in various tissues of the same organism and is twice that found in the 

 nucleus of the haploid sperm cell (Boivin, et al., 1948; Mirsky and Ris, 

 1949). Furthermore, cytophotometric measurements made on individual 

 Feulgen-stained nuclei have shown that the amount of nuclear DNA is 

 directly proportional to the number of chromosomes in the nucleus of 

 any given cell of the organism (Swift, 1953). Somatic interphase nuclei 

 with DNA content approximately two or four times the more common 

 diploid amount, are encountered in tissues such as liver where polyploidy 

 is known to occur (PoUister, 1952). Variations in the quantity of DNA 

 would also be expected to occur, particularly in differentiated tissues, 

 as a consequence of polyteny. Cytophotometric studies have also indi- 

 cated that the amount of nuclear DNA is the same at the late interphase 

 (following DNA synthesis preparatory to cell division) and metaphase 

 stages of the mitotic cycle (Swift, 1953). Present evidence suggests that 

 in many cells the quantity of DNA is doubled at a specific time during 

 the interphase (Howard and Pelc, 1951; Walker and Yates, 1952), but 

 replication of DNA may occur in some cells as early as the previous 

 telophase (Brachet, 1957) or as late as prophase (Moses and Taylor, 

 1955). In brief, the diploid telophase nucleus in somatic mitosis contains 

 half the DNA content (2C value) of the subsequent prophase (4C) 

 and twice the amount of DNA present in the haploid (C) sperm nucleus 

 of the same species. 



The finding that the amount of nuclear DNA was related to ploidy 

 and was doubled during the mitotic process, has led to the belief that 

 DNA is a relatively stable and more or less metabolically "inert" com- 

 ponent of the cell nucleus. In support of this view is the negligible rate 

 of incorporation of radioactive phosphorus into the DNA of nondividing 

 cells (Ris, 1957). There are, however, a number of observations which 

 suggest that the DNA of the nucleus is not quite so stable and metaboli- 

 cally inert as was once believed. For example, disproportionate increases 

 in DNA content of specific bands of giant dipteran chromosomes have 

 been demonstrated (Ficq, et al., 1959; Pavan, 1958; Rudkin and Cor- 

 lette, 1957; Stich and Naylor, 1958). These studies, especially those in 

 which uptake of DNA precursors was followed, suggest that there is a 

 fraction of the DNA which shows a high turnover as well as a fraction 

 which is relatively stable (Figure 4-23). Further evidence comes from 

 the fact that a small but detectable uptake of isotopically labeled DNA 

 precursors occurs in some nondividing cells (Koenig, 1958; Pelc, 1959). 

 Also it has been shown that fractions of DNA isolated from different 

 tissues differ in chemical composition and metabolic activity (Bendich, 



104 / CHAPTER 4 



