Comparative Analysis of the Kinetics of DNA Synthesis after Exposure during Different Phases of the Cell Cycle S Period

Abstract

The kinetics of DNA synthesis in the mitotic cycle of mouse corneal epithelial cells was studied after a single γ-irradiation of cells in a dose of 4 Gy at different S-phase points. Normally, corneal epitheliocyte S phase consists of S1 and S2 phases separated by an interval during which no DNA is synthesized. The duration of each phase was lengthened after single irradiation due to reparation of injuries in the cells at the expense of the time normally occupied by g1 period of the mitotic cycle. The first event during reparation is excision of damaged complex from the DNA molecule; this complex consists of labeled daughter fragment and matrix site of DNA chain that was used for the synthesis of the daughter fragment. Presumably, the entire reparation process in the cell consists of two stages: "reparative" synthesis and "additional" synthesis. The reparative synthesis, in turn, includes two stages: de novo synthesis of matrix fragment in the DNA chain at the site of the gap formation and de novo synthesis of the daughter fragment after the synthesis of the new matrix fragment is over.

Dynamics of DNA Synthesis Disturbance and Recovery after Fractionated Irradiation of S Phase Cells

“…the interval between 38 and 7 h after the isotope instillation) decreased to 22 h after double exposure (interval between 28 and 6 h). According our previous data [5], the total duration of S period decreased after irradiation at the expense of disappearance of DNA synthesis break between S 1 and S 2 phases during S period of the mitotic cycle 1. The S period was shorter because of some reparation stages unfolding in the exposed cell fraction during this period.…”

“…where t m was the duration of mitosis, which could vary in the mouse corneal epitheliocytes from 1.1 to 4.1 h throughout 24 h [5]. The duration of g 1 period under these conditions was 55-58 h. Experimental data suggest that normally S period in the mouse corneal epitheliocytes consisted of several phases divided by periods, during which DNA was not produced.…”

“…The fact that the percentage of cells with one silver grain was extremely low or minimum during the early periods of fi xation after irradiation indicated that most likely, the entire complex including the damaged labeled daughter fragment of the DNA matrix chain was removed from the DNA molecule during this period [5]. Excision of the complex took about 3 h in S 2 phase and about 4 h during S 1 phase.…”

“…The mean time of stage 1 of damage reparation (excision of damaged fragments) was therefore no longer than 4 h. On the other hand, the damage reparation process in the cell was not over at the stage of fragment excision. Complete recovery of the cell consisted in de novo synthesis of the matrix fragment in the DNA chain to replace the fragment excised by the reparation enzymes and de novo synthesis of the daughter fragment after the synthesis of the new matrix fragment was over [5,6].…”

“…Importantly, DNA replicated at the beginning of the S phase is enriched with G-C pairs, while DNA synthesized at the end of S period is characterized by high content of A-T pairs; this suggests different radiosensitivity of the cells during different time of the S period. Exposure to a single γ-irradiation in a dose of 4 Gy during different time of the S period prolongs each of these phases, because reparation processes take the time normally occupied by g 1 period [5,6]. This fact indicates common mechanism of reparation after injuries -during the g 1 period -after a single exposure during different phases of the S period.…”