MATERIALS AND METHODS 95 



diate effects on cellular activities, although the latter decrease 

 sooner or later. For Mazia (1952), two alternatives are possible. In 

 the first, the nucleus is the site of enzyme synthesis, as in Wilson's 

 (1925) hypothesis. Removal of the nucleus should then lead to a 

 continuous drop in the enzyme content of the cytoplasm. Such a 

 drop would not necessarily occur at the same rate for all cytoplas- 

 mic enzymes, since the latter might be synthesized in different parts 

 of the nucleus. It is also possible, according to Mazia, that enzymes 

 which are functionally related decline together if their replacement 

 mechanisms in the nucleus are somehow related. Still another pos- 

 sibility is that the replacement of coenzymes is a function of the 

 nucleus. 



In the second of Mazia's (1952) alternatives, the product of nu- 

 clear activity might be a cytoplasmic unit. The latter, which would 

 be comparable to a plasmagene, would play a role in cytoplasmic 

 synthesis ; but it would require continuous replacement by the nu- 

 cleus for its maintenance in the cytoplasm. A similar idea had been 

 expressed before by Wright (1945) and by Marshak (1948); one of 

 its consequences is that the losses of activity after removal of the 

 nucleus would be discontinuous instead of gradual. The disconti- 

 nuities v/ould occur at the time when the synthetic units were 

 exhausted. 



2. AVAILABLE MATERIALS AND METHODS 



The ideal method is, of course, to work with living intact cells. This 

 has become feasible now that good autoradiography techniques 

 have become available. It is possible to introduce into the cell a 

 specific precursor of nucleic acids or proteins and to follow its in- 

 corporation in the nucleus and the cytoplasm. As we shall see, this 

 method has already yielded a number of important results. But the 

 drawbacks of autoradiography, especially for quantitative work, 

 should be kept in mind. It is seldom possible to measure the specific 

 radioactivity by this method, because it is often unknown how much 

 of the free precursor is present in the part of the cell which is being 

 studied ; furthermore, the exact composition of the nucleic acids or 



References p. 133/135 



