114 ROLE OF THE CELL NUCLEUS 



removal of the nucleus. However, later work by Richter (1959) and 

 by Naora, Richter and Naora (1959) failed to confirm this conclu- 

 sion; these workers found that there is little change in the RNA 

 content of enucleate fragments during the first weeks after opera- 

 tion. In any event, enucleate cytoplasm of Acetabularia markedly 

 differs from enucleate fragments of amoebae where, as we have seen, 

 removal of the nucleus is quickly followed by a marked drop in the 

 RNA content. In Acetabularia, enucleate cytoplasm is capable of 

 retaining its RNA store for a long time and to a surprising extent. 



The reason for this marked difference between the alga and the 

 amoeba might well be the presence of chloroplasts in the former. It 

 has just been found by Naora, Brachet and Naora (1959) that RNA 

 is still synthesized in the chloroplast fraction after removal of the 

 nucleus ; it decreases in the other fractions (microsomes and super- 

 natant liquid), just as it does in amoebae. The contradictory results 

 obtained in the past are probably due to the fact that various 

 strains and culture conditions were used in the different experi- 

 ments so that the importance of chloroplast multiplication was 

 highly variable. At any rate, the conclusion of the last experiments 

 made on this difficult material is that the total RNA content of 

 enucleate fragments does not markedly change, but there is a net 

 synthesis of chloroplast RNA at the expense of the RNA present in 

 the other cell fractions. 



It is therefore not surprising that the incorporation of orotic 

 acid, a labeled precursor of RNA, remains very active in the ab- 

 sence of the nucleus (Brachet et al, 1955). In fact, the incorporation 

 is from the beginning 50 % higher in the nucleate than in the enu- 

 cleate halves, but there is no striking fall in the latter, even 70 days 

 after the operation. This situation merely reflects the fact that the 

 nucleus itself (in particular the large nucleolus) is the site of an 

 especially active RNA metabolism (Stich and Hammerling, 1953; 

 Hammerling and Stich, 1956; Vanderhaeghe, 1957). The more 

 recent experiments of Naora, Brachet and Naora (1959) have con- 

 firmed that enucleate fragments can easily incorporate such pre- 

 cursors as ^*C-adenine and ^^COg in their RNA. They further show 

 that the alga is capable of synthesizing free adenine and guanine in 



