Spare the (Elastic) Rod

Physicists love emergence. From a welter of complex details about a system's constituents, simple and universal rules sometimes emerge that adequately describe the collective behavior of the components. Even if these rules are not completely universal, they often have only a few relevant parameters, a vast simplification compared to the many that describe the constituents individually. But as Vafabakhsh and Ha remind us on page 1097 of this issue (1), emergent behavior can conceal important aspects of a system. Using a beautiful application of fluorescence microscopy, the authors provide the clearest evidence to date that the elastic-rod model for DNA mechanics, an emergent description that works well on long length scales, breaks down on shorter length scales relevant to cell biology.