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Interphase Human Chromosome Exhibits Out of Equilibrium Glassy Dynamics

By Guang Shi, Lei Liu, Changbong Hyeon, D. Thirumalai

Posted 24 Sep 2017
bioRxiv DOI: 10.1101/193375 (published DOI: 10.1038/s41467-018-05606-6)

The structural organization of the condensed chromosomes is being revealed using chromosome conformation capture experiments and super-resolution imaging techniques. Fingerprints of their three-dimensional organization on length scale from about hundred kilo base pairs to millions of base pairs have emerged using advances in Hi-C and super-resolution microscopy. To determine the poorly understood dynamics of human interphase chromosomes, we created the Chromosome Copolymer Model (CCM) by representing the chromosomes as a self-avoiding polymer with two loci types corresponding to euchromatin and heterochromatin. Using advanced clustering algorithms we establish quantitatively that the simulated contact maps for chromosomes 5 and 10 and those inferred from Hi-C experiments are in agreement. Ward Linkage Matrix (WLM), constructed from spatial distance information, shows that the Topologically Associated Domains (TADs) and compartments predicted from simulations are in agreement with inferred WLM computed using data from super-resolution microscopy experiments. Glassy dynamics is manifested in the stretched exponential relaxation of the structure factor and caging in the mean square displacement of individual loci, Δi(t)~tα with 0 < α < 1. Remarkably, the distribution of α, is extremely broad suggestive of highly heterogeneous dynamics, which is also reflected in the large cell-to-cell variations in the contact maps. Chromosome organization is hierarchical involving the formation of chromosome droplets (CDs) on short genomic scale followed by coalescence of the CDs, reminiscent of Ostwald ripening. We propose that glassy landscapes for the condensed active chromosomes might provide a balance between genomic conformational stability and biological functions.

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