15 



Recent Evidence on the Mechanism 



of Chromosome Aberration Production 



by Ionizing Radiations * 



NORMAN H. GILES, JR. 



Biology Division 



Oak Ridge National Laboratory 



Oak Ridge, Tennessee 



Understanding the mechanism by which ionizing radiations produce 

 chromosome aberrations is one of the fundamental problems in radiation 

 genetics. The present discussion will deal with recent evidence on this 

 subject, obtained for the most part from experiments with plant chro- 

 mosomes, especially those of the spiderwort, Tradescantia. At the out- 

 set it appears desirable to review briefly the interpretation of aberration 

 production in Tradescantia as proposed originally by Sax (24, 25) and 

 developed in quantitative form primarily by Lea (20). After this, more 

 recent results, stemming principally from the observations by Thoday and 

 Read (28, 29) on the effect of oxygen on aberration frequency, will be 

 presented. Finally the implications of the oxygen effect for an interpre- 

 tation of the biochemical mechanism of radiation-induced chromosomal 

 changes will be considered, in particular the extent to which this effect 

 requires a revision of previous views as to the mechanism involved. 



The Production of Chromosomal Aberrations in Tradescantia 

 BY Ionizing Radiations 



The general experimental technique for observing the effects of ion- 

 izing radiations on Tradescantia chromosomes as developed by Sax (24) 

 has been described in detail previously by Sax (25), Catcheside, Lea, and 

 Thoday (6, 7), and Lea (20) and will be considered here only briefly. 

 The typical procedure is to expose entire inflorescences, consisting of 

 several buds containing microspores in various stages of development, 

 to penetrating radiations such as x-rays or fast neutrons. Cytological 



* This work was done under Contract W-7405-Eng-26 for the Atomic Energy 

 Commission, Oak Ridge, Tennessee. 



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