Units of DNA 

 per cell 



Fig. 2. DNA synthesis during the cell division cycle. 



dumbbell shape. The dumbbell constricts in the middle and the two 

 daughter nuclei pinch apart. This is called amitosis. Amitotic division is 

 restricted to a few microorganisms and some cells of higher animals. As 

 you may know, certain tissue cells grow only in the young animal and 

 cease dividing in the adult, and it is interesting to note that their last few 

 divisions are very often amitotic. Degenerating or abnormal tissue cells 

 (e.g., tumors) also appear to divide amitotically. 



EXTRA-NUCLEAR ELEMENTS 



Some organelles of the cell seem to be partly or completely inde- 

 pendent of the nucleus. For example, the long whip-like flagellum seen 

 in many protozoa is formed from a small granule at its base called a 

 blepharoblast or basal granule. When certain protozoa divide, the old 

 flagellum disappears and the basal granule splits in two. Each daughter 

 cell receives a daughter granule and at the completion of cell division 



9 



