(a) The frequency of chromosome changes which can be considered 

 as "one hit" events is directly proportional to dose. 



(b) The capacity of broken ends of chromosomes to rejoin is retained 

 for some time, depending on the material irradiated. In Tradescantia 

 microspores it appears to be about one hour. 



(c) The yield of "one hit" events is, within limits, independent of 

 intensity. 



Unit of Breakage Diagrom 



Unit of Rejoin Conf igurotion 



Chromosome 



-• k- 



Chromosome 



Chromosome 



Chromatid 



DC 



Chromatid 



Pairs of 

 half-chromotid 



ZXZl 



Chromatid 



Holf-chromatid 



Figure 9-1. Schematic Drawing Showing Units of Breakage and Rejoining 

 Following Irradiation. 



(d) As a general rule irradiation of nuclei in interphase produces 

 chromosome breaks. Irradiation of early prophase results in chromatid 

 breaks, and of late prophase in rearrangements indicating the 4-stranded 

 condition of the chromosomes (Wilson and Sparrow, 1960; Wilson, 

 et al., 1959). These conditions are illustrated in Figures 9-1, 9-2, 9-3. 



(e) Although breaks usually seem to be randomly distributed along 

 chromosomes, there is some evidence of higher sensitivity in hetero- 

 chromatic regions. 



196 / CHAPTER 9 



