Chromatin fiber structure: morphology, molecular determinants, structural transitions.

Despite more than 20 years of research, the structure of the chromatin fiber and its molecular determinants remain enigmatic. Recent developments in high-resolution microscopic techniques, as well as the application of mathematical modeling to chromatin fiber structure, have allowed the acquisition of some new insights into the structure and its determinants. Here we present some of the newest data on the structure of the chromatin fiber in both its extended and compacted states, and bring together this new knowledge with older data in an attempt to provide a unified view of how chromatin components interact with each other to form its various conformations. The structural transitions that are believed to take place during transcriptional activation and its cessation are also discussed. It becomes obvious that despite some progress in our understanding of the fiber structure and its dynamics, huge gaps continue to exist. Bridging these gaps will require further improvements in already available techniques and the introduction of completely new approaches.