Loop extrusion driven volume phase transition of entangled chromosomes

Abstract

Mitotic chromosomes without nucleosomes have been reconstituted in recent experiments. When topo II is depleted from the reconstituted chromosomes, these chromosomes are entangled and form 'sparklers', where DNA is condensed in the core with linker histone H1.8 and condensin is localized at the periphery. To understand the mechanism of the assembly of sparklers, we here take into account the loop extrusion by condensin in an extension of the theory of entangled polymer gels. The loop extrusion stiffens an entangled DNA network because DNA segments in the elastically effective chains are translocated to loops, which are elastically ineffective. Our theory predicts that the loop extrusion by condensin drives the volume phase transition that collapses a swollen entangled DNA gel as the stiffening of the network destabilizes the swollen phase. This is an important element to understand the mechanism of the assembly of the reconstituted chromosomes.