Radiation-Induced Structural Chromosome Changes 



181 



penetrating radiation (such as fast elec- 

 trons), producing this 1.8 X 10 !) ion pairs 

 in a given cm :{ of air, can also produce this 

 amount in successive cm' 5 of air because only 

 a very small fraction of the incident radiation 

 is absorbed at successive depths. If not very 

 energetic X rays are used ("soft" X rays of 

 relatively long wavelength — also called Grenz 

 rays), all radiation may be absorbed near 

 the surface of the medium, keeping the 

 deeper regions free from ionization. The 

 amount of energy left at any level depends 

 not only upon the energy of the incident 

 radiation, but also upon the density of the 

 medium through which the radiation passes. 

 Thus, in tissue, which is approximately ten 

 times as dense as air, a penetrating high- 

 energy radiation produces about one thou- 

 sand times the number of ion pairs per cm 3 

 as it does in air. Knowing this, it can be 

 calculated that one r (always measured in 

 air) produces about 1.5 ion pairs per cubic 

 micron (/a 3 ) of tissue. Since the volume 

 of the Drosophila sperm head is about 0.5 

 ju 3 , one r is able to produce, on the average, 

 less than one ion pair in it. Since ions occur 

 in clusters, one r may place dozens of ion 

 pairs in one sperm head and none in dozens 

 of other sperm heads. The r unit measures 

 only the absorbed energy which produces 

 ions; another unit, the rad, measures the 

 total amount of radiant energy absorbed by 

 the medium. In the case of X rays, about 

 90% of the energy left in the tissue is used 

 to produce ions; the rest produces heat and 

 excitation. Since ultraviolet radiation is non- 

 ionizing, its dosage is measured in rads and 

 not r units. 



The number of chromosome breaks pro- 

 duced by X rays increases linearly with the 

 radiation dose (r) (Figure 13-2). This re- 

 lationship means that X rays always produce 

 at least some ion clusters large enough to 

 cause a break. Moreover, clusters of ions 

 from different tracks of ions do not combine 

 their effects to cause a break. (If there were 



co7 



or 

 0Q5 - 



t- 

 o 



(T2 



10 20 30 40 50 60 70 80 90 100 110 120 130 

 DOSAGE IN R UNITS 



figure 13-2. The relation between X-ray dos- 

 age and the frequency of breaks induced in 

 grasshopper chromosomes. (Courtesy of J. G. 

 Carlson, Proc. Nat. Acad. Sci., U.S., 27:46, 

 1941.) 



such cooperation between clusters, the break 

 frequency at low doses would be lower than 

 what has been found because of the waste 

 of clusters too small to break; the frequency 

 at higher doses would be higher because of 

 the cooperation among such clusters.) Cer- 

 tain radiations, like fast neutrons, produce 

 fewer breaks per r than X rays because one 

 r of these radiations produces larger — and, 

 hence, fewer — clusters of ions than do X 

 rays. These larger clusters more often ex- 

 ceed the size needed to produce a break, and 

 therefore, are relatively less efficient in this 

 respect. 



Ion clusters can produce breaks either di- 

 rectly by attacking the chromosome itself, 

 or indirectly by attacking oxygen-carrying 

 molecules (which, in turn, react with the 

 chromosomes) or other chemical substances 

 (which, in turn, affect the chromosome or 

 oxygen-carrying molecules). In any case, 

 this indirect pathway must be of nearly sub- 

 microscopic dimensions; otherwise, differ- 

 ent ion clusters would be able to cooperate 

 in causing breakage. Thus, only ion clus- 

 ters in or very close to the chromosome can 

 produce breaks in it, as has been visibly 

 demonstrated by using beams of penetrating 



