The eukaryote chromosome, a two-state system with interconversion by a volume phase transition.

The formation of metaphase chromosomes requires the activity of condensins, DNA translocases that reduce chromosomal length through the process of loop extrusion. Reduction of length, however, does not account for the increase in density of chromatin in condensed states. Here, we show that eukaryotic chromosomes are ionic hydrogels and that the chromosomal material is a two-state system, with interconversion between condensed and decondensed states through a volume phase transition, analogous to a liquid-gas transition. Ionic contacts between histone tails and DNA, formed and broken by histone acetylation and deacetylation, may control the transition and be responsible for condensation in heterochromatin and decondensation in euchromatin.