Nucleosome unwrapping may be easier than you think

Nucleosome unwrapping is essential in order to provide access to eukaryotic DNA otherwise tightly wrapped into nucleosomes. Unwrapping several tens of base pairs from one side of the nucleosome is known to require several kT of free energy. Here we show that for a nucleosome that is fixed on both ends such as in the context of higher order chromatin structure, changes in the end-to-end distance by tens of base pairs are possible at a free energy cost of only a few kT due to the entropic contribution of both ends of the nucleosomes unwrapping simultaneously. We derive this result in the context of quantitatively modeling recently published experimental data on nucleosomes attached to a DNA origami caliper and verify our model through analogous experiments on hexasomes, nucleosomes that are missing one of their histone heterodimers.