470 ROLLIN D. HOTCHKISS 



dissipated in a second infection cycle. This might suggest that the geneti- 

 cally "functional" portion of the DNA is actually the half or more, the 

 isotope of which is liberated as low-molecular substances during the process 

 of reduplication. Once making this assumption, one might go so far as to 

 suppose that all of the parent DNA is broken down in exerting its genetic 

 functions, and a portion of the isotope only is reabsorbed from the metabo- 

 lite pool. Although the studies with mammalian tissue already cited suggest 

 that mitotic division also involves a wasteful synthesis of an extra duplicate 

 portion of DNA, for the reasons already given this conclusion is not safely 

 established. 



Also more or less in opposition to these last views is the finding that 

 carbon, nitrogen, and phosphorus of DNA are, in general, extraordinarily 

 stable toward exchange, even during growth when incorporation of iso- 

 topically marked atoms is most rapid. Furthermore, the cytochemical 

 studies of nuclei, described above and in Chapter 19, have led to the infer- 

 ence that the regularities in DNA content from cell to cell are a direct 

 consequence of the regularities with which genetic material is distributed 

 through known processes, none of which requires or suggests a breakdown. 

 It does not, therefore, seem probable that DNA is generally broken down, 

 in whole or in part, either during cell division or in performing its genetic 

 functions. Neither do we find, on those occasions when a part of it is broken 

 down, that this degradation has an obvious relation to genetic functioning, 

 or that. the one part of the DNA is in any obvious way less "essential" 

 than the other. 



1. Other than Genetic Roles for DNA 



Nevertheless, the distribution of DNA in cells and tissues does not always 

 seem to fit an exclusively genetic role. It is true, as already described, that 

 the irregularities as well as the regularities in nuclear DNA distribution 

 can generally be correlated with the sometimes irregular replications or 

 distributions of the genetic complement. There is sufficient variability in the 

 DNA of individual tissue nuclei, however, to raise the questions whether 

 the rather striking fixed levels of DNA content reported actually represent 

 the aggregate diploid set of genes, or chromosomes bearing nongene DNA 

 also, and, if so, whether before or after duplication in preparation for cell 

 division; or if they are merely modal values of a statistical distribution^^ of 

 different degrees of replication. Indeed, it has been calculated that a 

 Drosophila cell contains enough DNA for many replicates of each of the 

 several thousand expected pairs of genes, if these latter are single molecules 

 of molecular weight 10*.^'^ In the many-stranded "giant" chromosomes of 

 larval Drosophila, there appeared to be among the individual nuclei an 



213 N. B. Kurnick and I. H. Herskowitz, /. Cellular Comp. Physiol. 39, 281 (1952). 



