Simulations of mitotic chromosomes. Top: Simulation timeline. Center: Sketch of the model ingredients. Left: A string of blue beads (blue line) representing a mitotic chromosome is formed into a BBP with consecutive loops, which are assumed to be created by the action of looping condensins (gray segments, modeled as springs). Condensin loop anchors and bridging condensins are shown as red and green beads, respectively; the latter experience a purely steric interaction with the polymer in the first part of the simulation. Right: Starting from ∼5 min, bridging condensins can bind reversibly to chromatin, weakly to blue beads, and strongly to red ones, generating a SAC. Bottom: Snapshots from computer simulations showing typical structures for the BBP (left) and SAC (right) regimes. The transition between the two is driven by condensin-mediated bridging. From left to right, the three insets correspond to backbone (condensin loop anchors) in the bottlebrush regime, an image of a mitotic chromosome from RPE1 untreated cells with SMC2 staining (scale bar = 2.5 μm, see Materials and methods for additional information), and backbone with condensin bridges in the SAC regime. The experimental SMC2 staining image reveals an inhomogeneous profile as emerges from our simulations.