458 ROLLIN D. HOTCHKISS 



ject, it is characteristically found that, once labeled, tissue DNA retains its 

 phosphorus and other markers through long periods of growth. In general, 

 it may be said that cell DNA goes through changes primarily only in con- 

 nection with cellular division processes, and then principally only aug- 

 mentation. 



a. DNA Content of Tissues 



Analytical studies at the tissue, rather than at the cellular, level have 

 provided a great number of measurements relating the DNA content to 

 the dry or wet weight of various tissues under different physiological, nu- 

 tritional, or metabolic conditions. If we choose to accept the average DNA 

 content per nucleus, or per cell, as constant (as discussed above and in 

 Chapters 16 and 19) at all times in a given tissue, then the DNA analysis 

 per weight of tissue can be taken as an inverse measure of the way cell 

 weights are changing under the experimental conditions being studied. 



Other factors, such as replication of chromosomes or of chromosome 

 strands, and differences in ploidy, would tend to complicate this picture 

 somewhat. The cell DNA content increases in cyclical fashion from a basal 

 amount to the double amount and back as the cell goes through a whole 

 division cycle. For mass analyses of adult animal tissues, the average over 

 this period does not represent a large source of variation, since divisions 

 are relatively rare; it may be the explanation for the reported higher DNA 

 content of tumor tissues'^" '^^ and embryonic tissues of sheep, ^^^ mouse, ^'^ 

 grasshopper,'^* and sea urchin. '^^ In general, it appears that the DNA con- 

 tent represents the least variable parameter of a tissue — so that it is in 

 fact reasonably correct to use analytical changes of percentage DNA con- 

 tent as expressions of the changing average nmss of the tissue cells. Thus, 

 the apparently higher DNA content of livers of female rather than of male 

 rats turns out to signify the smaller average mass of the female rat liver 

 cell, the total DNA per cell and per animal being the same for both sexes. '^' 

 The equivalent is true for apparent decrease of DNA during pregnancy, 

 and for the increase of DNA found after fasting or the feeding of various 

 deficient diets. 



Similarly, when another parameter is expressed as a ratio to DNA, e.g. 

 PN A -phosphorus to DNA-phosphorus, the ratio may be a fairly direct 



"0 W. C. Schneider, Cancer Research 5, 717 (1945). 



'" R. Y. Thomson, F. C. Heagy, W. C. Hutchison, and J. N. Davidson, Biochem. J. 



53, 460 (1953). 

 132 J. N. Davidson and C. Waymouth, Biochem. J. 38, 39 (1944). 

 1" M. Alfert, J. Cellular Comp. Physiol. 36, 381 (1950). 

 "4 H. Swift, Intern. Rev. Cyiol. 2, 1 (1953). 

 135 R. D. McMaster, Thesis, Zoology Dept., Columbia University, New York, 1952. 



From L. C. Sze, J. Exptl. Zool. 122, 577 (1953). 



