STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 47 



Althougli sperm do have one-half of the DNA content of the diploid cell, 

 it has been difficult to obtain complete data on ova. Marine invertebrates 

 have provided most of the gametes studied. In general, chemical methods 

 have revealed an apparent deoxyribose content in eggs greatly in excess of 

 that found in the sperm. Frog sperm, for instance, contain 8.6 X 10' i- gm. 

 DNA; the eggs are stated to contain almost 10,000 times as much DNA 

 (Hoff-Jorgensen, 1954). In the frog's egg, large amounts of the excess 

 deoxyribose exist in the cytoplasm (Hoff-Jorgensen and Zeuthen, 1952) and 

 the DNA content of the frog embryo does not increase until about the 

 5000-cell stage (Hoff-Jorgensen, 1954). Thus, cytoplasmic DNA and deoxy- 

 ribosides provide a store for the production of nuclear DNA. Frog ova nuclei 

 have recently been isolated; prior to a reductional division to the haploid 

 state their DNA content was twice that of the haploid sperm (England and 

 Mayer, 1957). 



Numerous studies have been made on the DNA content of nuclei of tumors. 

 In most instances it was found that, with certain exceptions attributable to 

 polyploidy, the values were essentially identical with those of normal cells. 

 However, in the case of the Ehrlich ascites mouse tumor, the DNA content 

 was about twice that of the normal mouse cells or other mouse tumors. In 

 the development of crown gall tumors of tomato, a marked increase of DNA 

 also occurred prior to the appearance of j)roliferation (Klein, 1952). This 

 increase in DNA content is followed by cell divisions, however, which reduce 

 this substance to a value approaching that in normal tissue. 



The nuclei of maUgnant tissue show a greater scatter of DNA content than 

 do those of normal tissue, a result attributed to pol}^3loidy and a higher 

 proportion of dividing cells in tumors. Nevertheless, observers have stated 

 that even in populations of presumably normal nuclei, exammed cyto- 

 chemicaUy, the highest value may be 50 % larger than the lowest. This may 

 arise from technical difficulties, such as the existence of optical inhomogeneity 

 of individual nuclei, or to an actual variation of DNA content from nucleus 

 to nucleus. If the latter is the case, we are faced with the problem of deciding 

 whether the observed differences arise from a variation per chromosome set 

 from nucleus to nucleus, or from examining nuclei in different stages in the 

 production of chromosomal substance during the interphase. 



Although the apparent anomahes displayed by eggs relative to the prin- 

 ciple of DNA constancy seem to be resolved by the discovery of cytoplasmic 

 DNA and the apparent equahty of DNA in sperm and egg nuclei, some 

 anomalies in DNA contents during embryological development have not yet 

 been explained. For example, Moore (1952) has studied the DNA content of 

 embryonic tissues and has observed that so-called diploid nuclei exhibit a 

 wide range of DNA content, i.e., 1.5 to 2.5 times that of haploid nuclei. She 

 has concluded that the rule of the constancy of DNA per chromosome set is 

 VOL. I — 5 



