Three-dimensional intranuclear DNA organization in situ: three states of condensation and their redistribution as a function of nuclear size near the G1-S border in HeLa S-3 cells.

Characteristic variations in nuclear morphology occurring with variations in the physiological state of the cell have been observed in a number of systems to date. In this paper, we have critically examined the relationship between nuclear morphology and intranuclear DNA organization near the G1-S transition in HeLa S-3 cells, by the study of both the spatial distribution of optical density values and the optical density histograms for individual Feulgen-stained nuclei. Our results demonstrate that the majority of the DNA is located in a narrow shell surrounding the nuclear and nucleolar borders, and present evidence for at least three discrete states of chromatin condensation. Greater than 90% of the genome appears distributed among the two classes with larger density, and a redistribution between these two classes occurs as a function of changing nuclear size. Numerical simulations indicate that the observed distribution does not arise as an artifact related to overlapping but, in fact, actually represents discrete states of condensation. Interestingly, the extrapolated nuclear area at which the fraction of DNA in the state of highest density is reduced to zero, corresponds closely to the nuclear size shown elsewhere as representing the critical size that HeLa S-3 nuclei must exceed in order to initiate S phase.