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CHAPTER 21 



tions produce clusters of ion pairs within 

 submicroscopic distances. In other words, 

 no amount or kind of high-energy radiation is 

 known at present which produces only single 

 ions, or single pairs of ions, evenly spaced 

 over microscopic (hence relatively large) 

 distances. Since we cannot obtain one ion 

 or a pair sufficiently separated from the next, 

 the genetic effects of ionization must be de- 

 termined from the activity of clusters of ions. 

 Ions undergo chemical reactions to neutral- 

 ize their charge, and, in doing so, clusters of 

 them are capable of producing breaks in 

 chromosomes. 



The amount of ionization produced by a 

 radiation can be measured in terms of an 

 ionization unit called the roentgen, or r unit, 

 which is equal to about 1.8 X lO'* ion pairs 

 per cubic centimeter of air. A sufficiently 

 penetrating radiation, which produces this 

 1.8 X 10'^ ion pairs in a cm^ of air, may also 

 produce this amount in successive cm'' of air 

 because only a very small part of the total 

 energy of the incident radiation is left at suc- 

 cessive depths. The amount of energy left at 

 any level depends upon the density of the 

 medium through which the radiation is pass- 

 ing. Thus, in tissue, which is approximately 

 ten times as dense as air, this penetrating 

 high-energy radiation would produce about 

 one thousand times the number of ion pairs 

 per cm^ So, it can be calculated that Ir 

 (always measured in air) produces about 

 1.5 ion pairs per cubic micron (/x^) of tissue. 

 Since Drosophila sperm heads are about 

 0.5/i^ Ir would produce, on the average, less 

 than one ion pair in each. But remember 

 that these ions occur in clusters, so that Ir 

 may place dozens of ion pairs in one sperm 

 head, and none of these in dozens of other 

 sperm heads. While the r unit only measures 

 energy left in the form of ionization, another 

 unit, the rad, measures the total amount of 

 energy of the radiation which is absorbed by 

 the medium. In the case of X rays, about 

 90% of the energy left is in the form of ioniza- 



tions, the rest as heat and excitations. Ultra- 

 violet radiation can be measured in rads, but 

 not r units, since it is nonionizing. 



It has been found, for X rays, that the 

 number of chromosome breaks produced in- 

 creases in direct proportion to the dose ex- 

 pressed in r (Figure 21-5). This means that 

 break number increases linearly with X-ray 

 dose. This also means that X rays produce 

 at least some ion clusters which are large 

 enough to cause a break, and that different 

 clusters of ions do not combine their effects 

 to do so. (Had there been such a cooperation 

 between clusters, the break rate at low doses 

 would have been lower than found, due to 

 the waste of smaller clusters having no others 

 with which to cooperate, while the rate at 

 higher doses would have been higher, due to 

 the cooperation among smaller clusters.) 

 Certain radiations, like fast neutrons, pro- 

 duce fewer breaks per r than do X rays. 

 This is explained by the fact that one r of 

 these radiations produces larger (and hence 

 fewer) clusters of ions than do X rays, these 

 larger clusters more often exceeding the size 

 needed to produce a break, and being, there- 

 fore, relatively less efficient in this respect. 



Ion clusters may produce breaks either 

 directly, by attacking the chromosome itself, 

 or indirectly, by changing oxygen-carrying 

 molecules, which in turn react with the chro- 

 mosomes, or by influencing chemicals which 

 affect oxygen-carrying molecules. In any 

 case, this indirect pathway must be of sub- 

 microscopic dimensions, otherwise there 

 would be cooperation among the chemical 

 effects of different ion clusters to cause 

 breakage. Thus, only ion clusters in or very 

 close to the chromosome can produce breaks 

 in it. This has been visibly demonstrated by 

 the use of beams of radiation of microscopic 

 dimensions. Such a beam passing through a 

 metaphase chromosome breaks it, but fails 

 to do so when directed in the plasm just adja- 

 cent to the chromosome. 



From what has been stated in the previous 



