Phase Transition in Reconstituted Chromatin

By observing reconstituted chromatin by fluorescence microscopy (FM) and atomic force microscopy (AFM), we found that the density of nucleosomes exhibits a bimodal profile, i.e., there is a large transition between the dense and dispersed states in reconstituted chromatin. Based on an analysis of the spatial distribution of nucleosome cores, we deduced an effective thermodynamic potential as a function of the nucleosome-nucleosome distance. This enabled us to interpret the folding transition of chromatin in terms of a first-order phase transition. This mechanism for the condensation of chromatin is discussed in terms of its biological significance.