IV. CELLULAR CONTROL OF DNA BIOSYNTHESIS 155 



interest in the role of DNA in regulating the cell's physiological behavior. 



A large amount of information is now available relating to the 

 control of DNA biosynthesis, but little of it has been obtained with this 

 end in view. In what follows, material from different sources, investi- 

 gated originally in a variety of contexts, is discussed. This will lead to 

 the inevitable criticism that results are not strictly comparable and 

 that, therefore, general conclusions may not be drawn. However, what 

 follows presents a definition of the problem, the current status of its 

 investigation, and the direction in which it appears that further investi- 

 gation can profitably be directed. 



It is intended to represent one individual's analysis of the situation 

 rather than a comprehensive review of the literature. 



A. THE DNA CYCLE 



Recent studies have led to the general conclusion that during the 

 division cycle of higher cells there exists a period following mitosis 

 during which DNA synthesis does not take place (Gi), a subsequent 



DNA Synthesis 



Division 



Fig. 1. A representation of tlie DNA synthetic cycle in relation to the cell 

 division cycle. Division, or the end of mitosis, marks the beginning of the cycle. 

 A gap, Gi, occurs before the onset of DNA synthesis, S. DNA synthesis is followed 

 by another period, Gi, prior to the onset of mitosis. (Adapted from Stanners and 

 Till, 1960.) 



period of DNA synthesis during which the DNA content of the inter- 

 phase nucleus is doubled (S), a second period during which DNA is not 

 synthesized iG-z), and, finally, a period of mitosis {M) (Howard and 

 Pelc, 1953) (Fig. 1). Although the duration of these periods may vary 

 from system to system, they are often remarkably constant from cell to 

 cell within the same system. [In bacteria, it has been possible to demon- 

 strate that DNA is synthesized throughout most of the cell cycle 

 (Schaechter et al., 1959; Young and Fitz-James, 1959; jVIcFall and 

 Stent, 1959; Abbo and Pardee, 1960). However, recent evidence has 

 indicated that even in these systems a DNA duplication cycle may exist 



