Description
In preparation for mitotic cell division, nuclear DNA is compacted into X-shaped metaphase chromosomes. This dramatic metamorphosis, which is known to require Condensins and Topoisomerase IIα (TOP2A), has been observed using microscopy for over a century, and yet remarkably little is known about the structural organization of condensed human chromosomes. Here, we introduce a novel system to interrogate the structural organization of human chromosomes based on optical manipulation. This allows high-resolution force measurements and fluorescence visualization of native chromosomes to be conducted under tightly controlled experimental conditions. We have used this method to conduct the first extensive characterization of chromosome mechanics. Notably, we find that under increasing mechanical load, chromosomes exhibit nonlinear stiffening behavior, distinct from classical polymer models. To explain this anomalous stiffening, we introduce a Hierarchical Worm-like Chain (HWLC) model describing the chromosome as a heterogeneous assembly of nonlinear WLCs. Moreover, through inducible degradation of TOP2A only after chromosome condensation had occurred, we provide support for a role of TOP2A in the maintenance of a compacted chromosome structure. Taken together, our new approach opens the door to a wide array of new investigations of the structure and dynamics of both healthy and disease-associated chromosomes.
