Removal of histone H1 exposes a fifty base pair DNA segment between nucleosomes.

Micrococcal nuclease has been used to prepare chromatin from HeLa cells and to probe the structure of HeLa chromatin under various ionic conditions and after the removal of chromatin proteins by salt extraction. The results suggest that (1) HeLa chromatin DNA exists as 150-160 base pair beads interspersed with 40-50 base pair bridges;(2) the bead and bridge conformation exists at physiologic salt concentrations; and (3) removal of histone H1 renders the 40-50 base pair bridge, but not the 150-160 base pair bead, more nuclease susceptible.