THE PRODUCTION OF CHROMOSOME BREAKAGE BY IONIZING RADIATIONS 



demonstrated by Thoday and Read' (1949) for alpha rays and by Giles, 

 Beatty and Riley« (1952) for neutrons. It must therefore be considered 

 that restitution as defined by Schwartz ^^ (1952) cannot be the process 

 principally affected by oxygen tension in Tradescantia. 



Figure 1 is an operational scheme adopted, in modified form, from 

 Thoday12 (1953), and applicable only to the data obtained from the study 

 of chromatid aberrations in Tradescantia. In agreement with Gray^ (1953) 

 that a variety of initial events can take place in the chromosome as the result 

 of exposure to ionizing radiations, it is suggested that both potential (latent) 



PHASE I 



IRRADIATION 



r-». STRUCTURALLY INTACT CHROMOSOME 



POTENTIAL BREAKS 



} 



PRIMARY BREAKS 



Og INDEPENDENT- 

 RELATIVE FREQUENCIES OF 

 POTENTIALS TO PRIMARIES 

 DEPENDENT ON ION DENSITY 



REPAIR, 



FAVORED BY 



ANOXIA 



PHASED 



PHASE m 



SEPARATION OF 

 BROKEN ENDS, 

 FAVORED BY O2 



NOT AFFECTED 

 BY 02 



PRIMARY BREAKS 



V 



FAILURE OF BROKEN 



ENDS TO ADHERE, 



(HEALING?) 



OBSERVED BREAKS 

 (CHROMATID DELETIONS) 



r 



OLD WAY 



i 

 RESTITUTION 



(INTACT 



CHROMO- 

 SOME) 



ADHESION OF 

 BROKEN ENDS 



1 



NEW WAY 



\ 

 RECOMBINATION 



(ISOCHROMATIDS IF 

 SISTER REUNION, 

 EXCHANGES IF NON- 

 SISTER REUNION) 



FATE OF POTENTIALS 

 DEPENDENT ON O2 TENSION 



O2 INDEPENDENT 



Figure 1. A schematic diagram representing in brief the major events taking place in the chromosomes of 

 Tradescantia during and after radiation, with suggestions as to the most likely manner by which 

 oxygen affects the frequency of aberrations. 



and primary breaks are formed, and that in Phase I their formation is inde- 

 pendent of oxygen tension. Valid objections to the terminology employed 

 may be raised, but these are terms of convenience and may perhaps best be 

 considered as the termini of a continuous spectrum of damage inflicted in 

 the chromosome, the primary breaks being sites of irretrievable damage 

 and of subsequent rupture, and the potential breaks, lesions or sites of lesser 

 damage capable of repair or of complete disruption, as the cellular circum- 

 stances might determine. Furthermore, it is proposed that the relative 

 frequencies of the two types of breaks would be a function of ion density, 

 for it is inconceivable that radiations of such quantitatively diflferent ioniza- 

 tion characteristics as gamma and alpha rays could produce the same 



258 



