Abstract
The organization of chromatin is critical for gene expression, yet the underlying mechanisms responsible for this organization remain unclear. Recent work has suggested that phase separation might play an important role in chromatin organization, yet the molecular forces that drive chromatin phase separation are poorly understood. In this work we interrogate a molecular model of chromatin to quantify the driving forces and thermodynamics of chromatin phase separation. By leveraging a multiscale approach, our molecular model is able to reproduce chromatin's chemical and structural details at the level of a few nanometers, yet remain efficient enough to simulate chromatin phase separation across 100 nm length scales. We first demonstrate that our model can reproduce key experiments of phase separating nucleosomal arrays, and then apply our model to quantify the interactions that drive their formation into chromatin condensates with either liquid- or solid-like material properties. We next use our model to characterize the molecular structure within chromatin condensates and find that this structure is irregularly ordered and is inconsistent with existing 30 nm fiber models. Lastly we examine how post-translational modifications can modulate chromatin phase separation and how the acetylation of chromatin can lead to chromatin decompaction while still preserving phase separation. Taken together, our work provides a molecular view into the structure and dynamics of phase-separated chromatin and provides new insights into how phase separation might manifest in the nucleus of living cells.