Light-dark (L-D) synchronized Chlamydomonas grow during a 12-hr light period and divide by a series of mitoses into 4 or 8 daughter cells during the early part of the following 12-hr dark period. Sensitivity to the lethal effects of 9108 R X-irradiation varies throughout the L-D cycle: Mortality rises from 20% at the 1st hour to 40% at the 9th hour, to 70% at the onset of the dark; it reaches a peak of about 85% at about the 14th hour, just before the first cytokinesis, and then returns to a level of about 45% when cell division has been completed (after data correction for multiplicity of targets per colony-forming unit). Most lethally affected cells complete at least one set of divisions (into 4 or 8 daughter cells) before they die; however, exposure shortly before the first nuclear division results in two sets of divisions before death, suggesting that these cells were committed in some way at the time of irradiation to divide again 24 hr later. Some single cells exposed prior to cytokinesis exhibit mixed-colony formation: About half of their progeny die and half survive, indicating that prior to cytokinesis there are perhaps two radiation-sensitive "targets" per cell.

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