  Chromatin is a complex of DNA and specific proteins forming an intermediary
level of organization of eukaryotic genomes, between double-stranded DNA and
chromosome. Within a generic modeling of the chromatin assembly, we investigate
the interplay between the mechanical properties of the chromatin fiber and its
biological functions. A quantitative step is to relate the mechanics at the DNA
level and the mechanics described at the chromatin fiber level. It allows to
calculate the complete set of chromatin elastic constants (twist and bend
persistence lengths, stretch modulus and twist-stretch coupling constant), in
terms of DNA elastic properties and geometric features of the fiber. These
elastic constants are strongly sensitive to the local architecture of the fiber
and we argue that this tunable elasticity might be a key feature in chromatin
functions, for instance in the initiation and regulation of transcription.
Moreover, this analysis provides a framework to interpret micromanipulations
studies of chromatin fiber and suggests further experiments involving
intercalators to scan the tunable elasticity of the fiber.
