Scientists discovered the structure of DNA through a combination of experimental research and theoretical modeling, primarily relying on X-ray crystallography and insights into its chemical composition.
The Key Players and Their Contributions
The discovery of DNA's structure was a collaborative effort involving several key scientists:
- Rosalind Franklin and Maurice Wilkins: Conducted X-ray diffraction experiments at King's College London. Franklin's sharp X-ray diffraction image, known as "Photo 51," was crucial in revealing DNA's helical shape.
- James Watson and Francis Crick: Used Franklin's X-ray data (without her direct permission in the earliest stages), along with their understanding of chemistry and base-pairing rules, to build an accurate model of the DNA molecule. They proposed the double helix structure.
The Process: From Observation to Understanding
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X-ray Crystallography: Rosalind Franklin used X-ray diffraction to bombard DNA crystals with X-rays. The way the X-rays scattered produced a diffraction pattern that could be analyzed. Photo 51 clearly showed a helical structure. The pattern suggested a repeating structure and dimensions within the DNA molecule.
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Building the Model: Watson and Crick, understanding the work of Linus Pauling (who had discovered the alpha-helix structure of proteins) and utilizing the X-ray data from Franklin and Wilkins, began building physical models of DNA.
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Base Pairing Rules: They initially struggled to create a stable model. Crucially, they learned from Erwin Chargaff’s findings, which stated that the amount of adenine (A) in DNA is always equal to the amount of thymine (T), and the amount of guanine (G) is always equal to the amount of cytosine (C). This observation led them to propose that A pairs with T and G pairs with C.
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The Double Helix: Watson and Crick realized that DNA was made up of two chains of nucleotide pairs that encode the genetic information for all living things. By combining the X-ray data, the base-pairing rules, and model building, they finally proposed the double helix structure of DNA, with the sugar-phosphate backbone on the outside and the base pairs on the inside, held together by hydrogen bonds.
Why It Matters
The discovery of DNA's structure was a landmark achievement in biology. It explained how genetic information is stored, copied, and transmitted, paving the way for advancements in genetics, medicine, and biotechnology.
