Chinese hamster ovary cells, arrested in late S or early G1 phase, were exposed to various doses of membrane-associated125 I (administered as125 I-labeled, monovalent concanavalin A) and DNA-associated125 I (administered as <tex-math>${}^{125}{\rm I}\text{-iododeoxyuridine}$</tex-math>. Cell division was evaluated by monitoring the formation of duplet cells in monolayer. On the basis of radionuclide incorporation data, cellular dimensions, and subcellular radionuclide distributions, the cumulative dose to various subcellular regions was calculated from the known decay properties of125 I. In terms of125 I decays/cell, plasma membrane-associated125 I was very inefficient in producing division delay (0.01 min/decay), whereas DNA-associated125 I was extremely effective (1.45 min/decay). In terms of radiation dose to the cell nucleus, membrane-associated125 I caused exactly the same division delay as external X irradiation (0.45 min/rad), but nuclear125 I remained much more toxic (1.80 min/rad). These findings indicate that radiation-induced division delay results primarily from damage to the cell nucleus rather than from cytoplasmic or membrane damage. An analysis of the data suggests that the ultimate target organelle is not the nucleus as a whole, but a much smaller substructure within the cell nucleus. This substructure appears to be identical or closely related to the structure responsible for radiation-induced cell death.

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