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CHAPTER 21 



matin, as compared to euchromatin, cannot 

 be decided. Nevertheless, this means that in 

 many rearrangements, at least one of the 

 breakage points is located in the heterochro- 

 matin near the centromere. It is not unex- 

 pected, therefore, that whole arm reciprocal 

 translocation (which is involved in forming a 

 V-shaped chromosome from two rods) is the 

 most frequent type of reciprocal transloca- 

 tion. Moreover, the likelihood of the change 

 from a V to two rods is enhanced in Dro- 

 sophila by the fact that the Y is almost en- 

 tirely heterochromatic, as is one entire arm 

 of chromosome IV. 



You should recall that our motivation for 

 studying energetic radiations was based upon 

 their ability to induce many breaks and subse- 

 quently many structural changes. It was be- 

 cause this great supply of rearrangements was 

 readily available that many of the other fac- 

 tors influencing breakage and joining, which 

 we have just been discussing, were originally 

 discovered. Other important discoveries 

 were made possible by the study of structural 

 changes, including, for instance, that the 



centromere has a genetic basis and that cross- 

 ing over near it is always reduced, that the 

 telomere has a genetic basis, and that there 

 are genetic elements (called coUochores) near 

 the centromere which are especially important 

 for synapsis. Perhaps the most fundamental 

 contribution was the finding, via structural 

 changes, that the genes have the same linear 

 order in the chromosome as they have in 

 crossover maps. However, the spacing of 

 these is different in the two cases (Figure 21- 

 6). Thus, in view of the reduction of cross- 

 ing over near the centromere, the genes 

 nearest the centromere, which are spaced far 

 apart in the metaphase chromosome, are 

 found to be close together in the crossover 

 map. 



While we have restricted our attention to 

 the factors influencing the production and fate 

 of breaks produced by ionizing radiation, 

 these factors would be expected to operate 

 upon breaks produced by any other sponta- 

 neously occurring or induced mechanism. 

 For, in general, broken ends produced in 

 different ways all possess the same properties. 



SUMMARY AND CONCLUSIONS 



Pericentric inversions and whole arm reciprocal translocations have been frequent in the 

 past evolutionary history of Drosop/iila, the former changing chromosome shape, the latter 

 leading to changes in chromosome number. 



The components of structural change, breakage and exchange union, are readily studied 

 through the use of ionizing radiations. Such radiations induce breaks primarily by means 

 of the clusters of ion pairs they produce. These clusters form tracks of ionization, whose 

 thickness and length determine the number and location of the breaks. Tracks of ionization 

 must occur very close to, or inside of, the chromosome that is broken. Whether they result 

 from one or from two breaks, all chromosomal rearrangements produced by a single track 

 increase linearly with radiation dose, have no threshold dose, and show no effect of pro- 

 tracting or concentrating the dose. Two-or-more-break structural changes, produced by 

 ion clusters in separate, independently initiated tracks, increase in frequency faster than the 

 dose, and have a threshold dose. If joining of ends produced by breakage can take place 

 during the course of the irradiation, the latter types of rearrangement are reduced in fre- 

 quency by protracting the delivery of the total dose. 



Since both the breakage and joining processes involve chemical changes, their frequencies 



