  Apart from being the gateway for all access to the eukaryotic genome,
chromatin has in recent years been identified as carrying an epigenetic code
regulating transcriptional activity. The detailed knowledge of this code
contrasts the ignorance of the fiber structure which it regulates, and none of
the suggested fiber models are capable of predicting the most basic quantities
of the fiber (diameter, nucleosome line density, etc.). We address this
three-decade-old problem by constructing a simple geometrical model based on
the nucleosome shape alone. Without fit parameters we predict the observed
properties of the condensed chromatin fiber (e.g. its 30 nm diameter), the
structure, and how the fiber changes with varying nucleosome repeat length. Our
approach further puts the plethora of previously suggested models within a
coherent framework, and opens the door to detailed studies of the interplay
between chromatin structure and function.
