194 KARL n. LARK 



lIowi'NiT, ii'crnt t'\'ifloii('e has (k'lnun.-^tratiMl that ladiatioii can, uiidcr 

 certain circunii-tanccs, initiate DNA synthesis in onion root ti])S before 

 the normal onset of S (Das and Alfert, 1961a). Moreover, the rate of 

 DNA synthesis early in 5 can be stimulated in both plant and animal 

 cells by X-radiation (Alfert and Das, 1961). A similar ob.servation was 

 made on irradiated intestinal epithelium (Sherman and Quastler, 1960) 

 who observed that cells in N matuicd early following irradiation. These 

 results are consistent with tlie hypothesis that some chromosomes are 

 synthesized earlier in S than otliers and that replication of umhiplicated 

 chromosomes may he initiated by irradiation. Presumably tliis could 

 occur by conversion of their DNA to a primer state. 



More infomiation should become available with the study of the 

 effects of physical changes in the environment of cells that have been 

 synchronized by selective methods. Such studies have been initiated by 

 Terasima and Tolmach (1962), who have studied the effect of tempera- 

 ture and X-radiation on cultures of HeLa cells synchronized by a 

 selective method. Preliminary results have indicated that a period of 

 reduced temperature (29°C) during Gi, or S will delay mitosis, whereas 

 Go does not appear to be as temperature dependent. X-radiation during 

 S w'ill prolong this period of DNA synthesis, whereas radiation during 

 Gi is without effect. In general, radiation was found to delay or block 

 mitosis. Most striking, perhaps is the finding that cells about to enter 

 S are most sensitive to the lethal effect of X-radiation and tliat tliis 

 sensitivity can be greatly increased by the addition of dcoxyadenosine 

 late in Gi to block DNA synthesis and entry into S (Terasima and 

 Tolmach, 1962). 



Measurement of the rate of DNA synthesis during S in synchronized 

 HeLa cells has shown that it is not constant (Terasima and Tolmach, 

 1962). This result con-fii'ms the indirect observations of Howai'd and 

 Dewey (1961) and Painter et al. (1960), who came to a similar conclu- 

 sion on the basis of variation in the grain count of cell labeled during 

 random growth. 



D. THE EFFECT OF THE NUTRIENT ENVIRONMENT 



Temporaiy deprivation of thymidine will result in synchronization 

 of cell division and DNA synthesis in both bacteria (Earner and Cohen. 

 1956) and in animal cells, i.e., HeLa (Rueckert and Mueller, 1960). In 

 the latter case, cell division ceases upon treatment with the thymidine 

 analog fluorodeoxyuridine (FI'^DR), whereas attempted DNA synthesis 

 continues for a short period resulting in the formation of a "DNA-like 

 material." Readdition of thymidine results in resumption of DNA 

 synthesis in two steps, just before and shortly after cell division. This 



