Changes in both the DNA and histone content of the nuclei isolated from rat thymus were followed for up to 24 hr after lethal whole-body irradiation of the animals. The number of nuclei or nuclear masses remained quite constant after the irradiation although both the DNA and histone content fell to approximately 20% of that found in the nonirradiated controls. Electron photomicrographs of the nuclei and nuclear remains are presented. The ratio of the DNA to histone remained constant in the nucleoprotein during the course of this study. From gel electrophoresis analyses, it appears that there is no significant selectivity in the manner or sequence in which the histone fractions are lost from the nuclei. The loss of both DNA and histone from the thymus seems to result from a gradual and nonspecific degradation of the individual nuclei influenced by the irradiation.

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