114 



ments have used 2537 ^ . This is not enough evidence. 



HOLLAENDER: Dr. Mazia is correct that there is a different type of 

 effect produced by ultraviolet and X-rays in regard to chromosomes. You have 

 a larger number of chromosome breaks produced by X-rays and a greater pre- 

 dominance of gene effects with ultraviolet. The difficulty with ultraviolet, of 

 course, is the question of penetration and how much the cytoplasmic material 

 protects the nucleus from radiation damage. Of course this is no problem with 

 250-kv. X-rays. 



ALLEN: This would seem to suggest that the effect of ionizing radia- 

 tion on this calcium bond might be via an indirect effect, whereas the ultraviolet 

 having a direct effect, is manifested more inside the organic part. 



There were some old experiments in which it was shown that the mobil- 

 ity of colloidal particles of gold and graphite, dispersed in water, was affected 

 by extremely small doses of X radiation. 



ZIRKLE: These particles are a little long to be genes, are they not? 



MAZIA: They are about the length of one DNA molecule. 



This brings in some interesting work. In recent years, there have 

 been a lot of developments in the study of suballeles or sub-genes, members of 

 a genetic locus that have related effects but that can function independently. I 

 understand -- and I could be wrong -- that Stadler has done some work showing 

 that while these genetic subunits can change independently of their fellows in 

 spontaneous situations, when you irradiate them, they all are affected together; 

 there is an all-or-nothing result. If this is so, it might be concluded that radia- 

 tion affects the grosser discontinuities in the genetic system. Such a result 

 would make sense if we supposed that the groups were held together to form a 

 chromosome by bonds that were different irom and more radiosensitive than 

 the bonds holding together the subunits within each group. 



KAPLAN: Do these large particles exist in the resting cell? 



MAZIA: We think so. At least we can extract particles having simi- 

 lar properties from resting cell nuclei and from condensed nuclei such as we 

 find in sea urchin sperm. 



POTTER: How do you see them? 



MAZIA: With the electron microscope. 



BENNETT: Is there any way that you can determine if radiation 

 causes breaks in these, or aren't the breaks of that nature? 



MAZIA: Dr. Bernstein, who is now working with Kauffman at Cold 

 Spring Harbor, has been studying X-ray effects on solutions of these particles. 

 I know that he finc^s them very sensitive but I do not know the nature of the 

 effects he observes. I'm pretty sure that the nucleoprotein as a whole is sensi- 

 tive at doses that are biologically interesting. The question 1 am raising is 

 whether breaks might not occur between relatively large subunits of chromo- 

 some structure, rather than within them. 



CHARGAFF: But you could, of course, have them stacked so that the 

 units are not next to each other but in different positions. You really have to 



