2^2 NUCLEIC ACIDS AND GROWTH 3 



the DNA molecules of the cell nucleus or only in a few and, finally, we should not 

 forget the following very sensible remarks of Hotchkiss (1955b) "there is httle 

 reason to hold, for example, that all of a pneumococcus-transforming DNA prepa- 

 ration is made up of genetic determinants, whether identified or not identified. 

 It may very likely be as unjustified as assuming all proteins to be enzymes." Hotch- 

 kiss (1955b) also reminds us of the existence of heterochromatin, the significance 

 of which has been discussed earlier in this chapter: it might well be that the in- 

 corporation of 5-bromouracil, in Zamenhof and GriboflT's (1954) experiments, 

 occurs mostly — or perhaps entirely — in this metabolically more unstable fraction. 



One last, but very important, mvitagenic agent and well-known mitotic 

 inhibitor is of course X-radiation: once more, X-irradiation produces depolymeri- 

 zation of DNA both in vitro (Taylor et al., 1948) and in vivo (Errera, 1947). And 

 again, DNA synthesis is exceedingly sensitive to X-irradiation (Mitchell, 1942; 

 Hevesy, 1949; Holmes, 1949; etc.). 



Summing up, it can be said that most of the evidence reviewed in this section 

 strongly supports the view that DNA is of the utmost importance in chromosome 

 duplication and in genetic activity. If we take into account the existence of hetero- 

 chromatin and admit that its role is diflferent from the genetically more active 

 euchromatin, the evidence becomes even more convincing. We should therefore 

 think of DNA as a very essential growth factor, whenever growth is closely linked 

 to cell division. 



Fresh and important evidence in favour of this view has just come from 

 recent work by Miller et al. (1955), who have succeeded in isolating a new growth 

 substance for plant cells. This substance, which has been called kinetin and which 

 seems to be identical with the "wound hormone" of the plant physiologists, is a 

 derivative of DNA. While freshly prepared DNA is inactive in stimulating 

 mitosis, kinetin activity is present in all preparations or in DNA which has been 

 degraded by autoclaving for half an hour at pH 4.3. Kinetin is biologically 

 active at very low concentrations (0.0 1 "/qq) ; further work on its chemical nature 

 will be awaited with great interest. But, whatever its nature, the close association 

 of kinetin with DNA brings additional support for the view that DNA is an 

 essential factor for cell division and growth. 



IV. RNA AND growth: protein synthesis 



Growth is, of course, not necessarily a consequence of mitotic activity : we have 

 just seen that, according to Kelner (1954) and to Kanazir and Errera (1954), 

 growth and protein synthesis continue in U.V. irradiated bacteria, although DNA 

 synthesis has stopped. Other similar cases will be discussed later. In animal cells, 

 striking instances of growth without cell division may be found, for instance, in 

 the oocytes and the neurones. The tremendous increase in size and dry weight 

 of the growing oocyte is of course very well known ; in the neurones, elegant 

 ligation experiments of the axones by Weiss and Hiscoe (1948) have shown that 

 continuous synthesis of fresh cytoplasm occurs in the cell body. 



During the last 15 years, considerable evidence has accumulated in favour of 



