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CHAPTER 13 



kind of high-energy radiation presently 

 known can produce only single ions, or 

 single pairs of ions evenly spaced over 

 microscopic (hence, relatively large) dis- 

 tances. Since one ion or a pair sufficiently 

 separated from the next does not exist, the 

 genetic effects of ionization must be deter- 

 mined from the activity of clusters of nega- 

 tively and positively charged ions Ions 

 undergo chemical reactions to neutralize 

 their charge to reach a more stable con- 

 figuration. It is during this process that ion 

 clusters are able to produce chromosome 

 and chromatid breaks (Figure 13-1). 



The amount of ionization produced by 

 radiation is measured in terms of an ioniza- 

 tion unit called the roentgen, or /• unit, one 

 r being equal to about 1.8 X 10° ion pairs 

 per cubic centimeter of air. A sufficiently 



figure 13—1. Structural changes X-ray-induced (75-150 r) in normal human male fibro- 

 blast-like cells in vitro. Arrows show: (A) broken chromosomes, (B) translocation {cen- 

 ter) and dicentric (lower left), (C) ring chromosomes. A, B are in metaphase (see Fig. 

 11-9); C is late prophase. (Courtesy of T. T. Puck, Proc. Nat. Acad. 5c/., U.S., 44: 

 776-778, 1958.) 



