70 Discussion 



DISCUSSION 



Roller: Why do you think that mutation may involve damage to the 

 gene molecule? Why not assume that mutation is caused by changing 

 the sequence of purine-pyrimidine bases in the DNA ? 



Butler: You would have to break it up in order to change the sequence. 

 There is no easy chemical way of changing the sequence. 



Haddow : You were thinking of cytology ? 



Roller: Yes. Owing to the fact that mutation can be reversed and the 

 original gene structure reformed, the term "damage ", which is commonly 

 used by chemists and physicists, should be more clearly specified. 



With regard to the question of hydrogen bonding, can we assume that 

 reduplication of the chromosome would involve the breaking of all the 

 hydrogen bonds in the DNA, which is a very high number? Are there 

 other possibilities to explain chromosome duplication and separation? 



Spiegelman: I think that the doubled molecule has really been elimin- 

 ated by the Levinthal experiment which shows that you don't get 

 randomization of the ^^p making duplicates. 



Butler: I heard that Mazia has the opposite results. 



Spiegelman : I don't think the two types of experiments can be com- 

 pared. Mazia studied chromosomal multiplication and Levinthal the 

 duplication of viral DNA. Levinthal's results are consistent with the 

 obvious duplication mechanism deducible from the Watson-Crick 

 structure. He starts out with virus particles heavily labelled with ^^p 

 and examines the distribution of ^^p in the progeny particles produced. 

 An electron- sensitive photographic emulsion is used for the measurement 

 of the radioactivity of a single virus particle or a single DNA molecule. 

 He finds that 40 per cent of the DNA is contained in one large piece 

 which replicates to produce two particles, each containing half of the 

 original atoms. No further distribution of the original atoms takes 

 place with subsequent replication. 



Alper: This whole experiment seems a bit mysterious if it is done with 

 phage, when it is considered to be fairly well established that the first 

 thing that happens when a phage gets inside a bacterium is that it just 

 breaks up completely. 



Spiegelman: No, all that happens is that DNA goes in and leaves the 

 protein of the phage behind. 



Alper: But it is not recognizable as phage chromosome for quite a 

 while, so that it is hard to see how things should come together, the 

 hot with the hot and the cold with the cold, so to speak. 



Spiegelman: I don't understand why you believe that it falls apart 

 completely. 



Alper: It depends on what one means by completely. You cannot 

 pick up phage recognizable as phage, or indeed any virus as virus, for 

 quite a long period. 



Spiegelman: One cannot find infectious virus particles until the coats 

 and tails have been synthesized and put together, since these are neces- 

 sary for attachment and infection. However, the virus DNA is im- 

 mediately recognizable in such instances, e.g., T2 which contains 5-OH- 

 methylcytosine . 



