Effect of Irradiation on DNA Synthetic Period of the Mitotic Cycle in Cells

Kinetics of DNA synthesis in mitotic cycle of mouse corneal epithelial cells after single γ-irradiation (4 Gy) at the end of S period was studied by the method of radioautography. Normally, S period of corneal epithelial cells consists of several stages separated by intervals without DNA synthesis. The estimated mean duration of the first (S 1 ) and second (S 2 ) phases of S period was 16 and 10 h, respectively, and the interval between them was 7 h. Single irradiation at the end of S period changed the duration of mitotic cycle periods: S 2 phase became 2.2-longer than S 1 phase and the duration of g 1 period decreased, because the time for reparation in irradiated cell increases at the expense of g 1 period. Shortening of g 1 period is a factor promoting the appearance of transformed cells.