Limits of chromosome compaction by loop-extruding motors

Abstract

During mitosis, human chromosomes are linearly compacted ~ 1000-fold by loop-extruding motors. Recent experiments have shown that condensins extrude DNA loops, but in a “one-sided” manner. We explore whether one-sided extrusion can compact chromosomes by developing a mean-field model for polymer compaction by motors that actively extrude loops and turnover. The model establishes an upper bound of only ~ 10-fold for compaction by one-sided extrusion. Thus, it cannot be the sole mechanism of chromosome compaction. However, other, effectively two-sided mechanisms can achieve sufficient compaction.