Active Mitosis 

 Wilson and Hyppio (1955) pointed out that active mitosis involves 

 a series of events normally integrated but more or less independent of 

 one another. These include chromosome movement, spindle formation 

 and function, changes in the nuclear membrane, and chromosome mor- 

 phology. Experimental cytology provides the possibility of considering 

 these various events separately, thus determining their individual roles 

 in the total process. So far, the evidence indicates that a sequence of 

 changes in the nuclear boundary is associated both with the onset of 

 mitosis and with the transition from prophase to prometaphase. This 

 phase of active mitosis apparently occurs independently of an organized 

 spindle. The transition from prometaphase to metaphase to anaphase 

 appears to be, in part at least, a function of the spindle in relation to the 

 kinetochores of the chromosomes. Chemical treatments such as exposure 

 to actidione, colchicine, and iodoacetic acid can all be interpreted as 

 disrupting nuclear membrane changes, spindle organization, kinetochore 

 cleavage, or any combination of these. Morphological changes in the 

 chromosomes themselves, once initiated, appear to go to completion 

 spontaneously. 



Synthetic Phase 

 In general, following active mitosis, daughter cells are essentially 

 "half" cells. Before such cells can re-enter mitosis they must grow and 

 in the process synthesize many essentials. Exactly what is synthesized 

 and what the location and manner of synthesis may be are questions for 

 which we have no clear answer. The fact that nucleoli reappear in the 

 telophase or early interphase nucleus suggests synthetic activity involv- 

 ing both protein and RNA. The use of radioactive tracers has tended to 

 confirm the synthesis of RNA in the nucleolus during telophase - inter- 

 phase. Autoradiographic studies using tritium-labeled cytidine have 

 shown that incorporation into the nucleolus occurs in root tip cells of 

 Vicia faba as soon as it appears in late telophase, and continues during 

 all stages of interphase and into prophase (Taylor and Woods, 1959). 

 Many experiments with radioactive precursors, especially tritiated thymi- 

 dine, indicate that DNA synthesis reaches a peak fairly early in inter- 

 phase. The situation with regard to cytoplasmic or nuclear protein 

 synthesis has not been studied in detail. Cells and nuclei do apparently 

 increase in volume through most of the interphase (Leak and Wilson, 

 1960); however, how much of this increase is the result of actual 

 synthesis of protein or RNA is not known. The most active period of 

 protein synthesis during meiosis in Lilium longiflorum, as measured by 



MECHANICS AND PHYSIOLOGY OF CELL DIVISION / 139 



