The nucleolus is generally described as being either a fluid, or a semi- 

 solid body. Most evidence favors the view that it is a colloidal structure 

 with the properties of a complex concervate. It may, however, possess 

 a relatively high density and protein content and should, in such cases, 

 be a more or less solid structure. RNA may exist in both a stable and a 

 labile form, and is apparently a consistent but variable component of the 

 nucleolus (Vincent, 1955). It also contains protein and lipid but there is 

 disagreement as to the amounts and types present. Its chemical composi- 

 tion probably varies widely from one cell type to another and with the 

 metabolic activity of the cell. Isotope-incorporation studies of both plant 

 and animal cells indicate a high rate of uptake of isotope into the RNA of 

 the nucleolus (see Figure 4-26), suggesting that this structure is an 

 active site of RNA synthesis in the cell (Brachet, 1957). The nucleolus 

 is apparently also a center of some protein synthesis as indicated by its 

 uptake of labeled amino acids. In certain cells, the enzyme involved in 

 the synthesis of the coenzyme diphosphopyridine nucleotide (DPN) is 

 located in the nucleolus (Baltus, 1954), which may implicate this struc- 

 ture in the synthesis of nucleotide coenzymes (Brachet, 1957). 



Chromocen+ers 

 There is some confusion between so-called prochromosomes, which 

 are specific segments of the chromosomes and appear as condensed, 

 deeply staining bodies during interphase, and true chromocenters, which 

 appear to be more or less separate globular bodies of similar staining 

 reaction scattered throughout the interphasic nucleus. Whether the latter 

 structures are continuous with or separated from the chromosomes is by 

 no means clear. In some species (e.g., the plant Trillium sessile) their 

 number and size are relatively stable, while in others both are highly 

 variable. There is some evidence that the total volume of chromocenter 

 material is constant and increases with an increase in number of chro- 

 mosomes. 



Chromosomes 



tAorphology 



When a cell is viewed in the living state, aside from nucleoli, the 

 nucleus frequently appears to be optically empty. In the fixed state, 

 however, coiled threads usually appressed to the inside of the nuclear 

 envelope may be detected quite readily (Figure 4-1). These struc- 

 tures are the chromosomes. The detailed morphology of the chro- 

 mosome varies from cell to cell and major changes are associated with 



STRUCTURE AND FUNCTION OF THE NUCLEUS / 87 



