Radiation-Induced Structural Chromosome Changes 



185 



tion causes a greater degree of synchromy 

 in division than occurs in the absence of the 

 radiation. Accordingly, starting with a pop- 

 ulation of cells in various stages of nuclear 

 division, the chromosomal targets for muta- 

 tion are different in the later stages of re- 

 ceiving a protracted dose and of receiving a 

 concentrated dose. 



The capacity to produce recoverable struc- 

 tural changes is not the same in euchro- 

 matic and heterochromatic chromosomal 

 regions. Recovered radiation-induced struc- 

 tural changes involve heterochromatic regions 

 more frequently than they do euchromatic 

 ones. It has not been determined whether 

 this excess is due to heterochromatin hav- 

 ing a greater breakability, a lesser resti- 

 tutability (which might be associated with 

 the general ability of different heterochro- 

 matic regions to synapse with each other), 

 or both. Nevertheless, in many rearrange- 

 ments, at least one of the points of breakage 

 is located in the heterochromatin nearest the 

 centromere. This is one reason why whole- 

 arm reciprocal translocation is the type most 

 frequently observed. 



The present discussion has been motivated 

 by the ability of energetic radiations to in- 

 duce many breaks and, subsequently, many 

 structural changes. The great supply of re- 

 arrangements readily available via radiation 

 treatment has made it possible to discover 

 many of the factors influencing breakage and 

 joining. Many other important discoveries 

 were made possible by the study of structural 

 changes, including 



1 . The genetic basis of the centromere 



2. The reduced incidence of crossing over 

 near the centromere 



3. The genetic basis of the telomere 



4. The existence in some species of ge- 

 netic elements (collochores) near the 

 centromere of special importance to 

 synapsis. - 



^ See K. W. Cooper (1964). 



X CHROMOSOME 



cv ct 

 y sc br pn wrb 



Centromere 



H CHROMOSOME 



/ I0\20 30 40^50 



17? ^^ \ \ 



,60-^70^80 90 100 107 



al dp 



7 v" 



b Bl/\cn vg c 

 It tk 



P* sp 



Centromere 



IE CHROMOSOME 



Q__jg_Aig__3Q_l40?t ,5Q^60 70 80 90 IQO 



D th cu sr e 



Centromere 



figure 1 3-4. Comparison of chromosome 

 (hollow bar) and crossover (solid bar) maps 

 in D. melanogaster. 



Perhaps the most fundamental contribution 

 was the finding, via structural changes, that 

 the genes have the same linear order in the 

 chromosome, that is, in chromosome maps, 

 as they have in crossover maps. The spac- 

 ing of these, however, is different in the two 

 cases (Figure 13-4). Thus, for example, 

 because of the reduction in crossing over 

 near the centromere, the genes nearest the 

 centromere — spaced far apart in the meta- 

 phase chromosome map — are found to be 

 close together in the crossover map. 



Although our subject matter has been re- 

 stricted to the factors influencing the origin 

 and joining of breaks produced by ionizing 

 radiation, these factors are expected to oper- 

 ate on breaks produced by any other spon- 

 taneously occurring or induced mechanism. 

 For. in general, no matter how broken chro- 

 mosomes are produced, all possess the same 

 properties. 



