  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.
