115 



make a lot of assumptions. 



MAZIA- Oh, yes. All I am proposing -- and it seems to be reasonable 

 -- is that there is a kind of discontinuity in the structure of the chromosome 

 that will make the chromosome sensitive to variables -- such as the ionic en- 

 vironment -- which we ordinarily would not think of when we consider the effects 

 of irradiation of nucleic acids or proteins. 



CHARGAFF: Except that this would make it even more mysterious how 

 a chromosome can split so regularly. 



PLATZMAN: Why do you say that? 



CHARGAFF: If -ou assume a simple ionic binding between blocks it is 

 very hard to visualize it mechanically. 



PLATZMAN: Wouldn't you say that about any picture whatsoever? 



CHARGAFF: That is why I hesitate to put mechanisms on the black- 

 board. That is where you have to be very careful. 



PLATZMAN: Crystals reproduce each other. They grow. 



CHARGAFF: Crystals grow in a saturated solution, it is true. 



MAZIA- Dr Chargaff, we are not putting down a mechanism to account 

 for how the chromosomes do anything positive. We are considering how they 

 can be broken apart, which is a little easier problem. 



SPIEGELMAN: How about the chemistry? Is there evidence for the 

 regular spacing of divalent ions. 



CHARGAFF: I don't know. You will have to ask Dr. Mazia. 



MAZIA- There is a lot of evidence showing that the concentration of 

 divalent ions in the nucleus is high. Some workers, such as Allgen, have 

 stressed the finding that it is very difficult to get rid of these in the purification 

 of DNA. Dr. Chargaff may have had some experience with this. 



CHARGAFF: Magnesium is really everywhere. It is a good assumptior 

 that it is also in these proteins. Some divalent metals are easy to find any- 

 where. 



ALLEN: What about depolymerization of these acids on irradiation? 

 Could that also be connected with the calcium bridge? 



MAZIA- I don't know. For the moment, I am trying to focus attention 

 on the situation between the macromolecular units; on whether the chromosome 

 is discontinuous. 



CURTIS- I think it would also be worth mentioning that Dr. Steffensen 

 at Brookhaven National Laboratory has been growing plants in media which are 

 deficient in calcium, magnesium, and other ions. He finds that there are many 

 more spontaneous chromosome breaks than normal in plants grown on calcium- 

 deficient media. 



