Mitotic Effects of Monochromatic Ultraviolet Radiation at 225, 265, and 280 nm on Eleven Stages of the Cell Cycle of the Grasshopper Neuroblast in Culture: 1. Overall Retardation from the Stage Irradiated to Nuclear Membrane Breakdown

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Abstract
Neuroblasts of Chortophaga viridifasciata (DeGeer) in culture were exposed to different doses of 225, 265 or 280 nm UV radiations at 11 different stages and substages of the mitotic cycle and individually selected cells were timed to breakdown of the nuclear membrane. Comparisons of the effectiveness of different wavelengths on the different stages were based on the dose that reduced the cell progression rate to 67% of normal (D67) and the slope of the regression line, i.e., the control to treated time (C/T) ratio change/erg per mm2 at the D67 level. Cells of the prereplication period (metaphase + anaphase + early telophase) and the S phase (middle and late telophase + interphase + very early prophase) were equally sensitive to UV and contrasted sharply with the much lower sensitivity of those in the postreplication period (early and middle prophase). Chromosomal DNA was probably the chromophore for UV-induced mitotic retardation. Cells in the prereplication period at exposure showed no wavelength effect. In the S phase all stages, except middle telophase and all stages combined, were significantly more sensitive to 265 and 280 nm than to 225 nm. Of the postreplication stages, early prophase was retarded significantly more by 280 than by 225 or 265 nm. The C/T ratio/erg per mm2 exhibited no consistent wavelength pattern during the postreplication period. Evidence based on the orientation of the neuroblast with respect to the UV source suggested that the chromophore for mitotic retardation did not reside within the centrosome and related structures, but was probably present, at least partly, in the nucleolus.