Stretching and imaging single DNA molecules and chromatin.

The advent of single-molecule biology has allowed unprecedented insight into the dynamic behavior of biological macromolecules and their complexes. Unexpected properties, masked by the asynchronous behavior of myriads of molecules in bulk experiments, can be revealed; equally importantly, individual members of a molecular population often exhibit distinct features in their properties. Finally, the single-molecule approaches allow us to study the behavior of biological macromolecules under applied tension or torsion: understanding the mechanical properties of these molecules helps us understand how they function in the cell. The aim of this chapter is to summarize and critically evaluate the properties of single DNA molecules and of single chromatin fibers. The use of the high-resolution imaging capabilities of the atomic force microscopy has been covered, together with manipulating techniques such as optical fibers, optical and magnetic tweezers, and flow fields. We have learned a lot about DNA and how it responds to applied forces. It is also clear that even though the study of the properties of individual chromatin fibers has just begun, the single-molecule approaches are expected to provide a wealth of information concerning the mechanical properties of chromatin and the way its structure changes during processes like transcription and replication.