Chromatin organization: the role of microphase separation

Chromatin has been broadly classified into two different types, active (euchromatin) which is accessible to the transcription machinery and inactive (heterochromatin) which is not. Having two types of constituents in chromatin, can lead to microphase separation into compartments that are enriched in either active or inactive chromatin. However, how such microphase separation is reflected in the global organization of chromatin is still elusive. Additionally, some regions of chromatin interact with soluble molecules in the nucleoplasm which can further regulate the solvent quality experienced by the chromatin blocks. Based on recent experiments and simulations showing that chromatin phase separates from the nucleoplasm, we propose a minimal, block copolymer model for chromatin in which each type of block experiences different solvent conditions which can also be regulated by soluble molecules. We analytically predict the length scales of the microphases and their dependence on the copolymer characteristics and interactions with the soluble molecules. We then compare our analytical results with Brownian dynamics simulations of multiblock copolymers in the presence of such soluble molecules and discuss the relevance of our results to recent experiments on chromatin organization.