The Gene: An Intimate History

by Siddhartha Mukherjee

To decipher the structure of DNA, Wilkins had decided to corral a set of biophysical techniques invented in nearby Cambridge—crystallography and X-ray diffraction. To understand the basic outline of this technique, imagine trying to deduce the shape of a minute three-dimensional object—a cube, say. You cannot “see” this cube nor feel its edges—but it shares the one property that all physical objects must possess: it generates shadows. Imagine that you can shine light at the cube from various angles and record the shadows that are formed. Placed directly in front of the light, a cube casts a square shadow. Illuminated obliquely, it forms a diamond. Move the light source again, and the shadow is a trapezoid. The process is almost absurdly laborious—like sculpting a face out of a million silhouettes—but it works: piece by piece, a set of two-dimensional images can be transmuted into a three-dimensional form. X-ray diffraction arises out of analogous principles—the “shadows” are actually the scatters of X-rays generated by a crystal—except to illuminate molecules and generate scatters in the molecular world, one needs the most powerful source of light: X-rays. And there’s a subtler problem: molecules generally refuse to sit still for their portraits. In liquid or gas form, molecules whiz dizzily in space, moving randomly, like particles of dust. Shine light on a million moving cubes and you only get a hazy, moving shadow, a molecular version of television static. The only solu...