On Friday 2 May 1952, at King's College London, Rosalind Franklin and her PhD student Raymond Gosling, took an X-ray diffraction image of DNA fibers. As Franklin reported in her lab notebook, the DNA was exposed to X-rays for a total of 62 hours. This photo, recorded as number 51, was the best image of a "wet" form, the B-form, of DNA. Based on this image, physicist Francis Crick and biologist James Watson built a 3-D model of the DNA molecule and revealed to the world "the secret of life".
A small angle X-ray diffraction pattern provides a view of the squared amplitude of the 2D spatial Fourier transform of the electron density integrated along the beam propagation axis. In the first panel to the left you see a double helix projected in 2D whose geometric parameters can be adjusted using the sliders below. The squared amplitude of the 2D Fourier transform of this image is displayed on its right. Activate the "show grid" checkbox and move the sliders trying to match the diffraction pattern of photo 51 displayed in the rightmost panel. You will learn how the helix radius and pitch are encoded in the angle and spacing of diffraction spots in the main "X" shaped pattern. Base pair spacing controls the position of the main top and bottom central diffraction peaks. A last hint comes from the missing fourth diffraction spot on the "X", this is due to interference of the diffraction amplitudes from the two strands and can be obtained by tuning the shift between the two helices. The DNA double helix parameters are:

• pitch: 3.4 nm
• radius: 1 nm
• basepair distance: 3.4. Å
• shift: 3/8~0.38 of a period.

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