540 ERIK ZEUTHEN 



at constant 28° C. 8-Azaguanine is nicely antagonized by guanine, guano- 

 sine, adenine and adenosine. 6-Methylpurine is antagonized by adenine 

 and by adenosine, not by guanine. 



The general picture invites the suggestion that base analogues interfere 

 much less readily with the cell's preparation for synchronous division at 

 the level of synthesis of the two nucleic acids than at the level of the co- 

 factors (GTP and ATP) involved in protein synthesis. This leads to the 

 hypothesis [13] that the temperature-shocked cells, while overcharged with 

 nucleic acids and proteins, are short of one or several proteins which are 

 specifically related to the process of cell division. According to the 

 hypothesis this situation is corrected by new synthesis after the termination 

 of the temperature shocks. 



3. Studies on DNA 



In view of suggestions made by Scherbaum [8] it is recalled that in 

 early work with Dr. E. Hofi-Jorsensen we [14] found close to constancy 

 of the ratio DNA /unit cell volume. As shown in Fig. 3 this is for the period 

 of shifting temperature when the average cell increases by a factor 2-5-3 

 and for the period of synchronous divisions during which the average cell 

 size regulates to normal. This work gave no indication of a special role 

 played by DNA in the induction by heat of the division synchrony. 



As a continuation of this work Dr. Rose Cerroni in our laboratory 

 independently made an observation also reported by Scherbaum [8]. 

 Tritiated thymidine (specific activity 1-9 c./mM, 5 /xc. per ml. cell 

 suspension, henceforth to be referred to as the standard dose) added to 

 populations of 50 000-100 000 cells /ml. label the nucleus of only a fraction 

 of all cells. We used the labelled compound undiluted with cold thymidine 

 externally added. The total amount of thymidine represented by the 

 standard dose is of the order of only i/io of the amount of DNA in all 

 cells in the sample. 



Whether logarithmic cells are studied in proteose-peptone (0-4 liver 

 extract) or whether they are transferred to the inorganic medium before 

 the addition of the labelled thymidine only 27-37% of all nuclei take 

 the label. These experiments suggest that the radioactive compound is 

 soon removed by the one-third to one-half of the cells in the ran- 

 domized population which can be expected (cf. [5] and [6]) to synthesize 

 DNA at any one time. This view is further supported by the observation 

 that all (98%) nuclei become labelled if three additional standard doses 

 of tritiated thymidine are given to the same cells for 4 min. at 50 min. 

 intervals. 



A standard dose of tritiated thymidine ofi^ered to the synchronized cells 

 in proteose-peptone again label only a fraction of the nuclei ; 20% when 



