STRAUSS: DISCUSSION OF SESSION II 177 



of phosphorus from the DNA chain constitutes a definite break in 

 that chain. How then does a break in the backbone chain cause 

 a change in the base order? Kaudewitz (personal communication) 

 suggests that a break in one of the two (replicating) chains of the 

 DNA molecule leaves the distance between these chains unfixed 

 permitting a mispairing, the actual mutation (change in base order) 

 in this case being a secondary effect. Somewhat similar is the obser- 

 vation of Freese that treatment of bacteriophage at 45° C and pH 

 5 results in the production of mutants. Freese (1) supposes that 

 these mutations are due to the formation of apurinic acid at lower 

 pH. The initial reaction at pH 5 is different from those mentioned 

 above, but once again the DNA is unstabilized in a way that permits 

 unusual base pairings. 



In such cases the mutation is not "fixed" and in fact by defi- 

 nition can not be considered established until a new DNA strand 

 has been synthesized with the new base order. In order to have a 

 mutation which is functional before DNA duplication it is neces- 

 sary to produce, as a result of the action of the mutagen, a strand 

 which is read differently by the deciphering mechanism. It is this 

 requirement of the basic hypothesis that makes Doctor Haas's 

 report of X-ray-induced mutation in the absence of DNA synthe- 

 sis so interesting. Spontaneous mutation can occur in the absence 

 of DNA synthesis, but an energy source is required before such 

 mutations can occur (12). Doctor Haas reports that reversions can 

 be induced in the absence of DNA synthesis but that these muta- 

 tions require protein and RNA synthesis for their expression. 

 Unfortunately, it is not possible to exclude the possibility of DNA 

 synthesis from some of the data as presented. The frequency of 

 prototrophs induced is of the order of 100 per 10" cells, or 0.01 per 

 cent. Doubling of the DNA of these particular cells would give 

 a change of only 0.01 per cent, three cell divisions of this selected 

 population would still give less than a 0.1 per cent increase. Con- 

 sidering the analytical methods which must be employed, it is 

 doubtful if the results are good to within 1 per cent. But 1 per cent 

 could represent over five divisions of a selected cell population. If 

 one accepts the fact that the thymine-requiring organisms do not 

 make DNA, then the results using these mutants do seem to indi- 

 cate mutation and mutation expression in the absence of DNA 

 reproduction. 



