II. The Effect of Ionizing Radiation on Chromosomes

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Abstract
Cytological studies of chromosome breakage induced by ionizing radiation at various stages of nuclear development have provided evidence regarding the time of chromosome duplication and the nature of chromosome breakage and reunion. The frequency of chromosomal aberrations induced by a given dose of X-rays is dependent upon the nuclear stage irradiated and can be modified by varying the temperature of the cells during irradiation, by varying the dosage intensity, by fractionated dosage, by centrifugation of the cells during irradiation, by irradiating the cells with infrared before or after X-irradiation, by anoxia, by irradiating the cells in atmospheres of CO or CO2, and by treating the cells with various chemical agents. Neutrons and a-radiation are more effective than x-rays in producing chromosomal aberrations because of the higher density of the ionization track. They produce the same types of chromosomal aberrations as do X- or a-radiation, but the dosage relationships are different. The modifying mechanisms or agencies which increase or decrease the frequency of chromosomal aberrations generally, although not invariably, also modify radiation injury to cells and organisms. Agents which augment radiation sensitivity, such as CO2, may be applicable to radiation therapy.