X-Ray-Induced Incorporation of Tritiated Thymidine into Deoxyribonucleic Acid of Grasshopper Neuroblast Chromosomes

Irradiated Chortophaga neuroblasts at identifiable stages of the cell cycle in the living condition were observed. Changes in chromosonre morphology which are characteristic of mitotic stoppage or delay were detected within 15 minutes after 32 r of x rays. Four mitotic responses of irradiated (32 r) prophase neuroblasts can be distinguished: nondelay, delay, stoppage, and reversion. The greatest number of types of response is seen in cells which were irradiated in late prophase, whereas the least variety of responses occurs after treatment of very early prophases. Preparations given 250 r of X-rays show reversion of prophase cells to an interphase-like condition. The observation of living neuroblasts exposed to tritiated thymidine (H/sup 3/T) and then irradiated in hanging-drop preparations and reidentification of the same cells in autoradiograms of sectioned material show that: prophase neuroblasts which are delayed, stopped, or reverted mitotically in response to 32 r of X-rays incorporate H/sup 3/T, incorporation is greater in cells irradiated in early prophase and less in cells treated in middle or late prophase, and no incorporation is seen in cells treated in very late prophase, incorporation is not seen in preparations treated with DNase before application of autoradiographic emulsion, labeling can be detected withinmore » 15 minutes after x irradiation of early or middle prophase neuroblasts, and after 250 r of x rays, prophase cells which do not divide within about 40 minutes do incorporate H/sup 3/ T during reversion to an interphase-like condition. ( auth)« less