The G1 period. Two cycles of chromatin conformational changes monitored by single cell dye intercalation.

In previous papers the existence of two cycles of chromosome condensation-decondensation per cell cycle was suggested based on experiments involving nuclear morphometry measurements of Feulgen-stained nuclei. This conclusion can be criticized since its assumption of a relationship between nuclear morphology and chromatin structure is derived from indirect evidence. In this paper, we report simultaneous measurements of nuclear area and nuclear fluorescence intensity on individual cells stained with the intercalating dye, acridine orange (AO). Using cells in various stages of G1 and synchronized by two different methods, our results demonstrate a linear correlation between nuclear area and fluorescence intensity. They also indicate two cycles of chromatin condensation-decondensation during the G1 period, as assayed by the number of chromatin primary, intercalating AO binding sites. Finally, they show that the first of these cycles involves a transition in early G1 from a very small condensed nucleus (immediately after telephase) to a relatively large, dispersed nucleus that occurs abruptly.