Compaction and swelling of single stretched DNAs driven by molecular crowding

We present a theory for the effects of osmotic pressure exerted by macromolecular crowders, on a double-stranded DNA (or any other semi-flexible polymer) extended by tension. Our results predict DNA force extension curves. The lowest-order effect is a crowder-dependent compression that counteracts the external stretching force. This compression effect can collapse the polymer at a crowder-dependent critical force f ^∗ . This compression is dependent on crowder radius r and density ϕ , 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.