IV. CELLULAR CONTROL OF DNA BIOSYNTHESIS 193 



properly spaced intervals results in phasing of DNA synthesis and in 

 synchronization of division. These results indicate that the population 

 of cells growing at 25° can be divided into two classes on the basis of 

 their ability to synthesize DNA when placed at 37° and that their DNA 

 cycles may be divided into portions of different temperature dependence. 

 A similar result was found with another bacterial system {B. mega- 

 therium) which continued to synthesize DNA at a rapid rate for a short 

 period of time when shifted from 34° to 15° (Falcone and Szybalski, 

 1956). In this case, all other cell growth parameters were immediately 

 affected by the lower temperature. Returning the temperature to 34° 

 resulted in synchrony of cell division and phasing of DNA synthesis 

 (Delamater, 1956). This again would indicate the existence of at least 

 two portions of the DNA cycle with different temperature dependencies. 



Hotchkiss (1954) found that the competence of pneumococci to be 

 transformed by DNA was increased by a short period of incubation at 

 25°. This resulted in a synchronization of cell division and a phasing of 

 competence. The latter, however, occurred in cycles whose frequency was 

 somewhat different from those of cell division. Whether the cycles of 

 competence are related to DNA synthesis, to adsorption of transforming 

 DNA (Lerman and Tolmach, 1957), or to the ability of the transforming 

 DNA to successfully survive within the cell until it is integrated (Fox 

 and Hotchkiss, 1960) is not known. Further investigation of this system 

 to ascertain the fate of the "infecting" DNA as a function of the host 

 cell DNA cycle should be extremely profitable. 



The DNA cycle of HeLa cells synchronized by cold shock differs 

 from that of randomly dividing cells not subjected to cold (Smith et al., 

 1959). These results indicate that tw^o populations of cells may be 

 created by their treatment — one whose DNA is synthesized as in cells 

 not exposed to cold (toward the end of the cycle), and the other com- 

 posed of cells in which Gi has been almost eliminated and DNA is 

 synthesized immediately after division. 



The meager information on the effects of temperature on the DNA 

 cycle indicates that different portions of the cycle may have different 

 temperature coefficients and that the DNA cycle may be dissociated to 

 a certain extent from the cell division cycle. 



As mentioned previously, similar suggestions have resulted from 

 studies with X-radiation. In general, exposure to X-radiation appears to 

 block cells in G2 and prevent mitosis. Cells which have not, at the time 

 of irradiation, synthesized DNA proceed to do so before the cycle is 

 halted although, in some cases, the onset of DNA synthesis is delayed 

 (for reference see Seed, 1961; Whitmore et al.. 1961; and Quastler, 

 1962). 



