THE MITOTIC CYCLE 



cells approaching necrosis, a high content of nucleic acids is said to 

 persist after the mechanism for protein production has broken down 

 (Caspersson^^). 



In the normal growth of the cell, Caspersson maintains that the 

 nucleoli and the adjacent heterochromatic chromosomal material are 

 particularly concerned with protein synthesis; this thesis is supported 

 in his paper in 1941 by the following assertions: 



1. Nucleoli are large in all the basophilic protein-forming cells 

 which we have listed above, and also in rapidly growing malignant 

 cells. 



2. That 'heterochromatin' is involved is shown by the fact that 

 the amount of cytoplasmic nucleic acid in an egg cell of Drosophila is 



■^ 



50.000 



Volume of oocyte (a ) 



Figure 4 Comparison of the amount of substances absorbing 

 ultraviolet light at 2,570 A in the oocytes of an XX and an XXY 

 female of Drosophila melanogaster ior varying volumes of these cells. 

 Crosses represent the values from XXY ; points, those from XX 

 ovaries. From Caspersson and Schultz** {By courtesy, Nature). 



increased when an extra Y-chromosome has been incorporated into 

 the nucleus (Caspersson and Schultz^^) (Figure 4). Whether this 

 argument, however, can be generally applied depends on how far 

 'heterochromatin' is a constant entity (p 47). 



3. In some cells a zone of high nucleic acid content surrounds the 

 nuclear membrane. Caspersson's view of the bearing of this on the 

 function of the nucleolus may best be given in his own words taken 

 from a later review.'^ 



A certain part of the chromatin, we call it the nucleoliis-associated chromatin, 

 secretes substances of a protein nature. These are very strong indications that they 

 contain considerable amounts of diamino acids. These substances accumulate and 

 form the main bulk of the large nucleolus. From the nucleolus they difTuse towards 

 the nuclear membrane, on the outside of which an intensive production of ribose 



14 



