CHAPTER II 



The Sites of Protein Formation within 

 the Living Cell 



A. EARLY CYTOCHEMICAL DATA 



The first data on the sites of protein synthesis in the cell emerged from 

 cytochemical studies by Brachet and by Caspersson. Brachet (1933) had 

 observed that the variations of DNA content during the development of 

 the sea urchin egg do not parallel the changes of 'nucleic phosphorus'. This 

 called his attention upon the possible existence of other nucleic acids beside 

 DNA in this material. But at the time, there was no method available for 

 detecting or for determining ribosenucleic acids. Plant nucleic acid, as 

 RNA was then usually named, was regarded as a biochemical curiosity, 

 so rich in phosphate that it was probably a phosphate storage form, which 

 occurs in wheat germ, in yeast, and curiously enough also in pancreas. 

 A few years later, the purification and isolation of pancreatic ribonuclease 

 (Kunitz, 1940) provided a means of destroying RNA specifically. Making 

 use of this new tool, Brachet tried to see which structures or regions of 

 animal cells would be affected by ribonuclease. Brachet (1941) established 

 that the basophilic substance of the cytoplasm of animal tissues is speci- 

 fically removed by pancreatic ribonuclease. The substance responsible for 

 cytoplasmic basophilia was thus clearly identified as RNA, and at the same 

 time a very simple method for the detection and localization of RNA in 

 tissue sections was introduced. A screening of animal tissues by this 

 technique demonstrated the presence of RNA in all types of cells and 

 showed that RNA is responsible for the basophilia of ergastoplasm and 

 nucleoli. During cell division, RNA is also found in chromosomes and in 

 the spindle. But by far the largest amount of RNA is in the cytoplasm. 



Caspersson, on the other hand, had developed a microspectrophoto- 

 meter with which he was able to measure light transmission at selected wave 

 lengths in the ultraviolet on small regions of a cell. With this apparatus, 

 Caspersson (1941) observed that the cytoplasm of animal cells contains 

 substances which strongly absorb ultraviolet light, with an absorption 

 spectrum similar to that of nucleic acid. Since well-known cytochemical 

 tests indicated that DNA was not in the cytoplasm, it was inferred that the 

 cytoplasm contains RNA. Later, the combined use of basic dyes, ultra- 



40 



