Institute for Chemical Reaction Design and Discovery, Hokkaido University, Sapporo, Japan.
Cluster of Excellence Physics of Life, TU Dresden, Dresden, Germany.
Biophys J. 2022 Jul 19;121(14):2742-2750. doi: 10.1016/j.bpj.2022.06.014. Epub 2022 Jun 15.
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.
重构染色体的实验表明，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) 从这种重构的染色体中耗尽时，这些染色体不会解开并形成“烟花”，其中 DNA 和连接组蛋白在核心中浓缩，凝聚蛋白位于外围。为了理解烟花形成的机制，我们在这里考虑了凝聚蛋白在纠缠聚合物凝胶理论延伸中的环挤出。环挤出使纠缠的 DNA 网络变硬，因为弹性有效链中的 DNA 片段被转移到环中，而环是无弹性的。我们的理论预测，凝聚蛋白的环挤出驱动了体积相转变，使肿胀的纠缠 DNA 凝胶坍塌，因为网络的变硬使肿胀相不稳定。这可能是理解有丝分裂染色体组装机制的重要一环。
