After DNA is purified, it is crystallized. After the crystal is formed (you can imagine something similar to salt particles, but much smaller), X-rays with synchrotron radiation are projected on the DNA crystal, and X-rays will be diffracted, and the diffraction conforms to Bragg formula. We can't "see" from the diffraction pattern, but calculate the structure of DNA according to this pattern and Bragg formula. Wilkins and Franklin laboratories at King's College London use X-ray diffraction to study the crystal structure of dna. When X-rays irradiate the crystal of biological macromolecules, atoms or molecules in the lattice will deflect the rays. According to the obtained diffraction image, the general structure and shape of the molecule can be inferred.

Irradiate DNA molecules with X-rays, observe the points (diffraction patterns) produced by the rays on photographic films, and calculate the dispersion angles of the points. (the dispersion angle of each point represents the position of one atom or several atomic groups of DNA molecules) Infer the molecular arrangement. The most crucial figure 5 1 is the following figure, which was taken in May 1952.

The images of X-ray reflection in the center of the photo (blackening the X-ray film) cross, indicating that it is spiral, and the darkest parts above and below indicate that purine base and pyrimidine base are perpendicular to the spiral axis, and a pair appears regularly every 3.4 angstroms. Many X-ray diffraction patterns have been taken for type A DNA and type B DNA, which are filamentous (chain-shaped) and can obtain 34 angstrom data. Franklin also found that it looked the same after flipping 180 degrees. After receiving this information, Watson and Crick realized that the two chains were opposite. When drawing 5 1 was obtained, some data were also obtained. 1On February 24th, 953, Franklin reached the conclusion of the double helix through calculation and analysis, while Watson and Crick tried to match these data with the double helix model. At that time, Nature published three papers at the same time, and the other two were Wilkins' and Franklin and Randolph's. It needs complicated mathematical calculation to explain the X-ray diffraction pattern of DNA crystal.