Section 5 — Mutagenesis 



young adult mice on total lifetime fertilities 

 and progenies, as distributed by litter sequences 

 and strains, have shown profound differences 

 between the sexes. Males of 8 inbred strains 

 fertilized up to 25 litters in 2390, whereas females 

 had up to 15 successive litters in 707 conceived. 

 Acute irradiations from to 320r scarcely 

 affected male fertility or the size of litters 

 obtained at each successive gestation. After 

 320r, male fertilities, the numbers of their litters 

 and of their progenies within litters, decreased 

 orderly on an irradiation dose-dependent basis. 

 Irradiated female fertilities, the numbers of their 

 litters and the numbers of their progenies, all 

 decreased regularly even with the lowest X-ray 

 dosages of 20r. Sex differences may seem sur- 

 prising for each sex has nearly equal genie 

 materials for ion pair absorption. However, 

 sperm potential of the males exceeds egg 

 potential of females by a large factor. A large 

 number of sperm may be inactivated, yet sperm 

 capable of fertilization are still available for full 

 fertility. After this point is reached dose depend- 

 ence of fertility for both sexes becomes com- 

 parable. Sex ratios for the progenies of either 

 irradiated males or females are similar. Dose de- 

 pendence of the traits is affected by strain. These 

 data are striking contrast to those of low daily 

 dosages, 0.1 to 0.03r per hour, of 22 hour per 

 day Cobalt-60 irradiation to be presented at 

 this Congress. 



1. Journal Paper of the Iowa Agri- 

 cultural and Home Economics Experiment 

 Station, Ames. Project Nos. 1180 and 1187. 

 Assistance has been received from Contract 

 AT (11-1) 107 Atomic Energy Commission, 

 U.S.A. 



5.73. Cytogenetic Radiosensitivity of Various Phases 

 of Nuclear Cycle of Human Cells in Tissue 

 Cultures. N. P. Dubinin (Moscow U.S.S.R.). 



The problem of the effect of ionizing radiations 

 on the heredity of man is one of the principal 

 ones in contemporary biology. In this respect 

 the data concerning radiogenetics of mammals 

 are of great importance. However, the main 

 direction in the experimental analysis of the 

 problem is determined by investigations of the 

 effect of radiation on the chromosomes in the 

 nuclei of human cells. 



The study of the effect of radiation on the 

 cellular nucleus necessarily runs the following 

 trends: (a) analyses of the dependence of various 

 types of nuclear changes on the chromosome 



structure in various phases of the cell cycle, as 

 well as on the relative capability or inability of 

 fragments to reunite; (b) various degrees of 

 sensitivity of chromosomes to the primary breaks 

 in various phases of the cell cycle; (c) the time of 

 appearance of different types of rearrangements 

 due to prolongation of the cell cycle as the 

 radiation effect. 



The effect of 50 r of X-rays on every phase of 

 the cell cycle of man was studied in tissue 

 cultures of l|-2-months-old embryos. It was 

 found that the highest sensitivity relates to the 

 early prophase and to the second half of the 

 postsynthetic phase. The first half of the post- 

 synthetic phase was 5 times less radiosensitive 

 as compared with the mitotic prophase. Through- 

 out the synthetic phase, radiosensitivity is 2-3 

 times higher as compared to the first half of the 

 postsynthetic phase. During the presynthetic 

 phase, radiosensitivity falls again to the level 

 of the first half of the postsynthetic phase. 



It was established that fusion and non-fusion 

 of the distal and proximal breaks, when they 

 occur at isoloci, proceed independently. The 

 fusing capacity of the distal fragments is at its 

 maximum at the early prophase and gradually 

 falls to a minimum towards the onset of the 

 presynthetic phase. The fusing capacity of the 

 proximal fragments is at its maximum at the 

 stage of synthesis of DNA and it falls both at the 

 onset of early prophase and the presynthetic 

 phase. 



Throughout the cell cycle the chromatid 

 rearrangements (100 per cent at the prophase 

 and during the whole postsynthetic period) 

 are being displaced by chromosomal rearrange- 

 ments (100 per cent at theonset of the presynthetic 

 period). The presence of a mixture of chromatid 

 and chromosome rearrangements at the syn- 

 thetic phase and for the greater part of the 

 presynthetic phase points either to the occurrence 

 of an asynchronous synthesis in various cells, 

 or to the presence of a mechanism of chromatid 

 rearrangements of chromosomal origin, and 

 perhaps to the simultaneous action of both 

 these mechanisms. 



The investigation of a great number of 

 chromatid rearrangements in the course of the 

 spontaneous mutation process, revealed the 

 occurrence of some real number of chromosome 

 rearrangements. 



The data obtained have shown that the 

 knowledge of quantitative and qualitative 

 chromosome changes, induced by radiation at 

 various phases of the cellular cycle, should 

 determine a new approach to the very method- 

 ology of the investigation of radiocytogenetics 

 of human cells. These data also point to the 

 necessity of studying the effects of the dose, type 



80 



