A quantitative map of human Condensins provides new insights into mitotic chromosome architecture

The two Condensin complexes in human cells are essential for mitotic chromosome structure. We used homozygous genome editing to fluorescently tag Condensin I and II subunits and mapped their absolute abundance, spacing, and dynamic localization during mitosis by fluorescence correlation spectroscopy (FSC)-calibrated live-cell imaging and superresolution microscopy. Although similar to 35,000 Condensin II complexes are stably bound to chromosomes throughout mitosis, similar to 195,000 Condensin I complexes dynamically bind in two steps: prometaphase and early anaphase. The two Condensins rarely colocalize at the chromatid axis, where Condensin II is centrally confined, but Condensin I reaches similar to 50% of the chromatid diameter from its center. Based on our comprehensive quantitative data, we propose a three-step hierarchical loop model of mitotic chromosome compaction: Condensin II initially fixes loops of a maximum size of similar to 450 kb at the chromatid axis, whose size is then reduced by Condensin I binding to similar to 90 kb in prometaphase and similar to 70 kb in anaphase, achieving maximum chromosome compaction upon sister chromatid segregation.