Normal human embryonic lung fibroblasts were grown in the presence of different concentrations of tritium-labeled thymidine ([3 H] dThd) for various periods of time, and the flow of cells from metaphase to anaphase was followed. Mitotic indices were evaluated as a function of the percentage of cells in metaphase and anaphase. They revealed that the presence of [3 H] dThd induces a disturbance of the metaphase-anaphase transition. The DNA content of cells in metaphase and anaphase was measured with cytophotometry after the cells were grown in the presence of the radioactive precursor. It was found that the latter interfered with the flow of cells from metaphase to anaphase, impairing the symmetric distribution of DNA between daughter cells.

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