Cytoplasmic & Nuclear Structure & Metabolism 17 



is the nucleolus which has the most rapid and considerable 

 uptake of the tagged precursors (adenine, phenylalanine) of 

 RNA and proteins. This is most marked in young, actively 

 growing oocytes in which considerable nucleoprotein synthesis 

 is going on. In such oocytes, the activity of the nucleolus can 

 be a hundred times that of the other constituents of the cell 

 (nuclear sap, cytoplasm). Similar results for RNA have also 

 been published by Taylor (1953) and by Stich and Hammerling 

 (1953), using ^sp as a precursor: the former using auto- 

 radiographic methods, the latter measuring the activity 

 in the giant nucleolus of Acetabularia, isolated by micro- 

 dissection. 



We can therefore conclude that the nucleus, especially the 

 nucleolus, is the site of a particularly active protein and RNA 

 metabolism ; this metabolism, however, can remain unaffected 

 for a long time in non-nucleated cytoplasm; net synthesis of 

 proteins and RNA can even take place in such cytoplasm. 

 In all known cases, a very strict parallel has been found 

 between the fate of RNA and that of protein anabolism. 

 Such a parallel lends support to the hypothesis put forth by 

 Caspersson (1941) and ourselves (1941) that RNA is directly, 

 involved in protein synthesis. 



Summary 



It has now been conclusively shown that the nucleus is 

 not a prime centre of cellular oxidations. It seems, neverthe- 

 less, that the nucleus plays a fundamental role in the synthesis 

 of nucleotides and of ribonucleic acid (RNA) and it may be 

 that the nucleus, and especially the nucleolus, is directly con- 

 cerned in the synthesis of nucle6tide coenzymes. 



It is certain that the nucleus plays an important role in 

 protein synthesis, although in the unicellular alga Acetabularia 

 protein synthesis can go on for long periods without a nucleus. 

 Indeed, non-nucleated fragments of Acetabularia are able to 

 synthesize RNA and proteins for some fifteen days. The rate 

 of these syntheses decreases afterwards, showing that the 



