CHROMOSOME BREAKAGE BY DIEPOXIDE AND BY X-RAYS 



Roots are induced by appropriate pruning and grown in culture at controlled 

 temperature (22° C.) and illumination (16hr/day) in aerated culture vessels. 



Treatment with diepoxide solution is carried out at the controlled tempera- 

 ture and for the last 5 hours before fixation the roots are grown in 0-05 per 

 cent colchicine, also at the controlled temperature. Treatment with X-rays 

 is carried out as nearly as possible at the temperature of culture. 



This material does not respond to the usual maceration methods used in 

 making Feulgen squash preparations after aqueous fixation. New methods 

 of maceration had to be worked out. The root tips are fixed in La Gour's 

 2 BD osmic fixative, bleached in hydrogen peroxide and placed in an 

 extract of Bacterium aroideae for 3 to 5 days, a method suggested by the work 

 of WooD^"*. This treatment greatly assists separation of the cells during 

 squashing and does not interfere with subsequent staining. The root tips 

 are stained by the Feulgen method, hydrolysis being carried on for 20 minutes 

 and staining for one hour. 



The chromosomes of Tradescantia paludosa are so well known that no 

 detailed description is necessary. There are six pairs of chromosomes in the 

 somatic cells all with approximately median centromeres, consequently the 

 individual chromosomes are not easy to distinguish from one another. One 

 pair has arms of approximately equal lengths and is fairly easily picked out. 

 In the other five pairs the length of the shorter arm is from about one-half 

 to four-fifths of that of the longer arm {Figure 2). There is no demonstrable 

 heterochromatin and no distinguishable nucleolar constriction. 



The material was given X-ray doses of 50r-100r (intensity 27r/min. 

 approx.) and fixed at various intervals from 6 hours to 4 days after treatment. 



Diepoxide treatment was given with an M/2,000 solution for 10, 20, 30 

 and 40 minutes. Material was fixed at intervals from 12 to 78 hours after 

 treatment. 



RESULTS 



X-rays— The effects of X-rays are similar to those shown in the pollen cells. 

 There is, as is to be expected, a complete overlap of the different effects 

 obtained by earlier or later treatment of the pollen grains. This is due to 

 the lack of any synchronization in the development of the cells and to the 

 varying rates of development. 



At 36 hours after 200 r (22° C.) most of the reunion is of the R' or SR 

 type but dicentrics and centric rings (R") and minutes (m) also appear in 

 considerable numbers together with Xg cells showing breaks and micro- 

 nuclei. Dicentrics and rings survive in Xg mitoses and at 3 days paired 

 cells showing equal-sized dicentrics or rings are quite frequent. 



Diepoxide— MicY a treatment of 20 minutes with M/2,000 diepoxide the 

 breakage is quantitatively approximately the same as that obtained by 

 Revell*'- ' with Vicia using the same dose {Table I and Figure 1). Reunion is 

 entirely between chromatids, either chromatid reunion (B') or sister reunion 

 (SR), at all the time intervals {Figure 2). No dicentrics or rings (R") like 

 those observed after X-ray treatment were observed even at the longest time 

 interval of 60 hours. 



Approximately half of the changes are simple chromosome breaks, the 

 majority of which show sister reunion in both centric and acentric fragments. 



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