Does loop extrusion alone build and stiffen mitotic chromosomes?
In plain words
Before division, each long chromosome is packed within minutes into a compact rod. Motors called condensins form loops around a helical axis, and whether loop formation alone explains the rod's shape, stiffness and the separation of the two copies is not settled.
Precise statement
Hi-C time courses support nested condensin II and condensin I loops arranged around a helical scaffold (Gibcus and co-workers, 2018). Determine whether a polymer model with measured condensin extrusion rates, densities and loop sizes, and no added crosslinks, reproduces mitotic Hi-C maps, the compaction ratio, the stretching elasticity of isolated chromatids, and sister-chromatid resolution together with topoisomerase II activity.
What would settle it
Parameter-free polymer simulation compared with Hi-C, imaging and micromechanical measurements of mitotic chromosomes under condensin and topoisomerase II depletion.
Status in the literature
The helical loop architecture was reported in 2018 (Science, doi 10.1126/science.aao6135); a quantitative account of chromosome mechanics from extrusion alone is not established, to this survey's knowledge.