CELL DIVISION AND PROTEIN SYNTHESIS 539 



less in parallel during the period of blocked cell division, and at least by 

 more than a factor of 2. The base ratios in both nucleic acids remain 

 constant [7]. 



2. Studies with base analogues 



While rather extended biochemical studies of the Tetiahxmena svstem 

 have vielded much valuable information, they have not greatlv helped us 

 to understand the biochemical mechanisms bv which temperature changes 

 induce the di\ ision synchrony. Figure i shows that no cells divide during 

 the first hour which at constant 29 C. follows the period of changing 

 temperature. This observation, extended in careful studies bv Thormar 

 [11], shows that recovery from temperature damage must take place before 

 the cells di\ ide. The damage brings all cells into a common situation with 

 respect to their preparation for subsequent division. We have studied the 

 recovery by the use of antimetabolites of various sorts. The cells are 

 synchronized in a 2% proteose-peptone medium, fortified with o-i",, 

 (more recently with o-4'^o) hver extract L (Wilson Labs.). In some 

 experiments we applied an extra period of elevated temperature (an extra 

 "temperature shock"). Before the shock the organic medium was replaced 

 with a simple inorganic medium [2]. The cells divide in standard time after 

 the last shock, whether this is applied in the proteose-peptone or in the 

 inorganic medium. Cells in the latter medium shall be referred to as 

 "washed cells". 



The time of maximum engagement in divisions i, 2, and 3 (proteose- 

 peptone medium) and in division i, often 2 ("washed cells") can be 

 determined with great accuracy. Consequently, it is possible to quantitize 

 the division-delaying effect of an antimetabolite which is added at a 

 defined time before division. The changes in the division index is followed 

 in up to twenty dishes each of which holds i ml. of the population. Fre- 

 quent visual inspection of each dish is made at noted times. The percentage 

 of cells in fission is quickly estimated. Curves through the estimates permit 

 that we fix accurately ( ± < 3 min.) the time when in a dish a maximum of 

 cells show fission. Counting is not necessary for sound estimates to be 

 made. As a check of the method we have established that the results of 

 tw^o or more independent observers agree nicely. The effect of an anti- 

 metabolite is given by the delay of division relative to the proper control. 

 Because many dishes are followed drug concentrations and other factors 

 are easily varied in parallel runs. 



Out of a considerable number of purine- and pyrimidine-analogues 

 tested [13] only 8-azaguanine and 6-methylpurine were found to inhibit 

 the first synchronous division in the washed cells. Furthermore, the two 

 analogues were inhibitors of this division only when added before the 

 lapse of about half the time which the controls require to prepare division 



