Nucleosome structure.

Electron microscopic and biochemical results are presented supporting the following conclusions: (1) Two molecules of each histone H2A, H2B, H3 and H4 are necessary and sufficient to form a nucleosome with a diameter of 12.5 +/- 1 nm and containing about 200 base pairs of DNA. (2) H3 plus H4 alone can compact 129 +/- 8 DNA base pairs into a sub-nucleosomal particle with a diameter of 8 +/- 1 nm. In such a particle the DNA duplex is under a constraint equivalent to negative superhelicity. (3) Chromatin should be viewed as a dynamic structure, oscillating between a compact structure (the nucleosome) and more open structures, depending on the environmental conditions.