To fit in a cell nucleus, a strand of DNA must be folded to a size orders of magnitude smaller than its stretched length. In the first step of that compaction, the DNA winds around 10-nm protein clusters to form units called nucleosomes; the whole DNA–protein complex is known as chromatin. Cell biology textbooks typically illustrate chromatin’s structure as in figure 1a: with the nucleosomes evenly spaced along the DNA strand, like pearls on a string, and then arranged into a regular, compact 30-nm fiber.
The evidence for that tidy picture is spotty—and, some have recently argued, wrong. But the traditional tools for probing cells have trouble with structures tens of nanometers large: Conventional optical microscopy doesn’t have the resolution, and electron microscopy and x-ray techniques lack the chemical specificity to distinguish the nucleosome proteins from other molecules.
Now Melike Lakadamyali, Maria Pia Cosma, and their colleagues at the...
