ACCELERATED HEAVY IONS ON ENZYMES AND YEASTS 265 



b. Formation of chromosome bridges leading to dominant lethality. 



c. Increased occurrence of somatic cross-over. This interesting process 

 will make a heterozygous genetic defect homozygous. When re- 

 cessive lethal damage occurs, the cross-over process can result in 

 a homozygous lethal daughter and another daughter cell which is 

 free from genetic damage. Observation of this delayed genetic 

 recovery process can be made ui genetic loci, \\'hich lead to 

 deficiencies causing colour in the affected cells. Adenine deficiency 

 and galactose requirement can lead to sectored colonies of this 

 type. 



d. "Point mutations". These occur at loci well defined on the genetic 

 linkage maps. These occur more frequently after ultraviolet, but 

 X- and a-rays can also produce them. They are normally spon- 

 taneously reversible. Many different point mutations are possible 

 at the same locus. 



e. A most interesting genetic process is one first found with u.v. 

 radiations by Roman. Here a homozygous but heteroallelic 

 diploid cell is tested, having an auxotroph requirement. Radiation 

 will allow the cell to become independent of the nutritional require- 

 ment. The explanation is either genetic recombination or "copy 

 choice" replicating mechanism. 



f. X-rays and particularly u.v. can cause the loss from the cell of 

 genetically controlled cytoplasmic particles, such as the cyto- 

 plasmic mutant of Ephrussi. This can occur either as a nuclear 

 effect or as direct result of cytoplasmic absorption of radiation. 



While each of the above classes of genetic damage forms the object of 

 special studies, some very major problems still await solution. We do 

 not know how initial molecular damage from a single ionizmg particle 

 is amplified to macroscopically observable or even lethal damage. It 

 would be important to discover whether recessive and dominant effects 

 originate from the same kind of initial lesion, with the difference in out- 

 come being merely an environment-dependent probal)ility factor, or 

 whether the different end results stem from different initial lesions. It 

 becomes more and more apparent that a cell, that has received near 

 lethal radiation also has many sublethal lesions, which can modify its 

 genetic and physiological characteristics. Lethality depends on environ- 

 mental metabolic and physiological events in the period following 

 radiation exposure. 



REFERENCES 



Alexander, P. (1957). Radn Res. 6, 653. 



BiRGE, A. C. and Sayeg, J. (1959). Radn Res. 10, 433. 



BiRGE, A. C, and Tobias, C. A. (1954). Arch. Biochem. Bmphs. 52, 388. 



