Ill DNA AND GROWTH MITOSIS 267 



components of cell nuclei, comparing tissues with high metabolic rates (mamma- 

 lian liver, kidney and pancreas) with cells endowed with a rather low rate of 

 metabolism (avian erythrocytes and Echinoderm sperm). The experiment showed 

 that the '^N uptake is much faster by the nuclear constituents of liver, kidney and 

 pancreas than by those of erythrocytes and sperm. When uptake was compared 

 with DNA, histone and residual proteins in the liver, kidney and pancreas, it 

 was fovmd that the uptake is low in DNA, higher in histone and much higher in 

 residual proteins. Of great interest is the finding that, in liver, pancreas and kidney 

 (where cells could not be dividing during Daly et aL'% short experiments), in- 

 corporation in DNA is much less in kidney than in liver and pancreas. Since the 

 DNA content of the nucleus is the same in the three tissues, what varies is the 

 activity of the DNA. Furthermore, in the pancreas, the '^N uptake into DNA of 

 a well fed animal is 50% higher than in a fasting organism. We may thus conclude, 

 with Daly et al. (1952), that chromosomal "activity" varies in different cells and 

 also in the same cell depending upon its over-all activity. 



The situation for DNA metabolic stability is therefore much the same as for 

 the constancy of DNA content per nucleus : this stability is considerably greater 

 than for any other known constituent of the nucleus, but DNA is not an absolutely 

 inert molecule. 



An important fact, which is not yet completely understood, is that, according 

 to Bendich (1952), it is possible to separate DNA from several organs in at least 

 two fractions showing significant metabolic differences. In experiments with label- 

 led precursors of the bases, one of the fractions is more active than the other; it has 

 been suggested by Sparrow (1952) that one of the fractions might be equivalent 

 to heterochromatin and the other to euchromatin. Such a suggestion is of great 

 interest, because it might explain two facts : first that the DNA content per nucleus 

 is not absolutely constant, and second that the metabolic stability of DNA is not 

 absolute. W^e know, from the cytological work of Darlington and La Cour (1940), 

 that the intensity of the Feulgen reaction may alter considerably in the heterochro- 

 matic segments of plant chromosomes under the influence of external factors like 

 temperature changes. Thus the heterochromatin, which genetically is relatively 

 inert, is apparently metabolically more active than the genetically active euchro- 

 matin. Variability in the DNA content of heterochromatin, under external and 

 physiological influences, might then very well explain the results of those workers 

 who, like Pasteels and Lison (1950), find that DNA content and ploidy do not 

 always run exactly parallel. Such a hypothesis would of course also explain 

 Bendich's (1952) results, if we assume that the metabolically active DNA is located 

 in heterochromatin. This hypothesis could easily be tested by autoradiography 

 methods, which should help in ruling out an alternative explanation, i.e. that 

 the two metabolically distinct DNA's of Bendich (1952) are localized indifferent 

 histological cell types in the same organ. 



To summarize the various hypotheses which have just been discussed, present 

 evidence indicates that DNA is a satisfactory candidate for genetic activity, certainly 

 better than any of the other known biochemical constituents of the chromosomes. 

 It is in the field of bacterial genetics, however, that the best evidence for a genetic 

 role of DNA can be found. 



Literature j>. zgg 



