L82 



CHAPTER 13 



radiation of microscopic diameter. Such a 

 beam passing through a metapliase chromo- 

 some can break it. but fails to do SO when 

 directed at the protoplasm adjoining the 

 chromosome. 



From what has been stated, it is reason- 

 able to assume that the number of breaks 

 produced by a given dose of a certain radia- 

 tion depends upon the volume which a chro- 

 mosome occupies. This volume is different 

 at different times in the nuclear cycle (for 

 example, it changes during chromosome 

 replication). Because of variations in poly- 

 nemy or gene activity, the same chromo- 

 some can occupy different volumes in dif- 

 ferent tissues of an individual and the vol- 

 ume of the same sex chromosome can be 

 different in the two sexes. Because break- 

 age requires energy, it is also reasonable to 

 assume that the number of breaks indirectly 

 produced increases if, during irradiation, 

 either the amount of oxygen is increased, or 

 the cell's reducing substances are poisoned. 

 And conversely, replacement of oxygen by 

 nitrogen during irradiation reduces the num- 

 ber of breaks produced. 



After this preliminary discussion of some 

 of the factors that influence the production 

 of radiation-induced breaks, we are ready to 

 consider the factors that influence the fate 

 of the ends produced by breakage. Just as 

 breakage involves a chemical reaction, so 

 does the union between two sticky ends. 

 The joining of break-produced ends appar- 

 ently involves adenosine triphosphate and 

 protein synthesis. 1 Joining is enhanced by 

 the oxygen (and inhibited by the nitrogen) 

 present after irradiation. Accordingly, resti- 

 tution is prevented if nitrogen replaces oxy- 

 gen after irradiation, thus increasing the 

 time that ends from the same break stay 

 open, and. therefore, the chance for cross- 

 union when the supply of oxygen is later 

 resumed. (Note that the presence of oxygen 



1 See J. G. Brewen (1963). 



has two contrary effects on rearrangement 

 frequency — during irradiation it increases 

 the number iA' breaks, whereas after irradia- 

 tion it increases restitution. ) 



Since, under given conditions, the num- 

 ber of breaks increases linearly with an ion- 

 izing dose — each part of the dose independ- 

 ently producing its proportional number of 

 breaks — clearly, the number of breaks pro- 

 duced is also independent of the rate at 

 which a given total dose is administered. 

 It also follows that all structural changes in 

 chromosomes resulting from single breakages 

 are also independent of the radiation dose 

 rate. Radiations such as fast neutrons which 

 produce long and dense ion tracks can fre- 

 quently induce two chromosome breaks with 

 the same track. In this case, if the same 

 chromosome — having folded or coiled tightly 

 — is broken twice by being twice in the path 

 of the track, then large and small structural 

 changes of inversion, deficiency, and dupli- 

 cation types can be produced. The fre- 

 quency of these rearrangements increases 

 linearly with fast neutron dose and is inde- 

 pendent of the dose rate. 



A single fast neutron-induced track of ions 

 can also break two different chromosomes 

 when chromosomes are closely packed to- 

 gether, as they are in the sperm head. The 

 linear increase with dose in the frequency 

 of reciprocal translocations obtained after 

 sperm are treated with fast neutrons pro- 

 vides evidence for concluding — as was done 

 in Chapter 1 2 — that proximity of sticky ends 

 favors their union. Such a linear dose-effect 

 can be obtained only if both breaks are pro- 

 duced by the same track and if the broken 

 ends capable of exchange union are located 

 near each other — broken ends produced by 

 different tracks being too far apart. 



When ordinary X rays are employed, 

 however, the clusters are smaller, and the 

 track of ions is shorter than fast neutron 

 tracks. Accordingly, two breaks in the same 

 chromosome are produced by the same 



