STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 



43 



Williamson and Gulick (1944) liave analyzed thymus cells and nonaqueous 

 nuclei for Ca, Mg, and P. Both divalent cations were concentrated in the 

 nuclei. Ca++ was present in amounts considerably greater than Mg++ and 

 approached the content of the diesterified phosphate present in DNA. 



In addition to an unusually high content of these metals, nuclei contain 

 the largest proportion of the manganese of the rat Hver cell, as well as 

 significant amounts of cellular zinc. Such data, as well as a comparison of the 

 cation contents of various ceU fractions, are presented in Fig. 4. 



Fig. 4. Sodium, potassium, magnesium, calcium, zinc, iron, manganese, and copper 

 in fractions of rat liver. The height of each bar represents the metal content of each 

 fraction, expressed as a percentage of the total content of that metal in liver, divided by 

 the same parameter for nitrogen. This normalization allo^^'s direct comparison between 

 different fractions and metals. The width of each bar represents the nitrogen content of 

 the fraction (Thiers and Vallee, 1957). 



Chromosome breakage in Tradescantia paludosa has been shown to increase 

 considerably when the plants are grown in decreasing amounts of magnesium 

 (Steffensen, 1953) or of calcium (Steffensen, 1955). These results were con- 

 sistent with the data given above, as well as that of Bernstein and Mazia 

 (1953), which indicated that the dissociation of chromosomes into constituent 

 deoxyribonucleoproteins was facihtated by citrate or other sequestering 

 agents. Thus, Ca++ and Mg++ seemed to be implicated in maintaining the 

 structural continuity of the chromosome. 



Levine (1955) has reported that altering cationic conditions, particularly 

 with respect to Ca++, markedly affects crossing over in Drosophila melano- 

 gasfer. However, King and Rubinson (1957) have summarized considerable 



