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DNA, RNA, and proteins are fundamental units of life, but the atoms that make them up are even more basic. As any engineer knows, the function of a machine depends on the way its parts are assembled, and the same is true for molecules. By the mid-20th century it had become clear that understanding genes would require learning about their chemical makeup and physical structure.

Chemists knew that DNA consisted of a sugar called deoxyribose, plenty of phosphate atoms, and the four nucleotide bases. Each component has a particular shape and chemistry that determine how it snaps onto the others. With very simple molecules, it is sometimes possible to guess how the parts fit together just by looking at the chemistry of the subunits, but in this case there were too many ways that the pieces might fit.

The details of DNA and other molecules such as proteins were too small to be seen through even the most powerful electron microscopes, so chemists were trying to understand DNA's structure by watching how other molecules changed it--a bit like ramming cars into each other to study their engines. Crystallography took another approach, turning molecules into crystals and exposing them to X-rays. This method, developed by physicists, had provided some important information about the shapes of proteins; perhaps the same thing would work with DNA.

When an X-ray beam passes through an object, some of the waves collide with atoms' electrons and are diffracted (they scatter off in a new direction). William Astbury (1898-1961) shined X-rays through molecules and captured the scattering patterns on photographic plates. Usually the resulting image was an unreadable smear. But a molecule whose atoms were arranged in precise, repeated groups scattered the waves in the same directions over and over again, creating a symmetrical pattern that hinted at the shapes of molecules. Astbury had been trying this with proteins that had formed crystals. In crystals, molecules are often arranged in precise lattices that repeat over and over again, billions or trillions of times. This creates the regular structures necessary to obtain a clear diffraction pattern.

Very pure DNA could either be made into crystals or pulled into fibers that also provided regular diffraction patterns. When Astbury examined DNA fibers with X-rays, he obtained some basic information about the size and architecture of the molecule. His interpretation was that the bases fit together into flat disks, squeezed very tightly together like dinner plates stacked in a column. He could measure the diameter of the disks and the height of each plate. However, many of the details remained blurred; without knowing it, he was working with two different forms of DNA. In his images they were superimposed. . .

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