JEAN BRACHET 



than other cell fragments, and that the integrity of RNA is essential for this process, 

 as digestion of RNA by the enzyme ribonuclease leads to a strongly decreased in- 

 corporation of the labelled amino-acids into the proteins of the homogenates. 



As regards the nuclei, their function is also obscure : they are very poor in respira- 

 tory enzymes, but they probably contain the enzymes which synthesize the nucleo- 

 tides. For instance, according to Hogeboom and Schneider (1952), synthesis of the 

 very important diphosphopyridinenucleotide occurs entirely in the isolated nuclei; 

 other enzymes concerned with nucleotide metabolism (nucleosidephosphorylase, 

 adenosinedeaminase) have also been shown to be concentrated in the nuclei 

 (Stern et aL, 1952). Metabolism of RNA, as studied with labelled isotopes ( 32 P, 

 14 C-orotic acid) is also much more active in the nuclei than in the cytoplasm 

 (Marshak, 1948; Jeener and Szafarz, 1950; Barnum and Huseby, 1950; Hurlbert 

 and Potter, 1952, etc.) : it might very well be that, as suggested by Marshak (1948) 

 and by Jeener and Szafarz (1950), the nuclear RNA is a precursor of the RNA in 

 the microsomes. It looks thus as if the nuclei were especially concerned with the 

 synthesis of nucleotides, whether in the form of respiratory coenzymes or of nucleic 

 acids. 



There is no doubt that the work done on the chemical composition of the various 

 fractions of homogenates, which was originated by Claude (1943) and by Brachet 

 and Jeener (1944), has great importance for an understanding of the role of the 

 various cell constituents. However, this type of work is open to many sources of error 

 which obviously restrict the meaning of the results obtained : for instance, one is 

 never sure that the various particles which are collected by differential centrifugation 

 of homogenized cells are really the ones which pre-existed in the intact, living cell. 

 During homogenization and centrifugation, soluble enzymes can be adsorbed on 

 the particles while, on the contrary, the cell granules may release enzymes which 

 are normally bound to them. These uncertainties become apparent when it is rea- 

 lized that the chemical properties of the cell constituents, especially the nuclei, are 

 not the same when the cells are crushed in different media. A further limitation of 

 the homogenate technique, obvious to any biologist who has a morphological back- 

 ground, lies in the fact that it is hardly possible to get information on the interactions 

 occurring between the different fractions: mixing in a test-tube nuclei and mito- 

 chondria and following some enzymatic reaction, for instance, is a valid experiment 

 from a biochemical viewpoint; but it is a meaningless and a possibly misleading 

 undertaking if what we want to know is the nature of the interaction occurring be- 

 tween the nucleus and the mitochondria in a normal living cell. 



It is because these shortcomings of the homogenate technique are so obvious that 

 some of my co-workers and myself decided to compare the biochemical changes 

 occurring in nucleated and non-nucleated fragments of unicellular organisms 

 (Amoeba proteus and Acetabularia mediterranea) : this is the only way to gain some un- 

 derstanding of the biochemical interactions between nucleus and cytoplasm in a 

 living cell. This type of work is obviously limited by the sensitivity of the methods 

 available, since relatively few organisms can be cut into nucleated and non-nucleated 

 halves; but, thanks especially to Linderstrom-Lang and Holter, many excellent 

 ultramicromethods of analysis now exist. The present paper will deal with the main 

 results of these experiments even though they are still, admittedly, quite fragmentary. 



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