incorporation of glycine — C", appears to be during premeiotic inter- 

 phase and leptotene, with a low level being reached by the beginning of 

 zygotene (Taylor, 1959). The synthesis of DNA was found to occur 

 only during premeiotic interphase, while RNA synthesis took place in 

 both the nucleus and cytoplasm during interphase and up to late lepto- 

 tene. After late leptotene practically all synthesis of RNA occurred in 

 the nucleus and at a slower rate until the end of prophase. 



An+ephase 

 In the model given in Figure 6-1, c-a is the segment of the mitotic 

 cycle which Bullough (1952) has designated as antephase. This stage 

 would appear to represent, as already noted, the stage at which the 

 cell is competent to enter active mitosis. Experiments with excised pea 

 roots have shown that even in the absence of an exogenous carbohy- 

 drate source this fraction of the population does indeed ultimately pass 

 into active mitosis. This presumably means that it is not a stage requiring 

 high energy output, though the rate of entrance of cells into active 

 mitosis can be greatly increased by the addition of a suitable carbohy- 

 drate source (i.e., glucose, fructose, sucrose). Furthermore, it should be 

 noted that the conditions which are favorable for rapid entrance of 

 antephase into active mitosis are only moderately suitable for transition 

 from the synthetic phase (b to c) to antephase (c to a). The exact 

 nature of any of these transitions is still largely unknown. Presumably 

 hormonal control plays a role in relation to the utilization of various 

 carbohydrate sources for support of mitotic activity. Bullough (1955) 

 has shown in animal cells that a number of hormones affect the utiliza- 

 tion one way or another of glucose but not of fructose. Our own experi- 

 ments indicate that certain trioses (i.e., D-glyceraldehyde) function very 

 well as carbon sources, but give a quite different pattern of response 

 than the hexoses (Wilson, et al., 1959). The fact that mitotic activity 

 can be initiated and maintained by a variety of carbon sources leads to 

 the possibility that, ultimately, comparative studies will uncover the 

 pathways by which these sources are utilized by the different segments 

 of the mitotic cycle. 



SPINDLE MECHANISM 



Historically speaking, the problem of the mechanism of mitosis has 

 been very closely associated with questions concerning the origin, na- 

 ture, and function of the mitotic spindle. Early work focused attention 

 on the centrosome as playing a leading role both in the formation of 



140 / CHAPTER 6 



