Neuroblasts of Chortophaga viridifasciata (DeGeer) in culture were exposed to different doses of 225, 265, or 280 nm ultraviolet 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 (<tex-math>${\rm D}_{67}$</tex-math>) and the slope of the regression line, i.e., the control to treated time (C/T) ratio <tex-math>$\text{change}/{\rm erg}/{\rm mm}^{2}$</tex-math> at the <tex-math>${\rm D}_{67}$</tex-math> level. Cells of the prereplication period (metaphase + anaphase + early telophase) and the S phase (middle and late telophase + interphase + very early prophase) are equally sensitive to uv and contrast sharply with the much lower sensitivity of those in the postreplication period (early and middle prophase). This can best be interpreted if chromosomal DNA is the chromophore for uv-induced mitotic retardation. Cells in the prereplication period at exposure show no wavelength effect. In the S phase all stages except middle telophase and all stages combined are significantly more sensitive to 265 and 280 nm than to 225 nm. Of the postreplication stages, early prophase is retarded significantly more by 280 than by 225 or 265 nm. The C/T <tex-math>$\text{ratio}/{\rm erg}/{\rm mm}^{2}$</tex-math> is greater after exposure to 265 nm at all prereplication and replication stages, but exhibits no consistent wavelength pattern during the postreplication period. Evidence based on the orientation of the neuroblast with respect to the uv source suggests that the chromophore for mitotic retardation does not reside within the centrosome and related structures, but may be present, at least partly, in the nucleolus.

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