On Diffraction and the Helical structure of DNA
This is the famous Photograph 51, which is the nickname given to an X-ray diffraction image of crystallized DNA taken by Raymond Gosling in May 1952.
Crick and Watson used the features of this photograph to develop the chemical model of the DNA molecule aka Double-helix.
A simple way to understand why a helix structure produces such a diffraction pattern is to take a spring of a ballpoint pen and illuminate with a laser pointer.
Image courtesy of D. Tierney and of H. Schmitzer, Xavier University in Cincinnati
Although the spring is not a Double Helix structure, the diffraction pattern produced as you can see is very similar to the famous Photo 51 of helical DNA.
It was also known at that time that DNAs obeyed the Chargaff’s rules: Adenines paired with Thymine and Guanosine paired with Cytosine.
As James Watson explains in this video, he was trying out different models for the DNA that satisfied Chargaff’s rules and also the Helical structure of DNA as Gosling had found out. This led him to the Double Helix model of DNA.
And this model has stood the test of time. In 2012, an electron microscope captured the famous Watson-Crick double helix in all its glory asserting that the DNA is indeed a Double Helix.
Electron microscope image of DNA
The story of the discovery of DNA and its study is captivating by itself and here’s a list of useful papers and web links if you like to venture more:
- Nature podcast of Raymond Gosling discussing the Photograph 51.
- How Rosalind Franklin Discovered the Helical Structure of DNA: Experiments in Diffraction
- The DNA Double Helix Discovery — HHMI BioInteractive Video (Includes discussion with Dr.James Watson)
- How we discovered DNA - TED talk by James Watson
- The Discovery of the DNA Double Helix - A historical overview
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