Chromatin structure and the cell cycle.

Pancreatic DNase I is used to probe the structure of chromatin isolated from synchronized HeLa cells. The degree to which DNA in chromatin is protected from DNase attack varies during the G(1), S, and G(2) phases of the cell cycle. In addition, the DNase sensitivity of chromatin from contact-inhibited African green monkey kidney cells differs from that of actively dividing, subconfluent cultures. These cell cycle-dependent chromatin changes were observed consistently at all enzyme concentrations (5000-fold range) and incubation times (15 min-2 hr) tested. The results indicate that the degree of complexing between DNA and chromosomal proteins changes during interphase, and they suggest that the chromosome coiling cycle of visible mitosis may extend in more subtle form over the entire cell cycle.