Chromatin Higher Order Structure: Chasing a Mirage?

The question as to how the strands of chromatin are packed into the eukaryotic nucleus has intrigued biologists and biochemists for many years. Although there is ample space in the nucleus for the eukaryotic genome, the enormous length of the DNA molecules requires that they be extensively folded. In addition, the folding must be such as to allow regulated access to certain regions of the genome. The discovery of the nucleosome as a basic organizing unit cast the problem in more defined focus. Even so, it was realized that the condensation afforded to the DNA by nucleosomes (about a 5:1 compaction ratio) could not account for the many thousand-fold compaction that must exist in vivo. Some kind of higher order folding of the nucleosomal fiber must therefore exist. Many models have been proposed for this folding, most of which predicate a regular helical folding of the fiber. It is the purpose of this review to critically re-examine the evidence concerning this condensed structure of chromatin. We must begin, however, with two caveats. First, most of our evidence concerning chromatin fiber structure has been based on in vitro studies of isolated fragments of chromatin. There is some reason for concern that these may not closely resemble the fibers as they exist in the nucleus. Only a few experiments have been conducted in which chromatin fibers have been examined in situ, with minimum perturbation of the nuclear structure. Second, past experiments have all too often utilized cells that are relatively inactive, for example chicken erythrocytes and invertebrate sperm. The folding of chromatin in such cells may not be typical of eukaryotic cells in general. Such concerns must be kept in mind in evaluating existing data.