Differential chromosomal radiosensitivity within the first G1-phase of the cell cycle of early-dividing human leukocytes in vitro after stimulation with PHA.

Human leukocyte cultures were irradiated with 200 RX-rays before the addition of phytohemagglutinin (PHA) in the Go-stage and at different times up to 25 h within the first G1-phase of the cell cyle after the addition of PHA. The results of the analysis of chromosomal aberrations show that the frequencies of dicentric chromosomes increase significantly when leukocytes leave the Go-stage, reaching a miximum yield of aberrations about halfway through the first G1-phase. After that, toward the end of the G1-phase, the frequencies of dicentric chromosomes decrease again to a level similar to that found in the Go-stage. Different possible explanations for the differential chromosomal radiosensitivity of human leukocytes within the first post-stimulation G1-phase are discussed.