STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 



109 



development of synchronized embryos, at least for early divisions. Such 

 systems were used for the isolation of mitotic figures in different stages of 

 mitosis (Mazia and Dan, 1952). In many such systems, e.g., fertilized sea 

 urchin eggs, etc., DNA synthesis proceeds without net KNA synthesis, since 

 chromosome sets are increased within the segmenting ova, which do not 

 significantly change their mass. However, as in the case of phage multiplica- 

 tion, we must ask if RNA synthesis and turnover of a small fraction of the 

 total RNA are not involved in this developmental process. 



In regenerating rat Hver, Hecht and Potter (1956a) were able to demon- 

 strate an apparent synchrony of DNA synthesis, as measured by the incor- 

 poration of orotic acid-C^* into DNA (cf. Fig. 19). There is essentially no DNA 



O 5 10 15 20 25 30 35 40 48^'" 72 144 

 Hours after partial hepatectomy 



Fig. 19. SjTichrouization of DNA synthesis in regenerating rat liver (Hecht and 

 Potter, 1956a). 



synthesis for about 18 hours, after which this function is suddenly activated, 

 rising to a maximum between 24 and 30 hours. In this system, incorporation 

 of isotope into nuclear RNA was maximal at 2 hours and mto cytoplasmic 

 RNA about 10 hours later. In an extension of the study of this system ^dth 

 injected orotic acid, the distribution of isotope in acid-soluble pyrimidine 

 nucleosides and nucleotides suggested that there was a conversion of low 

 molecular ribose derivatives to low molecular deoxyribose derivatives, which 

 were then incorporated very rapidly into DNA, 



D. Pathological Systems 



The control of differentiation is a subtle process inasmuch as the exaggera- 

 tion of one or another facet of metaboHsm must be effected without destroy- 

 ing the cell. A cytopathogenic effect in virus infection may be considered to 

 be an instance in which a deviation and exaggeration of metabohsm cannot 

 occur without destroying the ability of the cell to maintain itself. Cellular 

 integrity is maintained in liver and nervous tissue despite the growth and 



