TRANSITIONS BETWEEN IN-SITU AND ISOLATED CHROMATIN

We show that the mechanism by which chromatin displaying higher-order structure is usually isolated from nuclei involves a transition to an extended nucleosomal arrangement. After being released from nuclei, ch romatin must refold in order to produce the typical chromatin fibers o bserved in solution. For starfish sperm chromatin with a long nucleoso me repeat (222 bp), isolated fibers are significantly wider than those in the nucleus, indicating that the refolding process does not regene rate the native higher-order structure. We also propose that for typic al eukaryotic nuclei, the concept that the native state of the (inacti ve) bulk of the genome is a chromatin fiber with defined architecture be reconsidered.