Dresden 2026 – scientific programme
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BP: Fachverband Biologische Physik
BP 36: Statistical Physics of Biological Systems IV (joint session BP/DY)
BP 36.11: Talk
Friday, March 13, 2026, 12:30–12:45, BAR/SCHÖ
Universal loop statistics from active extrusion — •Anastasia Chervinskaya1 and Kirill Polovnikov1,2 — 1Moscow, Russia — 2Institute for Physics and Astronomy, University of Potsdam, Potsdam-Golm, Germany
Cohesin-dependent loop extrusion is a key active mechanism of genome organization, yet quantitative links between extrusion kinetics and measurable loop statistics remain incomplete. We develop an analytical model that predicts the mean loop scale, full loop-length distributions, state composition, and arm-arm correlations for one-sided versus two-sided extrusion. The theory maps the master equations onto diffusion on a state graph yielding state-resolved loop-length PDFs.
We show that one-sided extrusion yields a universal exponential loop-length distribution, whereas two-sided extrusion generates a sum of exponentials that approaches a gamma-like form at high barrier density. The model also predicts a strictly positive lower bound of 1/4 on arm-arm correlations.
Parameterized with independent measurements for HeLa G1 (cohesin residence and spacing; barrier densities and lifetimes), our model quantitatively accounts for the observed loop size. Also, it reproduces the experimentally measured distribution of CTCF-CTCF loop lengths under the assumption of two-sided extrusion, providing additional evidence that cohesin extrusion in living cells is predominantly bidirectional. Our results provide a compact route to infer biophysical parameters of active extrusion from experimental data.
Keywords: chromatin; loop extrusion; cohesin; polymer physics; statistical physics