204 CELLULAR METABOLISM 



threads are broken, synthetic processes taking place on the thread sur- 

 face may also be influenced or interrupted. The possibility has also 

 to be envisaged that enzymatic processes taking place in the cytoplasm 

 are governed by the happenings in the nucleus (129, 138). In that case 

 a chromosome break may also have far-reaching consequences. 



It is known that massive doses of roentgen rays in the range of 

 1,000,000 r may not only depolymerize desoxyribonucleic acid, but lead 

 to some fission of glycosidic linkages with the liberation of purine bases, 

 to some breaking of the ester linkages, to splitting of the internucleotid 

 linkages, and so on (62). Doses of the order of 50,000 r markedly de- 

 crease the viscosity and thus influence the degree of polymerization of 

 desoxyribonucleic acid (63). The rigidity of nucleoprotein thread is 

 diminished under the action of a few thousand roentgens (64) . Dielectric 

 investigations also indicate the depolymerizing effect of irradiation. Di- 

 electric dispersion curves for aqueous solutions of desoxyribonucleic acid 

 irradiated with 5000 r differ from those of the controls (65). It is con- 

 ceivable that irradiation with even a few hundred roentgens produces 

 some depolymerization of desoxyribonucleic acid which facilitates the 

 breaking of chromosomes. Chromosome constituents other than desoxy- 

 ribonucleic acid may also be changed in chemical composition, the change 

 leading to a breakage of the chromosome thread. 



The effects of hard roentgen rays, soft roentgen rays, neutrons, and 

 alpha particles on chromosomes in microspores of Tradescantia hradeata 

 can be accounted for (66) by assuming that all observed aberrations 

 arise from chromosome or chromatid breaks primarily produced by the 

 passage of an ionizing particle through the thread at the locus of the 

 break (67, 117). It is not without interest, however, to note that the 

 aberrations seen at metaphase 24 hr after a roentgen-ray dose of 150 r 

 were found by Catcheside to arise entirely from chromatid injuries in 

 early prophase when intense nuclear synthesis sets in (68). 



Furthermore, chromosome fragility is generally greatest at the end of 

 the resting stage and in the most actively growing and dividing cells. 

 It is virtually absent in the heterochromatin and in the prophase chromo- 

 somes (69). Death of cells was interpreted by Koller as a loss of chromo- 

 some fragments at the ensuing mitosis (70, 115). The chromosome 

 breakage observed in the roots of Vicia faha and Allium cepa was also 

 interpreted as resulting from changes taking place before the formation 

 of the chromosome matrix in the interphase and prophase (70a). 



Above several cases were mentioned in which the absence of oxygen 

 during irradiation markedly reduced radiosensitivity. Giles and Riley 

 (71, 72) studied the effect of oxygen on the frequency of x-ray-induced 

 chromosomal aberrations of Tradescantia microspores observed at a 



