DNA Synthesis and Changes in Radiosensitivity during the Cell Cycle of Vegetative Culture of Chlamydomonas Reinhardii

Summary

Vegetatively growing Chlamydomonas reinhardii cells, synchronized by the 12-hour light-12-hour dark cycle, synthesized their DNA between the fourteenth and twentieth hours of the 24-hour cell cycle. Synthesis of the DNA preceded the nuclear division and the maximum of the DNA per cell was reached in the seventeenth hour of the cell cycle. The amount of DNA per cell was 2·6 × 10⁻⁷ μg. The radioresistance throughout the cell cycle and its relation to DNA synthesis were studied. A 35 per cent increase of D₀ values of the survival curves following x-irradiation was found during the DNA synthesis. Simultaneously, the extrapolation numbers n changed in indirect proportion to the D₀ values. The shoulder of the survival curve nearly ceased in the seventeenth hour of the cell cycle; however, after the daughter cells had been released from the mother cell, the shoulder was fully restored. The possible role of the repair process and the changes of the target itself in the increase of the radioresistance are discussed.