IV. CELLULAR CONTROL OF DNA BIOSYNTHESLS 



159 



Wildy, 1959; Nygaard et at., 1960; Williamson and Scopes, 1960; Abbo 

 and Pardee, 1960; Burns, 1961; Terasima and Tolmach, 1962). Figure 4 

 (open circles) shows the course of DNA synthesis as well as of mitosis 

 in dividing microspores of the anther of Trilliiun (Erikson, 1948; Stern, 

 1960b; Hotta and Stern, 1961b). Chemical analyses were performed on 

 samples of isolated microsj^ores. It should be noted that the amount of 

 DNA synthesized is exactly equal to that initially present. 



.E 0.5 - 



0) w 



*" o 



Fig. 4. DX.\ syntliesis in tlie microspores and deoxyrilaoside production in the 

 antlier of Tnllium. Synchronous de\eloiiment of tlie microspores occurs and as a 

 result DNA doul^les during a defined period of interphase. A double peak of deoxy- 

 riboside production hy the anther is observed at this time. The possibility exists that 

 the second peak may represent an overshoot in microspore DNA precursor produc- 

 tion (see text, p. 163). (From Hotta and Stern, 1961b.) 



All of these experiments serve to demonstrate the temporal control 

 outlined in Fig. 1. A similar situation is sometimes seen within a single 

 cell containing more than one type of nucleus or in a single, very large 

 nucleus. Thus, in the ciliate Euplotes (Gall, 1959) synthesis of DNA 

 in the large U-shaped macronucleus (which is a functional nucleus in 



