Fig. 1 Beads-on-a-string model, volume exclusion and system entropy. A) Scheme of a beads-on-a-string coarse-grain polymer model of the chromatin fibre. Each bead (black empty circles) represents a portion of the chromatin fibre, which can be one or more nucleosomes. B) Thick (blue) and thin (green) polymers occupying peripheral or central positions inside a sphere. Both polymers lose possible conformations and entropy when squashed against the borders of the sphere, but the loss is bigger for the thinner and more flexible polymer [13]. C) A mixture of big and small particles. The excluded volume of the big particles is showed in light blue. It is the volume around each particle that is inaccessible to the others. When big particles come together, the sum of their excluded volumes decreases and more space is available for the smaller particles, increasing the entropy of the system ([16]).