray diffraction data, including Photo 51, James Watson and Francis Crick at the University of Cambridge proposed their double helix model of DNA in 1953. They integrated this structural evidence with existing biochemical knowledge. Franklin and Wilkins’ X-ray patterns guided their model building.
The double helix model describes DNA as two long strands wound around each other in a spiral, resembling a twisted ladder. The “sides” are alternating sugar (deoxyribose) and phosphate molecules, forming a sugar-phosphate backbone on the outside. The “rungs” are pairs of nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C).
A defining feature is the specific pairing of bases: adenine always pairs with thymine (A-T), and guanine always pairs with cytosine (G-C). These complementary base pairs are held together by hydrogen bonds, forming the core of the double helix. This structure suggested a mechanism for genetic information copying, as each strand could serve as a template for a new complementary strand. The model, with its precise dimensions and complementary pairing, provided a framework for understanding heredity.
The Legacy of the Discovery
The elucidation of the DNA double helix structure marked a watershed moment in science, impacting numerous fields. This discovery provided the molecular basis for heredity, transforming genetics and molecular biology. It explained how genetic information is stored, replicated, and passed down.
Understanding DNA’s structure paved the way for advancements in biotechnology and genetic engineering. Scientists could manipulate DNA, leading to new diagnostic tools, therapies for genetic diseases, and genetically modified organisms. The insights continue to drive research in personalized medicine, forensics, and evolutionary biology.
The discovery’s impact was recognized with the Nobel Prize in Physiology or Medicine in 1962, awarded to James Watson, Francis Crick, and Maurice Wilkins. Rosalind Franklin’s contributions were acknowledged posthumously. Her efforts were foundational to revealing DNA’s structure.