X-ray diffraction of DNA
How do you see the shape of a molecule far too small for any microscope of the 1950s? You cannot look at it directly — but you can shine X-rays through it and read the pattern of shadows they cast. That technique, X-ray diffraction, is how the double helix was first glimpsed.
When a beam of X-rays passes through an orderly array of DNA fibres, the regular spacing of atoms scatters the rays into a pattern of spots and arcs on a detector. From the geometry of that pattern, a trained eye can work backwards to the shape of the molecule that produced it. In Rosalind Franklin's famous image, known as Photo 51, the bold X-shaped cross of spots was a direct fingerprint of a helix, and the spacing of the marks revealed the helix's repeat (about ten bases per turn) and its width.
This matters as both science and history. Franklin's careful diffraction data, and her conclusion that DNA was helical with the phosphate backbone on the outside, were essential evidence for the Watson-Crick model — yet her image was shown to them without her knowledge, and her contribution was long under-credited. She died in 1958, before the 1962 Nobel Prize that the structure earned, and her role is now recognised as central to one of science's great discoveries.
The simple X of dark spots in Photo 51 is, to a crystallographer, an unmistakable signature: only a helical molecule scatters X-rays into that cross, and the gaps between the layers gave away the ten-base-per-turn repeat.
Photo 51: shadows of X-rays that revealed a helix.
Diffraction does not photograph a molecule directly; it records a scatter pattern from which the structure must be inferred. And the discovery was a collaborative, contested affair — Franklin's data were used without her consent, a now-famous episode in the ethics of science.