CHROMOSOME ABERRATIONS IN ANIMALS 



659 



tion is abandoned, the concept itself of a hit acquires a macroscopic sig- 

 nificance, while the possibility of establishing a direct atomic physical 

 link between the hit and its biologic consequence becomes more remote." 

 (Fano, Caspari, and Demerec, 1950.) 



3-lb. The Sphere of Action of an Ionizing Particle. Development of 

 the interpretation that a single ionizing particle, rather than a single ion- 

 ization, produces a chromosome break has modified the original concept 

 that methods using ionizing radiations are adequate to determine the 

 number of component strands of a chromosome at the time of treatment. 

 On the assumption that a single ionization would suffice to induce a 

 break, it was inferred that irradiation of the divided, or double, chromo- 

 some would produce breaks independently in the two chromatids (chro- 

 matid breaks), whereas irradiation of the undivided, or single, chromo- 

 some would produce breaks that would be transmitted in the subsequent 

 division equally to the daughter chromatids (chromosome breaks) . The 

 experimental results indicated in a general way that prior to a certain 

 stage in mitosis the chromosome reacts as a unit to the ionizing radiations 

 (as shown in Fig. 9-3, column 1) and that subsequently each of the two 

 chromatids is affected independently (Fig. 9-4, column 1). As an 

 example, Carlson (1941b) presented the data shown in Table 9-2, which 



Table 9-2. Percentage Neuroblasts of the Grasshopper, Chortophaga, Reveal- 

 ing Chromosome Aberrations at Stated Time Intervals after Treatment with 



125 r of X Rays 



(Carlson, 1941b) 



Hours after X irradiation 



Chromatid translocations 



Chromatid + chromosome translocations. 



Chromosome translocations 



Chromosome fragments 



12 



12 

 

 



38 



indicate the frequency of interchanges involving chromosome and 

 chromatid breaks in neuroblast cells of the grasshopper Chortophaga 

 viridifasciata. In cells irradiated 72 to 96 hours before they reached meta- 

 phase, at which stage they were examined cytologically, only chromosome 

 breaks were produced; and in those irradiated 12 hours before examina- 

 tion only chromatid breaks appeared ; but in the intervening period both 

 types of breaks were produced. In some cases chromosome and chro- 

 matid breaks were detected in the same chromosome. The underlying 

 structural basis for such response has been examined in detail by several 

 investigators (reviewed by Kaufmann, 1948b). There is convincing 

 evidence from studies on Tradescantia, in which cytological examination 

 sometimes permits determination of the number of strands in the chromo- 

 somes at the stage of mitosis during which they are irradiated, that two 

 chromatids may be severed conjointly by a single ionizing particle to 



