264 NUCLEIC ACIDS AND GROWTH 3 



photographs of cells at this wavelength, using a quartz or reflecting microscope 

 (Fig. 3, a). The technical difficulties have been overcome by Caspersson (1940, 

 1950), who has been able to measure the complete absorption spectrum of very 

 small parts of the cell. Since the method does not distinguish between the two 

 types of nucleic acids, it is necessary either to detect DNA with the Feulgen reaction 



3a 3b 



Fig. 3. a: U.V. photograph (2,650 A) of an amoeba; b: same after ribonuclease treatment: 

 nuclei of ingested flagellates still absorb the U.V., because of their DNA content. 



or to remove RNA with ribonuclease in the usual way in order to follow the dis- 

 tribution of the two nucleic acids. (Fig. 3, b). 



The results yielded by these different methods will be discussed in greater de- 

 tails in subsec{uent sections of this chapter. Here is, however, a very brief outline 

 of the main conclusions which have been reached. DNA, as shown by the Feulgen 

 test, is usually strictly localized in chromatin or, in dividing cells, in the chro- 

 mosomes; there is no great variation in the DNA content of the nuclei in different 

 tissues of the same species. On the other hand, RNA is mainly concentrated in the 

 nucleolus and in small basophilic, cytoplasmic particles (the microsomes). Chro- 

 matin contains some RNA, however, in amounts varying according to the cell 

 type. Furthermore, the RNA content of the nucleolus and the cytoplasm varies 

 considerably from one cell type to another, in the same organism: it is always 

 present in large amounts in cells synthesizing proteins. 



We will now discuss these general conclusions in more detail and try to find 

 out their significance for growth problems. 



