Chapter *13 



RADIATION-INDUCED STRUCTURAL 

 CHROMOSOME CHANGES 



I 



n the preceding chapter struc- 

 tural changes in chromosomes 

 were discussed with respect to 

 types and consequences, but little was said 

 about the events responsible for their pro- 

 duction, namely, breakage and cross-union. 

 Chromosomes break spontaneously; that is. 

 they occasionally break in cells exposed to 

 normal conditions. Because spontaneous 

 breakage is relatively rare, agents that are 

 able to produce great numbers of breaks are 

 very useful in studies of chromosome break- 

 age and its consequences. Our discussion 

 in this chapter is restricted to one of these 

 agents, radiation. 



The process of breaking a chromosome 

 is a chemical reaction requiring energy. 

 The biochemical effect of radiation depends 

 upon the type and amount of energy left in 

 tissue. Less energetic radiations (such as 

 visible light) leave energy in the form of 

 heat; more energetic radiations (such as 

 ultraviolet light) leave energy in the form of 

 heat and activation; the latter type of energy 

 makes an electron move from an inner to 

 an outer orbit of an atom. The more ener- 

 getic the radiation, the greater the likelihood 

 that the energy absorbed will lead to chem- 

 ical change. For example, ultraviolet light 

 produces more breaks in chromosomes than 

 does visible light. Radiations of energy 

 higher than ultraviolet light (X rays and 

 gamma rays; alpha and beta rays; electrons, 

 neutrons, protons, and other fast-moving 

 179 



particles) are even more capable of causing 

 breaks. Although such high-energy radia- 

 tions also heat and activate, most of the 

 energy left in the cells is in the form of ioni- 

 zation, and this leads to most of the chromo- 

 some breaks. Before discussing how ioniza- 

 tion energy leads to breakage, we should 

 first understand what ionization is and what 

 its consequences are. 



Like visible and ultraviolet light, X and 

 gamma rays are electromagnetic waves; how- 

 ever, they have relatively shorter wave 

 lengths and can penetrate tissue more deeply 

 than visible or ultraviolet light. When a 

 highly energetic wave is stopped (or a fast- 

 moving particle is captured or slowed down ) , 

 energy is absorbed by the atoms of the 

 medium. This energy can cause an atom to 

 lose an orbital electron, creating a charged 

 particle, or ion, by the process of ioniza- 

 tion. Such an electron, torn free of the 

 atom, goes off at great speed and can, in 

 turn, cause other atoms to lose orbital 

 electrons — to be ionized. All atoms losing 

 an electron, of course, become positively 

 charged ions, and atoms that capture free 

 electrons become negatively charged ions. 

 Since each electron lost from one atom is 

 eventually gained by another atom, ions 

 occur as pairs. In this way a track of ion 

 pairs, or an ion track, is produced which 

 often has smaller side branches. The length 

 of the main or primary ion track and its side 

 branches and the density of ion pairs differ 

 with the type and energy of the radiation 

 involved. Fast neutrons make a relatively 

 long, rather uniformly thick ion track; fast 

 beta rays or electrons make a relatively long, 

 uniformly thin or interrupted track of ions; 

 ordinary X rays make a relatively short track 

 sparse in ions at its origin becoming only 

 moderately dense at its end. It is sufficient 

 to say that all known ionizing radiations pro- 

 duce clusters of ion pairs within microscopic 

 distances. In other words, no amount or 



