6 J. Bracket 



molecular compositions (Crosbie, Smellie and Davidson, 1953; 

 Elson, Trent and Chargaff, 1955) give little probability to the 

 idea of nuclear RNA being the sole precursor of cytoplasmic 

 RNA. On the other hand, Goldstein and Plant (1955) recently 

 succeeded in grafting, in normal and in non-nucleated amoebae, 

 nuclei which had been labelled with ^^P. These experiments 

 strongly suggest that nuclear RNA can give rise to cytoplasmic 

 RNA, but they do not demonstrate that nuclear RNA is the 

 sole precursor of cytoplasmic RNA, nor do they prove that 

 nuclear RNA is not degraded prior to its conversion into 

 cytoplasmic RNA. It appears rather as if both forms of RNA 

 are synthesized independently, though at a faster rate in the 

 nucleus than in the cytoplasm. We shall see later that major 

 differences are also found in the fate of RNA in various 

 enucleated organisms. 



Let us next consider another aspect of the role of the nucleus 

 in the life of the cell, the possible relations of the nucleus with 

 protein synthesis. As early as 1881, Verworn had suggested a 

 control by the nucleus of the cell's anabolism, making this 

 hypothesis in order to explain the usual incapacity of non- 

 nucleated cytoplasm to regenerate. Caspersson (1941, 1950) 

 has taken up this old hypothesis of Verworn and extended it. 

 On cytochemical grounds he has postulated that the nucleus 

 plays a fundamental role in protein synthesis, a suggestion 

 we shall now consider in the light of recent experimental 

 results from a number of laboratories. 



The observation that cells in which an active protein 

 synthesis goes on have a particularly large nucleolus with a 

 correspondingly high content of RNA, has led Caspersson 

 (1941) to propose that the nucleus, and especially the nucle- 

 olus, is a key factor in protein synthesis. Simultaneously with 

 Caspersson (1941) but working independently, we proposed 

 the hypothesis that RNA plays a direct role in protein 

 synthesis (Brachet, 1941). This was suggested by the excep- 

 tionally high RNA content of all cells actively synthesizing 

 proteins. The hypothesis found further support in the results 

 of Hultin (1950) and of Borsook and co-workers (1952), who 



