Chinese hamster cells treated with such chemical mutagens as 4-nitroquinoline 1-oxide, captan, acridine dyes, and Cytoxan developed diplochromosomes, showing the induction of endoreduplication. This indicates that these mutagens not only cause damage to genes but also cause changes in ploidy. In this study, the maximum ratio of endoreduplicated cells, 6.6%, was obtained when cells were treated with Cytoxan, 5 mg/ml for 6 hr. Ethyl methanesulfonate, triethylenephosphoramide, and N-methyl-N′-nitro-N-nitrosoguanidine, all potent mutagens, showed little or no activity at the doses used. There seems to be no direct relationship between the induction of endoreduplication and mutagenic activity.

Further analyses were conducted using 4-nitroquinoline 1-oxide. Treatment with 4-nitroquinoline 1-oxide, 1.5 µg/ml, 7.9 × 10-6m, for 1 hr induced endoreduplication in 4.3% of cells. Cells synchronized by the double thymidine procedure gave the maximum ratio of endoreduplication when they were treated just prior to M phase. Autoradiography experiments indicated that endoreduplication was, after the first S phase (S1), mainly induced in G2 phase and then, omitting cell division unless otherwise specified, proceeded to the second S phase (S2). The S2 phase took a particularly long period, comparable to two S periods of an ordinary cell cycle.

Various chromosomal aberrations observed in diplochromosomes are presented, one of which shows that a symmetrical dicentric chromosome can be formed from an “arch” structure.

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This work was supported by research funds from the Nomura Research Institute.

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