Biophysical Journal 6: 247-260 (1966)
© 1966 the Biophysical Society

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Cell Cycle Kinetics and Radiation-Induced Chromosomal Aberrations Studied with C¹⁴ and H³ Labels

ABSTRACT

Chinese hamster cells in vitro were double labeled with C¹⁴TdR and H³TdR. At the time of irradiation with Co⁶⁰ gamma rays (600 rad), the cells in the G₂ phase were labeled only with C¹⁴, whereas cells in the late and middle S phases were labeled with both C¹⁴ and H³. The cells in early S phase were labeled only with H³ and the G₁ cells were unlabeled. Samples were fixed at various time intervals following irradiation and the metaphases were analyzed for chromosomal damage. The phase in which the cell was located at the time of irradiation was determined by counting grains in the first and second layers of autoradiographic film. In both control and irradiated cells some G₁ cells divided prior to some of the cells which were in the S phase denoting mixing of the populations. The G₂ phase sustained three times more chromosomal damage than the S phase. Little difference in chromosomal damage was found between the G₁ and S phases or among the different parts of the S phase. Cells in G₂ sustained a mitotic delay of 4 hr, while the other phases sustained a delay of 2 to 3 hr. Chromatid and chromosome (dicentrics) exchanges were induced in G₁ cells but only chromatid exchanges were induced in S and G₂ cells; this is consistent with the hypothesis that the chromosome consists of two subunits which separate either slightly before or immediately as the cell enters the S phase.