Ill 



ZIRKLE: Do you mean to say that you are forsaking the whole idea of 

 a protein chain skeleton for the chromosome? 



MAZIA: Yes, I am thinking of the protein continuum of which I was a 

 supporter. For the time being, I have had to give it up in favor of a particulate 

 structure. 



POLLARD: We ought to get that in writing. 



MAZIA: I can discuss it a little if you wish. The source of the trouble 

 is the fact that when we looked for methods of taking chromosomes apart -- I 

 mean literally trying to put them into solution -- it always seemed to be very 

 difficult. We had to use methods that broke down the proteins, and therefore 

 came to the notion of a continuous protein backbone. But we never had paid at- 

 tention to the ionic environment of the chromosomes. When we finally did ex- 

 periment with this variable -- a lot of the basic information was in the literature 

 but not much attention was paid to it -- Bernstein and I, working with sperm 

 cells, found that we could disperse the nucleus completely into a solution of de- 

 soxyribonucleoprotein particles. These were, in the case of sea urchin sperm, 

 about 4000 ^ long and 200 to 300 ^ wide, judging from what we saw with the 

 electron microscope. 



The chromosomes dispersed easily enough once the conditions were 

 met, but these were rather exacting. 



First of all, in the material we work with, we have to introduce a 

 chelating agent that will remove calcium and magnesium. After this, we have to 

 bring the chromosomes to an ionic strength below that of 0.05M NaCl. If you 

 just remove the Ca and Mg nothing happens. If you treat directly with distilled 

 water, the chromosomes swell but do not come apart. But if you apply the two 

 treatments in sequence, the chromosomes may be completely dissolved. I have 

 studied this phenomenon cytologically on salivary gland chromosomes and grass- 

 hopper spermatocyte chromosomes. Bernstein and I made the chemical studies 

 on the nuclei of sea urchin sperm. More likely than not, the exact requirements 

 for dispersing chromosomes will vary from one kind of nucleus to another. 



From this information on how a chromosome may be taken apart, let us 

 try to organize a picture of how it is put together. Let us say that these nucleo- 

 protein particles are the basic units. You might picture them as being held to- 

 gether by bridges of divalent ions. Once these were removed, the particles 

 could separate. But they would not necessarily separate unless they repelled 

 each other sufficiently. This may account for the fact that even after removing 

 the Ca and Mg, it is necessary to go below a certain ionic strength. Electrolytes 

 would tend to swamp out the repulsions; removing the electrolyte would enable 

 the charged particles to repel each other effectively and go into solution. 



Now what I would like to ask the panel of physicists here, is how one 

 can picture a primary radiation event as acting on this kind of ionic bonding. I 

 am suggesting that radiation-induced breakage of chromosomes is the result of 

 an action that permits these ionically bonded particles to come apart and not the 

 result of damage to the molecules within the particles. 



PLATZMAN: Do you mean how can a single ionization in one of the 

 particles snap the calcium? 



MAZIA: The question is what the radiation can do to the situation bet- 

 ween the particles. 



