156 CELLS, TISSUES, AND ORGANISMS 



of enzymes. Another possible suggestion, in fact made before (Zeuthen, 

 1958), is focused around the suggestion that the products which pile up 

 from division to division, and thereby condition division, are structur- 

 ally complex, hydrogen-bonded, and in their folding very sensitive to 

 temperature changes. Heat or cold might make a developing structure 

 useless by different physical mechanisms: cold might cause overfolding 

 and heat might disrupt enough weak bonds to make a structure useless 

 forever, or for some time. But heat and cold might also work on sepa- 

 rate structures, both of which are essential in cell division. Before a 

 synchronous division there is a rather critical time-point when the addi- 

 tion of p-fluorophenylalanine, 6-methylpurine, and 8-azaguanine no 

 longer stops or delays division. We have assumed that these agents in- 

 terfere with protein synthesis and that they do so rather immediately. 

 The critical time-point therefore signals completion of protein synthesis 

 in preparation for the following division. 



We have recently observed that in the synchronized cell there is a 

 sharp drop in sensitivity to heat shocks around the same critical point 

 at which the cells cease to be sensitive to the three analogues. In our 

 laboratory this has been confirmed by Dr. Joseph Frankel, who has also 

 found that the sensitivity to cold shocks (8° C, 40 minutes) follows 

 the same time pattern. It is an obvious possibility that at the critical 

 point two macromolecules, of which at least one is a protein, combine. 

 For this reaction to take place, the structural configuration of both 

 molecules is critical. The coiling or folding of both may be sharp func- 

 tions of temperature only with different transition points {i.e., from 

 folded to unfolded ) on the temperature scale. At increased temperature 

 the one partner is unsuitably unfolded; at decreased temperature the 

 other is too folded. In both cases reaction between the two is inhibited. 

 The combined molecule is insensitive to temperature changes, thus 

 structurally far more stable than the two molecules from which it is 

 composed. It conditions the later division and first visibly expresses it- 

 self by causing oriented movements of the kinetosomes at the level of 

 the new mouth. At division it would dissociate, otherwise dissolve, or 

 become incorporated in the structure of the new cells. 



References 



Chantrenne, H., 1958. "Nucleic-Acid Metabolism and Induced Enzyme Forma- 

 tion," Recueil des Trav. Chim. des Fays-Bos 77, 586. 



Christensson, E., 1959. "Changes in Free Amino Acids and Proteins During Cell 

 Growth and Synchronous Division in Mass Cultures of Tetrahymena pyri- 

 formis" Acta Physiol. Scand. 45, 339. 



Flavin, M., and Graff, S., 1951. "Utilization of Guanine for Nucleic Acid Biosyn- 

 thesis by Tetrahymena geleii," J. Biol. Chem. 191, 55. 



Hamburger, K., and Zeuthen, E., 1957. "Synchronous Divisions in Tetrahymena 



