Chapter 13 - Multilayer organization of chromosomes

Abstract

Diverse experimental evidences, including atomic force microscopy and cryo-electron tomography results, suggest that the chromatin filament of each metaphase chromosome is self-organized into stacked thin layers oriented perpendicular to the chromosome axis. In this multilayered structure, each individual layer has the width corresponding to a mononucleosome sheet. This avoids the entanglement of genomic DNA and allows the interaction of nucleosomes in adjacent layers. This structure justifies the mechanical properties of metaphase chromosomes and is compatible with the transverse orientation of the cytogenetic bands, with the existence of very thin bands, and with band splitting; it is also compatible with the orthogonal orientation and planar geometry of the connection surfaces observed in sister chromatid exchanges and in chromosome translocations. Further structural and cytogenetic studies indicate that chromatin layers are maintained during interphase. This may facilitate DNA replication and repair, and it has been suggested that gene expression occurs in compartments in which chromatin layers are unstacked.