156 



KARL G. LARK 



(Maalpc and Haiiawalt, 1961; Hanawalt et al., 1961; Goldstein et al., 

 1959). It seonis possible, therefore, that such systems may represent 

 situations in which Gi and Go may be of extremely short duration 

 relative to >S. This is made more probable by the finding that G'l or Go 

 may be extended in such systems by simple chemical or physical treat- 

 ments which also synchronize the cell population (Lark and ]\laal0e, 

 1956; Earner and Cohen, 1956; Maruyama, 1956; Scott and Chu, 1958; 

 Maruyama and Lark, 1959).] 



The existence of a cycle such as that in Fig. 1 has been demonstrated 

 with autoradiography (Howard and Pelc, 1953; Lajtha et al., 1954; 

 Hornscy and Howard, 1956; Taylor et al., 1957; Firket and Verly, 1958; 

 Mendelsohn et al., 1960; Painter and Drew, 1959; Stanners and Till, 

 1960; Prescott, 1960; Edwards et al, 1960; Sisken and Kinosita, 1961) 

 by labeling cells at a given time with a radioactive precursor of DNA 

 (usually tritiated thymidine) and then measuring the period of time 

 required for the first labeled mitotic figures to appear subsequently as 

 well as the proportion of cells so labeled or the rate at which radioac- 

 tivity, incorporated during interphase, appears in metaphase mitotic 

 figures. These measurements yield the time elapsing between the end of 



^250- 



2 4 6 8 10 12 14 16 18 20 

 Hours after addition of tritiated thymidine 



Fig. 2. The peak metajihase grain count values from autonuiiograplis of cells 

 sampled at various times after addition of thymidine-H^ to L-strain mouse cells 

 growing in tissue culture. The upper curve represents a film exposure of 15 days; 

 the lower, 8. Peak counts for each sample were obtained using the peak grain count 

 values from a frequency distribution of the grain count for each sample. (Number of 

 metaphases as a function of their inilividual grain count.) G2 is the period before an 

 increase in grain count is observed. Tiie grain count rises throughout S, after which 

 it is maintained at a constant value. (From Stanners and Till, 1960.) 



