96 ROLE OF THE CELL NUCLEUS 



proteins present in this part of the cell is usually unknown. To take 

 a specific example, let us suppose that autoradiography shows no 

 incorporation of phenylalanine into the nucleolus. This might mean 

 that the nucleolus is inactive in protein synthesis, but it might 

 equally be due to the fact that free phenylalanine cannot penetrate 

 into the nucleolus (lack of permeability to this amino acid) or to 

 the absence of phenylalanine in the proteins of the nucleolus. 



A second way of studying the biochemical role of the cell nucleus 

 is "merotomy" : a unicellular organism of sufficient size is cut into 

 two halves and the metabolic activities of the nucleate and enucleate 

 halves are compared. Merotomy is easy to perform on amoebae, on 

 the large alga Acetabularia, and on sea urchin or amphibian eggs. 

 Most of the results which are presented later in this chapter were 

 obtained by this method. One should also mention the interesting 

 case of the reticulocytes, for which the enucleation is performed by 

 nature itself during maturation of the red blood cells. 



Finally, it is of course possible to work on homogenates. The 

 advantage is that large amounts of nuclei can be obtained, so that 

 micromethods are not a prerequisite, as is the case for the unicellular 

 organisms. But the homogenate method suffers from several draw- 

 backs, such as possible loss or adsorption of enzymes {e.g. phos- 

 phatase) during isolation. In the absence of adequate electron 

 microscopic observations, it is difficult to be sure that the isolated 

 fractions are really identical with the preexisting intracellular 

 granules ; rupture of nuclei or mitochondria, as well as aggregation 

 of smaller cytoplasmic particles, are difficult to avoid completely. 

 Even after very careful work, it is therefore impossible to be certain 

 that the fractions obtained are cytologically homogeneous. 



The great limitations of the homogenate method become apparent 

 when, as in this chapter, the interest shifts from a mere description 

 of the chemical composition of the several types of cell organelles 

 to the more dynamic approach exemplified by the question: what 

 is the nature of the interactions which occur between the various 

 constituents of the cell, and between the nucleus and the cyto- 

 plasmic particles in particular? It is perfectly legitimate for a bio- 

 chemist to mix together the various fractions obtained by differen- 



