120 



action of the nucleus depends on its use of what you have aptly called the blue- 

 print. We are pretty certain on this point because another kind of nucleus can- 

 not be substituted; the right blueprint is required. 



KAMEN: It should be pointed out that there are more than just the four 

 pyrimidines. There are in addition a number of isomers. That is, there are 

 about three or four extra bases which come in, in small amounts, which are, 

 nevertheless, always associated with the very purest sample you can get of 

 nucleic acid. There has been, therefore, some slight flaw somewhere in the 

 structure. You cannot have a perfect setup. 



MAZIA: But in spite of all this variation the point still remains that the to- 

 tal purine to the total pyrimidine is a ratio of 1. That has not weakened with 

 all the variations and proportions that have been discovered. 



KAMEN: This is simply a repeating structure, a very simple structure, 

 isn't it? 



MAZIA: Not necessarily. Dr. Onsager's point was that the particular 

 sequence of bases along one chain implied a complementary sequence along the 

 chain that is paired with it. 



KAMEN: Was there a corresponding pyrimidine for the purine? 



MAZIA: A large number of analyses by Chargaff and others (21) indicates 

 that the ratio of the adenine (a purine) to thymine (a pyrimidine) in DNA is 1, 

 and, correspondingly, the ratio of quanine to cytosine and its substituents is 1. 

 Watson and Crick (20) have recently shown that in a two-helix model of DNA 

 only thymine will fit in juxtaposition to adenine, and only cytosine will fit with 

 quanine. Since the double-standard DNA would demand these uniformities in 

 composition, they argue that the experimentally demonstrated existence of these 

 regular nucleotide ratios in all kinds of DNA strongly supports their model. 

 What varies from species to species of DNA is the ratio of adenine and thy- 

 mine to guanine and cytosine. 



FAILLA: I am wondering what bearing the discussion of the last two days 

 might have on photo reactivation. Has anybody any ideas on it? 



BURTON: If a hypothesis is wanted, it is always easy to speculate. Con- 

 sider, as has been suggested, that a "semi-terminal" effect of high-energy is 

 radiation is the production of a triplet state which is fairly stable or, in more 

 general terms of a state which for one reason or another is not able to lose its 

 energy by radiation. (Let us call this stat e A ). On the other hand, this state 

 A may absorb energy (for example, ultraviolet light) and be excited up to a 

 state (Let us call it B) which intersects a high vibrational level of the lowest 

 lying single state. (IJT This system is shown schematically in Figure 4. The 

 transition from state _B_ to the low-lying singlet state I may be more or less 

 rapid, depending on the governing selection rules and the variety of possibili- 

 ties for internal conversion. The important point to note, however, is that by 

 optical excitation a possible path is provided for transition from state A_ to state 

 I. This is a stabilizing process. The alternative, if we wait long enough, may 

 be that state _A_ would disappear by chemical decomposition (for example, in a 

 rearrangement process to give ultimate molecules in a single elementary proc- 

 ess). Perhaps it will simplify matters if we look at figure 1 which corresponds 

 to the suggested model. For simplicity, I omit the ground covered by Dr. Kasha 

 and include only those states consideration of which is required in this model 

 for photoreactivation. Incidentally, it is importation to this picture that the 



