Universal Aspects of Chromosome Folding
R. Everaers, ENS Lyon, France  and A. Rosa, SISSA, Trieste, Italy

Human chromosomes are fiber-like complexes of histone proteins 
condensing DNA double helices of ~ 10^8 base-pairs. The dynamics of the 
mm-long chromatin fibers in the cell nucleus is subject to strong 
topological constraints. In particular, their incomplete equilibration 
during interphase results in territorial, entanglement-free crumpled 
globule-like chromosome conformations. We have further explored the 
subtle physics of solutions of non-concatenated ring polymers as a model 
for interphase nuclei. Our results indicate that not only the 
territorial confinement but also the fractal, large scale crumpled loop 
structure arise as a generic consequence of the polymeric nature of 
chromosomes. Using a multi-scale approach, we are able to map the rings 
system onto a melt of interacting lattice animals and to identify the 
physics underlying the emergent behavior. Our description allows for a 
direct mapping to experimental data and combines massive Molecular 
Dynamics simulations on the fiber level with Monte Carlo simulations of 
a lattice model of interacting, randomly branched primitive chains for 
the lattice animal model. Integrated with biological information on 
intra- and inter-chromosomal interactions, our results pave the way for 
the systematic modeling of the large-scale nuclear structure and dynamics.