Section 5 — Mutagenesis 



FUdR on chromosome structure is a result of 

 its inhibitory effect on the synthesis of thymidylic 

 acid. The experiments have shown that whereas 

 FUdR-induced thymidine deficiency produces 

 fragmentation of chromosomes, the incorpo- 

 ration of CUdR, BUdR and IUdR into chromo- 

 somal DNA does not itself cause chromosomal 

 aberrations, but makes the chromosomes more 

 sensitive to the chromosome-breaking effect of 

 X-rays. 



5.84. Ultraviolet Action Spectra for Chromosome 

 Aberrations of Thymidine Analogue-substituted 

 Mammalian Cells in Vitro. Ernest H. Y. Chu 

 (Oak Ridge, U.S.A.). 



A clonal quasidiploid line of Chinese hamster 

 cells grown directly in quartz dishes were 

 irradiated with monochromatic ultraviolet (u.v.) 

 light. Previous studies (Chu, 1962) had establish- 

 ed the average cell generation time and dose- 

 effect relationships. The u.v. -induced chromo- 

 some aberrations were qualitatively indis- 

 tinguishable from those induced by X-rays in 

 both plant and animal materials. The u.v. 

 action spectrum for chromosome aberrations 

 reaches a peak frequency at 2652 A and approxi- 

 mately parallels the u.v. absorption spectrum of 

 nucleic acids. 



For varying periods prior to irradiation, 

 different concentrations of 5-bromodeoxyuridine 

 (BUdR) or 5-iododeoxyuridine (IUdR) were 

 included in the growth medium. A few hours' 

 treatment with BUdR did not induce chromo- 

 some abnormalities. Prolonged exposures (24 and 

 48 hr) of cells to 10 5 m BUdR did not inhibit 

 mitosis and induced only infrequent chromosome 

 aberrations, although higher concentrations 

 caused increasingly severe damages to the cell. 



Cells pretreated with BUdR for 24 hr showed 

 an increased sensitivity to u.v. in terms of 

 induced chromosome aberrations. The most 

 significant modification of the u.v. action spec- 

 trum was observed in cells which had been 

 exposed to BUdR for 48 hr. Particularly note- 

 worthy is some thirty-fold and six-fold increase 

 of aberration rates at 2250 A and 3130 A, 

 respectively. IUdR-substituted cells also had a 

 modified action spectrum, but IUdR- or BUdR- 

 substituted cells responded differently to u.v. 

 at various wavelenghts. 



The facts that the u.v. action spectrum is 

 similar to nucleic acid absorption spectrum and 

 that direct chemical alterations of the DNA 

 molecules modify the u.v. radiosensitivity 

 strongly suggest that the initial DNA damages 



probably account for the majority of u.v.- 

 induced chromosome breakages. 



5.85. Actions of 5-bromouracil deoxyriboside on 

 Plant Chromosomes. F. K. S. Koo (Mayagiiez, 

 Puerto Rico). 



The thymidine analogue 5-bromouracil de- 

 oxyriboside (BUdR) upon its incorporation into 

 cellular DNA is known to increase the radio- 

 sensitivity of the cells. In the present experiments 

 with chromosomes in root tip cells of Zebrina 

 pendula, Rhoeo discolor, and Allium capa, the 

 following additional aspects of BUdR effect 

 have been observed in some or all of these 

 species: (1) BUdR often prevents the contraction 

 of the secondary constriction, giving an ap- 

 pearance of a stretched region between the 

 chromosome arm and the satellite. (2) BUdR 

 induces breakage more readily at the centromere 

 than at other regions of the chromosome. Since 

 the constituent of the centromere is presumably 

 not the same as that of the genetic material along 

 the chromosome arms, so the mechanism for 

 breakage induction could be also different. 

 (3) BUdR is capable of inducing breakages 

 either in one of the sister chromatids or in both. 

 Based on the semi-conservative hypothesis for 

 DNA replication and chromosome duplication, 

 each chromosome is supposedly composed of one 

 old and one new chromatids and BUdR is 

 expected to be incorporated into the newly 

 formed chromatid. If the breakages represent 

 the manifestation of weakness at the sites of 

 BUdR incorporation in the chromatid, they 

 should occur only in one of the sister chromatids 

 in a given chromosome. The detection of 

 breakages in both chromatids indicates that 

 BUdR is also capable of disrupting chromatids 

 by other means. (4) In the combined treatment 

 with gamma-rays, BUdR interacts with radi- 

 ation to produce a synergistic effect in the 

 induction of chromosomal aberrations. 



5.86. Experiments Utilizing Labeled Nucleic Acid 

 Precursors and Analogues in Plant Cell 

 Research. Harold H. Smith (Upton, N.Y., 

 U.S.A.). 



Investigations on the uptake and incorporation 

 of DNA precursors and analogues are being 

 made to study certain cytological, genetic and 

 morphological phenomena. Incorporation of 

 5-iododeoxyuridine (IUdR) and 2-aminopurine 

 into chromosomes of Vicia faba was demon- 



85 



