202 



Alma Howard 



of three tissues which have been studied by means of auto- 

 radiographs. In the bean root meristem, (Fig. 1 A), there is a 

 Gi period of up to 12 hours; the number of cells whose DNA 

 becomes labelled with ^^P remains normal for at least 6 hours 

 after irradiation (Howard and Pelc, 1953). In the Ehrlich 



TIMING OF MITOTIC CYCLES 



M 



M 



M 



BEAN ROOT MERISTEM 

 T-30 



B 



HUMAN BONE 

 MARROW IN 



CULTURE. 

 T- 40-45 



MOUSE EHRLICH 

 ASCITES TUMOUR 

 T»I8 



Fig. 1. Mitotic cycles deduced from autoradiograph studies. Time in hours. 

 M = metaphase ; D = mitotic division ; S = period of uptake of isotope into 

 DNA; Gj and G2 = periods in early and late interphase during which DNA 

 does not become labelled; T = total length of mitotic cycle. 



A. Bean root meristem. ^^p 

 (Howard and Pelc, 1953.) 



B. Human bone marrow. ^^P or [8-^^C]adenine 

 (Lajtha, Oliver and Ellis, 1954.) 



C. Mouse Ehrlich ascites tumour. [8-^*C]adenine 

 (Hornsey and Howard, 1956.) 



mouse ascites tumour (Fig. 1 C), no G^ is observed ; within 

 2 hours after irradiation there is a decrease in the num- 

 ber of cells taking up [8-^^C]adenine into DNA (Hornsey 

 and Howard, unpublished). If measured biochemically, the 

 mitotic delay in the tumour would appear as a reduction in 

 DNA turnover and specific activity, compared with controls, 

 although the amount of DNA per cell would increase for a 

 time equal to Gg (see Fig. 1), and remain slightly greater than 

 controls until mitosis reappeared. Such effects are not 



