BIOLOGICAL ROLE OF DEOXYPENTOSE NUCLEIC ACIDS 459 



measure of the PNA per cell. Within limitations imposed by questions of 

 ploidy, etc., one may expect a comparison of DNA contents between differ- 

 ent tissues of the same species to provide an indirect measure of the relation 

 between the average cell masses of these tissues. 



These questions have been discussed in some detail in Chapter 16 



h. DNA as a Measure of Growth 



If the percentage DNA content of a tissue gives information mainly 

 about such parameters as cell mass, the total amount of DNA present is an 

 important measure of cell number and reflects the occurrence of growth. 

 Among the systems which have been followed in this way are: the develop- 

 ment of fibroblasts,'*^ leucocytes,'" '** various organ tissues,'*^ and viruses'*" 

 in tissue cultures, and of malaria parasites in erythrocytes.''" The total 

 DNA per liver serves as a measure of liver regeneration; during the rapid 

 phase of growth shortly after hepatectomy, the DNA per nucleus becomes 

 higher than normal, so that for a time the DNA is increasing faster than 

 cell number, subsequently stopping at the point when the amount of DNA 

 originally present in the total liver has been restored. 



The usefulness of nucleic acid phosphorus (especially the DNA part) as 

 a measure of growth was recognized empirically, '^^"'^^ and then received 

 logical support when the constancy of DNA in the nucleus was discovered. 

 As has been pointed out in Chapter 16, it seems likely that DNA w411 come 

 to serve more and more either as a measure of cell number or, as a corollary 

 to this, as a basis of reference to which other analyzed components are 

 expressed in the form of an analytical ratio. 



There is some evidence that DNA duplication at the cellular level occurs 

 well in advance of cell division. Photometric measurements of individual 

 animal and plant nuclei have indicated that new DNA synthesis occurs 

 early in interphase of mitotic or meiotic cycles,'** or between interphase 

 and early prophase.'^ •'*^"'** Chemical analyses related to changes in mitotic 



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 95 (1951). 



38 E. E. Osgood, H. Tivey, K. B. Davison, A. J. Seaman, and J. G. Li, Cancer 5, 331 

 (1952) . 



39 H. W. Gerarde, M. Jones, and T. Winnick, /. Biol. Chem. 196, 69 (1952). 

 » L. T. Atlas and G. A. Hottle, Science 108, 743 (1948). 



1 P. R. Whitfeld, Nature 169, 751 (1952) ; Australian J. Biol. Sci. 6, 234 (1953). 



2 I. Berenblum, E. Chain and N. G. Heatley, Biochem. J. 33, 68 (1939). 



3 E. N. Willmer, J. Exptl. Biol. 18, 237 (1942). 



* J. N. Davidson, Cold Spring Harbor Symposia Quant. Biol. 12, 50 (1947). 



6 J. Pasteels and L. Lison, Arch. biol. (Liege) 61, 445 (1950). 



6 H. Ris, Cold Spring Harbor Symposia Quani. Biol. 12, 158 (1947). 



^ H. Swift, Proc. Natl. Acad. Sci. U. S. 36, 643 (1950). 



8 J. H. D. Bryan, Chromosoma 4, 369 (1951). 



