10 years of tension on chromatin: results from single molecule force spectroscopy.

The compact, yet dynamic organization of chromatin plays an essential role in regulating gene expression. Although the static structure of chromatin fibers has been studied extensively, the controversy about the higher order folding remains. In the past ten years a number of studies have addressed chromatin folding with single molecule force spectroscopy. By manipulating chromatin fibers individually, the mechanical properties of the fibers were quantified with piconewton and nanometer accuracy. Here, we review the results of force induced chromatin unfolding and compare the differences between experimental conditions and single molecule manipulation techniques like force and position clamps. From these studies, five major features appeared upon forced extension of chromatin fibers: the elastic stretching of chromatin's higher order structure, the breaking of internucleosomal contacts, unwrapping of the first turn of DNA, unwrapping of the second turn of DNA, and the dissociation of histone octamers. These events occur sequentially at the increasing force. Resolving force induced structural changes of chromatin fibers at the single molecule level will help to provide a physical understanding of processes involving chromatin that occur in vivo and will reveal the mechanical constraints that are relevant for processing and maintenance of DNA in eukaryotes.