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a. 10 minute talk

Have on hand a fine sieve or piece of gauze, a point source of light, a diffraction photograph of DNA fibres and a model of DNA. The talk begins with a demonstration of diffraction. This is done by having the students view the point source of light through the sieve or gauze. Then it is shown how the diffraction pattern is formed as a result of the regularity of the grid and constructive or destructive interference of scattered waves by this. If the mesh size is then varied, the reciprocal relationship between the mesh and the spacing of the diffraction pattern can be shown. This can also be demonstrated by showing diffraction photos of sodium chloride and a protein on the same scale.

The audience is then told that the whole experiment may be scaled down so that the sieve is replaced by a crystal (with spacings of the order of 10-8 cm) and that the visible light is replaced by X-rays (with wavelengths of the order of 10-8 cm). During this explanation it will be necessary to give some description of the regularity of a crystal as a result of the build-up of unit cells.

The next stage is to display a DNA model and Rosalind Franklin's DNA photo (mentioning the books The Double Helix by Watson and The Eighth Day of Creation by Judson). All regularities of structure that could cause a diffraction pattern are pointed out. There is a 3.5 Å spacing between the DNA bases along the helix axis, and this accounts for the large spot at the top and bottom of the X-ray photograph of DNA (Fig. 2). The parallel lines of backbone in the helix have a regularity that explain the cross in the middle of the photograph. Since the distances in DNA are larger for this regularity, the spots on the X-ray photograph are closer together.

Figure 2: (Photograph by courtesy of R. Langridge.)
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It is hoped that the audience will, on leaving, know what a diffraction effect is, and what DNA looks like.

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