438 ROLLIN D. HOTCHKISS 



tures, the chromosomes, as those the cytogeneticists have shown must 

 carry the genes. '' 



c. Quantitative Cytochemical Regularities in DNA Distribution 



The suggestion that DNA might be the genetically active material of 

 the cell has thus been supported to the extent that localization is at least 

 appropriate to this function. The recognition that chromosomes have 

 constituents other than DNA, especially histones and other proteins, 

 however, made it impossible to claim more than this. 



Quantitative cytochemical studies, stimulated in large part by the 

 bacterial transformations to be discussed in the next section, have recently 

 made a genetic function of DNA seem somewhat more convincing. Boivin 

 et al} showed that the DNA satisfied the additional criteria of being es- 

 sentially constant in amount in the (diploid) somatic nuclei of various 

 tissues within a species, and of being present in half quantities in the 

 (haploid) sperm cells (Chapter 19). Mirsky and Ris^ confirmed this rela- 

 tionship in general, extending the study to a number of new species. Mir- 

 sky'" has furnished the additional information that the various nuclear 

 proteins (histone, "residual protein" of the chromosomes, nuclear en- 

 zymes) did not — at least as defined by the analytical methods used — have 

 the distribution expected for gene material, and exhibited by the DNA. 



Differences in the DNA content of individual nuclei of rat liver""" 

 showed three levels of DNA in ratios 1:2:4, corresponding to three sizes 

 of nucleus, presumably representing diploid, tetraploid, and octaploid 

 nuclei. Ogur et al}^ have analyzed yeast strains varying from haploid to 

 tetraploid and have reported that DNA content is more consistently in 

 direct relation to the degree of ploidy than dry weight, PNA, or meta- 

 phosphate content. Smaller differences, within a size class, between indi- 



^ It is generally believed that the chromosomes maintain their essential DNA 

 localization and genetic integrity during the mitotic interphase, although they 

 are diffuse and difficult to observe at this stage. In any case, the correlation be- 

 tween DNA distribution and cytogenetic organization is applicable during the 

 more active nuclear stages, and the quantitative data to be discussed tends to 

 show that there is no gross discontinuity at interphase. 



8 A. Boivin, R. Vendrely, and C. Vendrely, Compt. rend. 226, 1061 (1948). 



9 A. E. Mirsky and H. Ris, Nature 163, 666 (1949). 



>"> A. E. Mirsky, in "Genetics in the 20th Century" (Dunn, ed.). Macmillan, New 



York, 1951. 

 " H. Ris and A. E. Mirsky, J. Gen. Physiol. 33, 125 (1949). 

 ^^ H. H. Swift, Physiol. Zool. 23, 169 (1950). 

 " C. Leuchtenberger, R. Vendrely, and C. Vendrely, Proc. Natl. Acad. Sci. U.S. 37, 



33 (1951). 

 " M. Ogur, S. Minckler, G. Lindegren, and C. C. Lindegren, Arch. Biochem. and Bio- 



p/iys.40,175(1952). 



