Summary
Within the nucleus, structural maintenance of chromosome protein complexes, namely condensin and cohesin, create an architecture to facilitate the organization and proper function of the genome. Condensin creates localized clusters of chromatin in the nucleolus through transient crosslinks. Large-scale simulations revealed three different dynamic behaviors as a function of timescale: slow crosslinking leads to no clusters, fast crosslinking produces rigid clusters, while intermediate timescales are optimal for producing flexible clusters that mediate gene interaction. By mathematically analyzing different relative scalings of the two sources of stochasticity, thermal fluctuations and the force induced by the transient crosslinks, we predict these three distinct regimes of cluster behavior. Standard time-averaging that takes the fluctuations of the transient crosslink force to zero can predict the existence of clusters, but not their timescale-dependent lifetimes. Accounting for the interaction of both fluctuations from the crosslinks and thermal noise with an effective energy landscape does capture the timescale-dependent flexible cluster lifetimes. No clusters are predicted when the fluctuations of the transient crosslink force are taken to be large relative to thermal fluctuations. This perturbation analysis illuminates the importance of accounting for stochasticity in local incoherent transient forces to predict emergent complex biological behavior.
