How Rosalind Franklin Discovered the Helical Structure of DNA

March Meeting 2010

When illuminated with a laser pointer, a spring of a ballpoint pen produces a diffraction pattern which is similar to the famous Photo 51 of helical DNA

Heidrun Schmitzer
Dennis Tierney
Gregory Braun

Xavier University
Department of Physics
Cincinnati, Ohio


The left photo shows the illuminated spring. The right photo is the diffraction pattern produced by the spring, when five turns are illuminated. It was taken at a distance of ~12m behind the spring. The pattern shows the same structure as Rosalind Franklin's X-ray diffraction image of helical DNA.

The angle of the upper and lower conical section are equivalent to the double pitch angle of the helix. The structure in the areas with maxima is caused by the multiple pitch diffraction and reveals the diameter of the helix. Note that the center of the right pattern was covered with a polarizer set almost to extinction to avoid over exposure at the very bright center maximum. Franklin used a lead disc for the same purpose in the X-ray photo.


1. F. Franklin, R. G. Gosling, Molecular Configuration in Sodium Thymonucleate, Nature, 171 (April 25) 1953, 740-741

2. D. T. Crouse, X-ray Diffraction and the Discovery of the structure of DNA, Journal of Chemical Education, 84 (5) 2007, 803-809

3. M. H. F. Wilkins, A. R. Stokes, H. R. Wilson, Molecular Structure of Deoxypentose Nucleic Acids, Nature, 171 (April 25) 1953, 738-740

4. F. Wang, Rosalind Franklin's X-ray Diffraction Photograph of DNA 

Usage Information

Reporters may freely use this image as long as they include the following credit: "Image courtesy of D. Tierney and of H. Schmitzer, Xavier University in Cincinnati".

For further information, contact:
Jason Bardi
(301) 209-3091