The 60th anniversary of Watson and Crick’s DNA structure paper is fast approaching (25th April). So I’ve been hunting for nice DNA demos. My favourite so far is a replication of Rosalind Franklin and Raymond Gosling’s diffraction experiment (which appeared in the same issue of Nature). Franklin and Gosling’s paper featured the now famous photo 51, which contained the tell-tale information that led Watson and Crick to build their double helical model.
The neat thing is you can demonstrate the relationship between the patterns seen in photo 51 and diffraction off a helix using a laser pointer and a spring from a retractable ball point pen.
Just shine the laser through the spring, onto a wall about 3 meters away and you end up with pattern that is strikingly similar to photo 51.
It makes for a great lecture demo or a full lab class, were students can work out the structure of a spring from the diffraction patter. You can find full details in a really nice paper published in The Physics Teacher.