Chromatin Changes during the Cell Cycle of HeLa Cells*

HeLa SO cells were synchronized by selective mitotic detachment and chromatin was isolated from synchronized populations at various times during the cell cycle. The isolated chromatins were studied by circular dichroism and for their ability to bind the intercalating dye, ethidium bromide. Chromatin from cells in the middle of the S phase showed an increase in maximum ellipticity in the 250to 300~nm region of circular dichroism spectra and an increased ability to bind ethidium bromide, when compared to chromatin isolated from mitotic cells. Chromatin from G1 cells had values intermediate between mitotic and mid-S chromatins.