BIOLOGICAL ROLE OF DEOXYPENTOSE NUCLEIC ACIDS 439 



vidual nuclei'^ '^^ have been variously interpreted as meaning that DNA 

 distribution is altered during tissue differentiation, or that the amount of 

 DNA per nucleus actually fluctuates about an average value, as evidences 

 of stages in chromosome duplication, or simply of experimental error 

 (Chapter 19). 



The elegant ultraviolet microspectrophotometric methods of Caspersson 

 allowed this worker and Schultz to observe that certain genetic position 

 effects in Drosophila could be correlated with cytochemical changes in the 

 nucleic acid absorption at the corresponding region of the giant salivary 

 gland chromosomes." 



The amounts of DNA found cytochemically to be exclusively associated 

 with the chromosomes, therefore, are appropriate for a genetic function. 

 A genetic role of DNA is a satisfactory assumption but is not a necessary 

 one on this basis. We have considered so far the indications provided from 

 cytochemistry and cytogenetics, two of the borderline fields relating the 

 three primary sciences of chemistry, cytology, and genetics. A more direct 

 and convincing kind of evidence will next be considered — that supplied 

 from the third borderline field, the one concerned with both chemistry and 

 genetics. 



2. Evidences of Genetic Functioning of DNA 



It is in the realm of genetic chemistry'^ that we find cause and efTect 

 information — direct evidence that this or that chemical substance is 

 involved in, or modifies, elementary genetic processes. This approach was 

 being developed more or less simultaneously with the cytochemical one 

 during the last two or three decades. 



a. Mutational Effects hy Agents Reacting with DNA 



Indication of in vivo genetic action attributable to DNA in intact cells 

 is furnished by the relatively specific agent, ultraviolet radiation. The 

 variation with wavelength (action spectrum) of the effectiveness for 



'* L. Lison and J. Pasteels, Arch. Biol. {Liege) 62, 1 (1951). 



'« B. C. Moore, Chromosoma 4, 563 (1952). 



" J. Schultz and T. Caspersson, Arch, exptl. Zellforsch. Gewebeziicht. 22, 650 (1939). 



1* The term genetic chemistry is used here to signify particularly the chemistry of 

 genetic processes, somewhat in distinction to the term chemical genetics. Al- 

 though Beadle clearly conceived the latter to include the whole chemical relation 

 between genetic determinants and their end-effects (G. W. Beadle, in "Genetics in 

 the 20th Century" (Dunn, ed.), p. 221. The Macmillan Co., New York, 1951, 

 the terms chemical (and biochemical) genetics have come in practice to be asso- 

 ciated with a large body of data in which it is primarily intermediary biochem- 

 istry that is illuminated. At the present moment the biochemical characters 

 unfortunately seem hardly more closely related to the gene than the previously 

 studied morphological ones. 



