Chromosomal aberration types in cells at the second division after irradiation in G1 or G2

Cultures of JU-56 cells were irradiated in either G1 or G2 and examined either in their first post-irradiation metaphase (in diploids) or in their second post-irradiation metaphase (in colcemid-induced tetraploids). The timing of fixation, together with tritiated thymidine pulse labelling, allowed selection for scoring only metaphases of cells that were in the G1 or G2 phase of the cell cycle during irradiation. With G1 irradiation it was found that many of the aberrations observed at the first division were not present (as derivatives) at the second division, and also that new aberrations were found at the second division, which were not derived from aberrations at the first division. Clonogenic survival was also measured in populations of cells irradiated in G1. It was found that cells containing chromosomal aberrations at the first division were not numerous enough to explain lack of survival. When frequencies of aberrations following G2 irradiation scored at the second division were compared with those scored at the first, there was a significant increase in dicentrics as compared with their progenitor asymmetrical chromatid interchanges, and of mirror-image dicentrics as compared with their progenitor sister unions. A substantial number of sister unions were also observed at the second division. We conclude that some aberrations are lost during the interphase between the first and the second post-irradiation metaphase and that new chromosomal aberrations arise during the second post-irradiation interphase.