124 



S. S. COHEN 



of polyteny, virus infection, etc.). A few years ago, several groups of workers 

 presented evidence to suggest that in animal tissues twice as much P^^ 

 entered DNA as could be accounted for by the net increase in DNA content 

 at mitosis (Stevens et al., 1953; Daoust et at., 1954; Barnum et al., 1953). This 

 was taken to mean that mitosis involves a synthesis of two new DNA mole- 

 cules accompanied by a breakdown of one old DNA molecule. A reexamina- 

 tion of the methods employed, particularly that of the estimation of mitotic 

 index, revealed that in regenerating rat liver the rate of formation ofP^^- 

 labeled DNA could be accounted for by the net increase of DNA (Daoust 



15 " 



U <M- 



Q- 



48 



96 



144 

 (hr) 



192 240 



Fig. 24. Retention of P"^ in the RNA and DNA fractions of L strain mouse cells pro- 

 pagated in suspension by the rotating culture technique (Siminovitcli and Graham, 

 1956). 



et al., 1956). The hypothesis that pre-existmg DNA is replaced at mitosis by 

 twice its amount of DNA was not correct. The carbon, nitrogen, and phos- 

 phorus atoms of DNA appear not to be replaced in the life of normal cells. 



The metabolic stabihty of DNA has been confirmed for fibroblast cultures 

 (Healy et al., 1956; Thompson et al., 1956; Siminovitch and Graham, 1956), not 

 only by the method of correlating incorporation of label with net synthesis, 

 but also by demonstrating that label once incorporated into tliis fraction is 

 not lost but only diluted by new synthesis in the absence of isotope. Such a 

 result is presented for suspended cell cultures in Fig. 24 (Siminovitch and 

 Graham, 1956). It can also be seen that in this system and in this sense RNA 



