198 BO THORELL 



division and of specific substances of enzymic character for the function of 

 the tissue elements. 



IV. Concluding Remarks 



In the preceding sections an attempt has been made to give a balanced 

 picture of what in the author's opinion, are the conspicuous facts about the 

 chromosomal nucleic acids during the cell cycle. The physiological impor- 

 tance of these compounds is, however, still unknown. Among the chromo- 

 somal substances there are representatives of the hereditary material. DNA 

 has been regarded as the most characteristic chromosomal constituent and 

 the view, which can be traced back to the work of Miescher, that DNA 

 represents the genes received strong support when the bacterial transform- 

 ing factors were discovered'** (see Chapter 27). The apparent chemical 

 homogeneity of DNA preparations from different sources, however, was for 

 long difficult to reconcile with the manifold genie characters. The work of 

 Chargaff and others'**"'" surmounted this obstacle (Chapter 10), and the 

 recent results of X-ray diffraction work by Watson and Crick and Wilkins 

 et a/.^*^"'^" (Chapter 13) leave very little to be desired as regards possibilities 

 of structural variations in the DNA molecule. 



The problem of how the chromosomal nucleic acids exert their effects on 

 the cell and the organism may be presented in two questions: Are they 

 connected with protein synthesis, and, in particular, with the formation of 

 specific intracellular enzymes? Is their main task structural and/or 

 catalytic? One obvious difficulty in getting an answer is the nature of the 

 substances themselves. The nucleic acids and the nucleoproteins prepared 

 according to the present available procedures, unlike, for instance, many 

 of the fairly well-known enzymic energy systems, are no doubt very differ- 

 ent from the compounds as they occur in the cell. The adequate study of 

 these questions, therefore, needs much further work of a fundamental 

 biochemical and biophysical character. 



" O. T. Avery, C. M. MacLeod, and M. McCarty, J. Expll. Med., 79, 137 (1944). 



5* E. Chargaff, Federation Proc. 10, 654 (1951). 



66 E. Chargaff, C. F. Crampton, and Rakowa Lipshitz, Nature 172, 289 (1953). 



" G. L. Brown and M. Watson, Nature 172, 339 (1953). 



58 J. D. Watson and F. H. C. Crick, Nature 171, 737 (1953). 



68 M. H. F. Wilkins, A. R. Stokes, and H. R. Wilson, Nature 171, 740 (1953). 



«o R. E. Franklin and R. G. Gosling, Nature 171, 745 (1953). 



