52 Nature of the Genetic Material 



not reflect upon what happens within the cell.) If some such scheme 

 is accepted — and I think this worthy of serious consideration — we 

 find ourselves at the same non liquet situation reached at the end of 

 the last section: the facts may be interpreted in terms of DNA either 

 as the genie material or as the scaffolding for the assemblage of the 

 proper protein molecules. Such an interpretation of the transformation 

 could suggest more decisive experiments, to be made with forms 

 which at present do not give 100 per cent clear results, experiments 

 which might also settle the biochemical details of the assumed sub- 

 stitutions. Consult at this point the facts relating to the transfer and 

 use of DNA from one nucleus to another one in growing egg cells, 

 mentioned in the next section. 



Further information comes from the behavior of bacteriophage 

 inside the bacterium, which somewhat parallels the basic features of 

 bacterial transformation. It is known (see review by Doermann, 1953) 

 that the phage entering the bacterium is stripped of its protein and 

 enters a vegetative phase, in which the phenomena of recombination 

 occur in one or another way. Hershey (see 1953) showed that during 

 this period of vegetative (non-infectious) multiplication of the virus 

 particle the genie apparatus of the bacterium is obliterated, though in 

 the beginning the bacterial DNA is in excess 100 times over that of 

 the initial content of the phage. The increase of phage DNA then 

 follows exactly the rules of phage particles increase until their 

 maturation. Thus DNA is again at the center of the whole process. 

 But Hershey, who has done so much work in this entire field, does 

 not conclude that this proves DNA to be the genetic material. He says, 

 "My ownn guess is that DNA will not prove to be the unique deter- 

 miner of genetic specificity, but that contributions to the question 

 will be made in the near future only by persons willing to entertain 

 the contrary view." 



Another possibility should not be disregarded, though it is 

 rather vague. The work on phenocopies in many organisms has shown 

 that the phenotype of any mutant may be produced as a non- 

 hereditary effect by treatment at the normal developmental stages 

 with various agents. For the present discussion the important fact 

 is Rapoport's discovery (confirmed by a number of workers) that 

 certain chemicals produce specifically phenocopies of one definite 

 mutant. It is obvious that here something is introduced which acts 

 in development in the same way as the mutant action does, directly 

 or indirectly. If, for example, a metal ion inhibits or slows a definite 

 enzymatic reaction, it may be assumed that both the mutant action 



