We present a theory for the effects of osmotic pressure exerted by macromolecular crowders on a double-stranded DNA or other semiflexible polymer extended by tension. Our results predict the force and crowder-density dependence of the polymer extension. The lowest-order effect is a crowder-dependent compression that counteracts the external stretching force, which can collapse the polymer at a crowder-dependent critical force $f^{*}$. This compression is dependent on crowder radius $r$ and density $\varphi$, with higher densities and smaller radii having greater effects on the force-extension curve. At first order in perturbation theory, we also find a fluctuation-dependent correction to the depletion volume, which can overwhelm the simple compression effect for large crowders, leading to expansion of the polymer.

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• Cell nucleus
• DNA