Active loop extrusion modulates the mechanical response of chromatin under tension

Chromosomes are complex biopolymers folded into dynamic loops via a loop-extrusion process and may experience various mechanical forces . We develop a force-dependent model of chromatin loop extrusion and investigate its mechanical consequences on chromosome organization using simulations and analytical theory. We show that loop extrusion alters the force-extension behavior of DNA in a nonmonotonic manner: Extrusion stiffens the chain at low forces but softens it at intermediate and high forces. Our model predicts hysteresis in pulling-recoiling cycles and out-of-equilibrium responses, qualitatively consistent with recent single-chromosome stretching experiments. We further find that loop extrusion provides mechanical robustness to chromatin by promoting compaction while enabling rapid structural recovery after stress. These results establish loop extrusion as a key regulator of chromatin mechanics.