EARLY AND LATE G(2) ARREST OF CELLS UNDERGOING RADIATION-INDUCED APOPTOSIS OR MICRONUCLEATION

Conventional flow cytometric DNA measurements combined with the micros copic detection of cells in the late G(2) phase of the cell cycle (cha racterized by the occurrence of paired kinetochores) enabled us to dif ferentiate and to quantify early and late G(2) cells 0-40 h after irra diation using a radioresistant (L929) and a radiosensitive (HL-60) cel l line. This approach provided us with (1) a new kind of G(2) arrest c haracteristic revealing changes in the G(2) phase which can not be det ected by flow cytometric DNA measurements, (2) cell line dependent dif ferences in the radiation-induced transition through G(2), accompanied by the occurrence of micronucleation and apoptosis, and (3) the chara cterization of apoptotic cells occurring probably during early G(2) an d bearing a rapidly reduced number of kinetochores in contrast to mito tic cells, suggesting processes different from those that operate in m itosis.