  It is well known that transcription can induce torsional stress in DNA,
affecting the activity of nearby genes or even inducing structural transitions
in the DNA duplex. It has long been assumed that the generation of significant
torsional stress requires the DNA to be anchored, forming a limited topological
domain, since otherwise it would spin almost freely about its axis. Previous
estimates of the rotational drag have, however, neglected the role of small
natural bends in the helix backbone. We show how these bends can increase the
drag several thousandfold relative to prior estimates, allowing significant
torsional stress even in linear, unanchored DNA. The model helps explain
several puzzling experimental results on structural transitions induced by
transcription of DNA.
