Early and late G2 arrest of cells undergoing radiation-induced apoptosis or micronucleation

Conventional flow cytometric DNA measurements combined with the microscopic detection of cells in the late G2 phase of the cell cycle (characterized by the occurrence of paired kinetochores) enabled us to differentiate and to quantify early and late G2 cells 0-40 h after irradiation using a radioresistant (L929) and a radiosensitive (HL-60) cell line. This approach provided us with (1) a new kind of G2 arrest characteristic revealing changes in the G2 phase which can not be detected by flow cytometric DNA measurements, (2) cell line dependent differences in the radiation-induced transition through G2, accompanied by the occurrence of micronucleation and apoptosis, and (3) the characterization of apoptotic cells occurring probably during early G2 and bearing a rapidly reduced number of kinetochores in contrast to mitotic cells, suggesting processes different from those that operate in mitosis.