[Structure of chromatin. I: Levels of DNA organization in the nucleus; nucleosome and chromatin fibres].

This bibliographic review, the first of a series of three on chromatin structure, discusses the current knowledge and hypotheses concerning chromatin organization in nucleosomes and fibers. Presently, the structural features of the chromatin arrangement are better known for the low levels of organization, e.g. nucleosome, but there is much disagreement about levels of organization higher than the 10 nm fiber. The published reports are consistent with the model that, in the nucleosome, the DNA is wrapped around the histone octamer. The histone octamer whose structure and dimensions were investigated for years is viewed today as a tripartite particle with the (H3-H4)2 tetramer flanked by two H2A-H2B dimers. In the chromatosome, two turns of DNA around of the histone octamer are sealed by the histone H1 that binds DNA at the points of entry and exit. A chain of nucleosomes, each with about 200 bp associated DNA, forms a 10 nm fiber with a DNA packing ratio of 6-7. The 10 nm fiber is then condensed into a 30 nm fiber, likely of solenoid structure, that is stabilized by histone H1. All the different models proposed for the structure of the 30 nm fiber are consistent with a DNA packing ratio of 35-40.