300 Action of the Genetic Material 



Does it follow that this assumed heterochromatization was the cause 

 of differentiation? I would conclude, just as before with polyteny, that 

 the consequence of differentiation, involving the ending of the need 

 for genie activity, becomes visible in chromosomal changes from the 

 specialized structure necessary for genie action to a generalized one 

 adapted for special synthetic action. Schultz' hypothesis demonstrates 

 that models of intranuclear differentiation could be worked out if 

 results of experiments should require such a view. Since this is not 

 the case, we had better not assume that the possibility of stating such 

 a theory in terms of very doubtful and confusing observations increases 

 its chance of being correct. 



From a very different angle, the same problem was attacked by 

 the group of researchers interested in the quantity of DNA in the 

 nucleus, the general results of which were discussed {I 2 B b aa) in 

 connection with the problem of whether or not DNA is the genetic 

 material. We saw that the authors (reviewed by Alfert and Swift, 

 1953) considered the constancy of DNA in all cells and the simple 

 ratios in polyploidy and polyteny as indication of such a role for DNA. 

 Lison and Pasteels (1951) found, however, some variation in different 

 tissues, and thought that this is associated with the control of morpho- 

 genetic processes, which then may be classed as intranuclear causes 

 of differentiation. Recently R. C. Moore (1952) made extensive 

 measurements, comparing the amount of variation of the DNA con- 

 tent in different tissues of haploid and diploid frog embryos. It was 

 found that the diploid values are always twice the haploid ones, as 

 expected. Furthermore, a considerable range of variability of the 

 DNA quantity was found, first, when comparing the same organ at 

 different times of differentiation, and second, between different tissues. 

 In a general way it was found that a wide range of DNA values is 

 correlated with differentiation. As differentiation proceeds, the range 

 of DNA values is narrowed, and finally they hardly fluctuate. From 

 this it is concluded that if DNA is the genie material, yet is variable 

 in amount in differentiating tissues, an explanation can be derived of 

 how cells with the same chromosome complement differentiate into 

 different tissues. "The genes controlling certain morphogenetic proc- 

 esses may produce different amounts and kinds of DNA at different 

 times, resulting in differentiation." This idea, then, evades the diffi- 

 culty of genie alterations by leaving the genes unchanged but their 

 activity in terms of "producing" DNA changing, and, in addition, 

 endows DNA with the rather mysterious quality of controlling differ- 

 entiation by its amount. 



