108 ROLE OF THE CELL NUCLEUS 



Ficq (1956), who used ^^C-phenylalanine as a precursor instead of 

 3^S-methionine, are in substantial agreement with those of Mazia 

 and Prescott (1955). However, the differences between the nucleate 

 and enucleate fragments were much less striking, the ratio between 

 them being in the neighborhood of 2 (instead of 20) on the tenth 

 day after removal of the nucleus. In fact, this difference is largely 

 due to higher incorporation in the nucleus than in the cytoplasm of 

 amoebae. 



Finally, Tencer and Brachet (1958) and Brachet (1959), in their 

 study of the incorporation of ^^COg in amoebae, were able to com- 

 pare this process in RNA and proteins. The result was that RNA 

 lies under much closer nuclear control than proteins. In all experi- 

 ments, the removal of the nucleus had a stronger effect on the in- 

 corporation of the precursor into RNA than on that into protein. 

 Such a result is of course in keeping with the fact that in amoebae 

 enucleate fragments lose the larger part of their RNA, while the loss 

 in total protein is moderate. 



We shall now see whether similar observations can be made on 

 Acetabularia. A few words will first be said about the biological 

 observations which have been made on this very interesting organ- 

 ism; we shall then examine its RNA and protein metabolism. 



c. Acetabularia 



By far the most important experimental work done on this organism 

 is that of Hammerling and his co-workers (review by Hammerling 

 in 1953). The life cycle oi Acetabularia mediterranea is depicted in 

 Fig. 36 (p. 112). The alga is made of a chloroplast-containing stalk 

 and of rhizoids during most of its life. A single large nucleus, with 

 an extraordinarily developed and basophilic nucleolus, is located in 

 one of the rhizoids. Later, the tip of the stalk forms a "cap" or 

 umbrella which serves for the reproduction of the alga. When the 

 umbrella is almost completely formed, the nucleus breaks down and 

 small daughter nuclei spread through the whole alga (Schulze, 1939), 

 including the cap, where resistant forms (the "cysts") are formed. 

 These cysts contain a few nuclei and are surrounded by a thick 

 capsule. After a maturation period, they can be stimulated to ger- 



Text continued on p. 112 



