Experiments on reconstituted chromosomes have revealed that mitotic chromosomes are assembled even without nucleosomes. When topoisomerase II (topo II) is depleted from such reconstituted chromosomes, these chromosomes are not disentangled and form “sparklers,” where DNA and linker histone are condensed in the core 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 because the stiffening of the network destabilizes the swollen phase. This may be an important piece to understand the mechanism of the assembly of mitotic chromosomes.

对重建染色体的实验表明，即使没有核小体，有丝分裂染色体也能组装。当拓扑异构酶 II (topo II) 从此类重组染色体中耗尽时，这些染色体不会解开并形成“火花”，其中 DNA 和连接组蛋白在核心中凝结，而凝缩蛋白则位于外围。为了理解烟花组装的机制，我们在这里考虑了凝聚素的环挤出，这是缠结聚合物凝胶理论的扩展。环挤出使缠结的 DNA 网络变硬，因为弹性有效链中的 DNA 片段易位到弹性无效的环中。我们的理论预测，凝缩蛋白的环挤出会驱动体积相变，从而使膨胀的缠结 DNA 凝胶塌陷，因为网络的硬化会破坏膨胀相的稳定性。这可能是理解有丝分裂染色体组装机制的重要部分。