Section 5 — Mutagenesis 



preted to be an effect by FUdR on DNA synthe- 

 sis. 



A series of experiments was undertaken in 

 which roots were treated 15 min with H 3 - 

 thymidine to label cells in S and then treated 

 with a mixture of 10~ 5 m FUdR and 10~ 4 m 

 uridine. Chromosome gaps and shattering were 

 found in those cells treated 2 to 4 hr. FUdR 

 markedly inhibited the mitotic index within 4 hr. 

 These results were similar to Taylor's. Autoradio- 

 grams of these cells, however, showed that the 

 broken chromosomes were not labeled and 

 thus must have been broken during the stage 

 following synthesis. Thus, controsy to what was 

 postulated previously, FUdR appears to break 

 chromosomes of Vicia independently of DNA 

 synthesis. 



When lateral roots were treated with FUdR 

 and uridine preceding treatment in H 3 -thymidine, 

 autoradiograms showed labeled nuclei. The 

 number of grains over the nuclei was not 

 significantly different from that in untreated 

 cells. In other experiments in which roots were 

 grown in FUdR, uridine, and 10 5 m thymidine, 

 there was almost as much chromosome breakage 

 as in the roots grown in FUdR and uridine only. 

 At concentrations of thymidine (10 3 m) 100 

 times greater than the concentration of FUdR, 

 the level of chromosome damage was only 

 slightly above the level of the control cells grown 

 in the absence of FUdR. 



1. S. S. Cohen et al., Proc. Natl. Acad. Sci., 

 U.S., 44, 1004-12, 1958. 



2. Proc. Natl. Acad. Sci., U.S., 48, 190-198, 

 1962. 



5.82. Cytological and Biochemical Analyses of 

 Chemically-induced Chromosome Breakage. 



Norman S. Cohn (Athens, U.S.A.). 



The roots of Vicia faba. Allium cepa, and 

 Pisum sativum have been treated with the 

 following agents: 5-fluorodeoxyuridine, 5-bro- 

 modeoxyurdidine, hydroxylamine, and certain 

 amino acids. Feulgen smears were prepared for 

 cytological analyses of chromosome breakage as 

 determined in metaphase and anaphase of 

 mitosis. In addition, chromatographic analyses 

 of certain of the treatments were performed using 

 specific methods of extraction for histone and 

 non-histone protein from treated and untreated 

 roots. It is our contention that breakage of 

 chromosomes must involve protein as well as 

 nucleic acid, and preliminary data tend to 

 support this. While biochemical analyses of all 



treatments are yet incomplete, observations on 

 Vicia and Pisum indicate quantitative as well as 

 qualitative differences in certain amino acids, 

 when comparing treated with untreated material. 

 Treatment with amino acids alone cast doubt 

 on their use as radiomimetic agents, contrary to 

 the work of Sharma and Sharma. Using relatively 

 short treatment times and various periods of 

 recovery, 5-FUdR and hydroxylamine produce 

 significant levels of chromosome breakage, but 

 5-BUdR has little effect. These results confirm 

 the studies of other workers (Kihlman; Taylor, 

 Haut, Tung; Somers and Hsu). In addition, these 

 studies have indicated that FUdR induces 

 breaks after DNA synthesis as well as during the 

 synthetic phase. While reunion of broken ends is 

 relatively low, significant reunion does occur. 

 This is supported by the presence of anaphase 

 bridges as well as exchanges and sister union of 

 Isochromatid deletions observed at metaphase. 



Supported in part by the U.S. Atomic Energy 

 Commission, contract AT (11-1) 826. 



5.83. The Effects of 5-fluorodeoxyuridine (FUdR), 

 5-chlorodeoxyuridine (CUdR), 5-bromodeoxy- 

 uridine (BUdR), and 5-iododeoxyuridine (IUdR) 

 on the Frequency of X-ray-induced Chromatid 

 Aberrations in the Root-tips of Vicia faba. 

 B. A. Kihlman (Uppsala, Sweden). 



Pretreatments with CUdR, BUdR, and IUdR at 

 concentration of 100u M increased the effect of 

 a given dose of X-rays by factors of 1.3, 1.6, and 

 1.9, respectively. 



IUdR increased the frequency of X-ray- 

 induced aberrations to the same extent whether 

 the roots were irradiated under anaerobic or 

 aerobic conditions. 



In contrast to the thymidine analogs CUdR, 

 BUdR, and IUdR, the deoxyuridine analog 

 FUdR produced chromosome damage in the 

 absence of radiation. 



When X-irradiation was combined with a 

 FUdR-treatment which by itself did not produce 

 any chromosome damage, the number of anapha- 

 se fragments was markedly increased and the num- 

 ber of bridges decreased, in comparison with the 

 X-ray control. The FUdR treatment was equally 

 effective when given after as when given before 

 and during irradiation. These results were 

 interpreted as an enhacnement of the chromo- 

 some-breaking effect of FUdR by X-rays. The 

 chromosome damage produced by FUdR was 

 reversed by thymidine and thymidine analogs; 

 this is consistent with the idea that the effect of 



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