Cell heterogeneity during the cell cycle

Abstract Using flow cytometry, populations of Chinese hamster ovary cells, asynchronous and synchronized in the cycle, were measured with respect to cellular RNA‐ and protein‐content, as well as cell light scatter properties. Heterogeneities of cell populations were expressed as coefficients of variation (c.v.) in percent of the respective mean values. Populations of cells immediately after mitosis have about 15% higher c.v. than mitotic cell populations, regardless of whether RNA, proteins, or light scatter are measured. These data indicate that cytoplasmic constituents are unequally distributed into the daughter cells during cytokinesis and that unequal cytokinesis generates intercellular metabolic variability during the cycle. An additional increase in heterogeneity, although of smaller degree, occurs during G 2 phase. Populations of S‐phase cells measured in the selective window equivalent to 15–60 min progression through the cycle, i.e., comparable with the mitotic and postmitotic populations, are the most uniform, having 20–30% lower c.v. than the postmitotic cells. Cell progression through S does not involve any significant increase in intercellular variability with respect to RNA or protein content. In unperturbed exponentially growing cultures a critical RNA content is required for G 1 cells prior to their entrance into S. Thus, the cells equalize in G 1 with respect to RNA and protein and, during the transition from the period (compartment) of equalization (G 1A ) to the prereplicative compartment (G 1B ), they exhibit minimal heterogeneity. The cell residence times in the equalization compartments are exponentially distributed, which may reflect the randomness generated by the uneven division of metabolic constituents to daughter cells during cytokinesis. The cell heterogeneities were presently estimated at two metabolic levels, transcription (RNA content) and translation (proteins). The most uniform were populations stained for RNA and the highest variability was observed after staining of proteins. This suggests that the regulatory mechanisms equalizing cells in the cell cycle may operate primarily at the level of DNA transcription.