192 KARL O. LARK 



cycle of the animal as a whole and thus be under sonic humoral control 

 mechanism which does not involve the actual synthesis of DNA itself. 



Althoupih variation among individual cells is much less for animal cells 

 growing in culture, Tcrasima and Tolmach (1962) have noted that most 

 of the randomization observed at M in a synchronized population of 

 HeLa cells occurs during d. Moreover, animal cells growing in culture 

 are halted in Gi when the medium in which they are growing is exhausted 

 (Whitfield and Rixon, 1959). The length of the DNA cycle may also 

 change for an entire cell population when the growth conditions in 

 culture are altered. When this occurs, the change is expressed as a 

 lengthening or shortening of Gi rather than S or Gn (Richards et al., 

 1956). A similar situation may occur in bacteria where it has been shown 

 that major increases in growth rate brought about by enriching the 

 growth medium are accompanied by an increase in the average DNA 

 content per nucleus (Schaechter et al, 1958). This would indicate that 

 more DNA is now being synthesized in the early stages of the cell cycle 

 thus raising the average DNA content per nucleus. 



The data available would indicate that under most conditions, Gi, 

 the portion of the DNA cycle prior to synthesis, is the most susceptible 

 to change. 



C. EFFECTS OF TEMPERATURE AND RADIATION ON THE DNA CYCLE 



Although several workers have noted the effects of temperature on 

 the division cycle, few studies have been made of the effect of heat or 

 cold on DNA synthesis and these have yielded little information. 

 Zeuthen and Scherbaum (1954) synchronized tetrahymena by repeatedly 

 shifting the temperature from 29° to 34°C. At the latter temperature 

 cell division appears to be blocked, whereas DNA synthesis continued. 

 Subsequent experiments demonstrated that during such repeated cycles, 

 the cells doubled their content of DNA without undergoing division 

 (Scherbaum, 1957; Scherbaum et al., 1959) but that upon being returned 

 to their optimal growth temperature, division took place before further 

 DNA synthesis was resumed (thus reducing the DNA content). These 

 experiments indicate that the temperature block (in this case a heat 

 block) was operative during the pre-raitotic, or Ga period. 



Randomly dividing bacteria (S. typhimurium) grown at 25°C 

 have the same content of DNA as when grown at 37°, although the rate 

 of synthesis at the former temperature is about half of that at the latter 

 (Lark and Maal0e, 1956). However, when cells grown at 25°C are 

 shifted to 37°, a sudden burst of DNA synthesis occurs equal to approxi- 

 mately 30% of the cellular content of DNA. Synthesis is then arrested 

 for a short period. Repeated application of such temperature shifts at 



