Synchronously growing sporelings of Oedogonium cardiacum treated with colcemide (500 μg/ml) form giant cells in their progeny which are morphologically similar to those induced by x- or uv-irradiation. The yield of giant cells in the progeny increases as the time of exposure to colcemide is lengthened and a maximum yield is observed for cells treated up to 20 hr. Cycloheximide given during the colcemide exposure prevents giant cell formation and concomitantly increases cell survival. The yield of giant cells from x- or uv-irradiated cells is not changed by postirradiation treatment with FUdR. On the other hand, the yield of giant cells is decreased by actinomycin D treatment after exposure of G2 cells to high doses of radiation, but cell survival is not altered appreciably. In contrast, the treatment of <tex-math>${\rm G}_{2}\text{-irradiated}$</tex-math> cells with cycloheximide prevents giant cell formation and consequently increases cell survival. These results imply that the radiation-induced lethal damage in G2 cells as manifested by giant cell formation is similar to the giant cells induced by long exposure to colcemide and may, primarily have been caused by the synthesis of incomplete or faulty proteins.

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