Direct imaging of single-molecules: from dynamics of a single DNA chain to the study of complex DNA-protein...

Recent years have seen significant advances in the characterization and manipulation of individual molecules. The combination of single-molecule fluorescence and micromanipulation enables one to study physical and biological systems at new length scales, to unravel qualitative mechanisms, and to measure kinetic parameters that cannot be addressed by traditional biochemistry. DNA is one of the most studied biomolecules. Imaging single DNA molecules eliminates important limitations of classical techniques and provides a new method for testing polymer dynamics and DNA-protein interactions. Here we review some applications of this new approach to physical and biological problems, focusing on videomicroscopy observations of individual DNA chains extended in a shear flow. We will first describe data obtained on the stretching, relaxation and dynamics of a single tethered polymer in a shear flow, to demonstrate that the deformation of sheared tethered chains is partially governed by the thermally driven fluctuations of the chain transverse to the flow direction. Next, we will show how single-molecule videomicroscopy can be used to study in real time DNA folding into chromatin, a complex association of DNA and proteins responsible for the packaging of DNA in the nucleus of an eukaryotic cell.