Condensed chromatin behaves like a solid on the mesoscale in vitro and in living cells

The association of nuclear DNA with histones to form chromatin is essential to the temporal and spatial control of eukaryotic genomes. In this study, we examined the physical state of chromatin in vitro and in vivo. Our in vitro studies demonstrate that MgCl2-dependent self-association of native chromatin fragments or reconstituted nucleosomal arrays produced supramolecular condensates whose constituents are physically constrained and solid-like. Liquid chromatin condensates could be generated in vitro, but only using non-physiological conditions. By measuring DNA mobility within heterochromatin and euchromatin in living cells, we show that chromatin also exhibits solid-like behavior in vivo. Representative heterochromatin proteins, however, displayed liquid-like behavior and coalesced around a solid chromatin scaffold. Remarkably, both euchromatin and heterochromatin showed solid-like behavior even when transmission electron microscopy revealed limited interactions between chromatin fibers. Our results therefore argue that chromatin is not liquid but exists in a solid-like material state whose properties are tuned by fiber-fiber interactions. ### Competing Interest Statement The authors have declared no competing interest.