152 



CELLS, TISStTS, AND ORGANISMS 



hours 



Figure 13. Synchronous cells pro- 

 duced by the standard procedure 

 (c/. text for Figure 1). Growth in 

 the broth is periodic for dry weight 

 and for phosphorus. During the cy- 

 cling temperature the cells suffered 

 a loss of 20 to 25 per cent in the 

 ratio of phosphorus to dry weight. 

 The percentage loss was identical 

 for all fractions. This figure shows 

 that correction after E.H. is gradual 

 through several division cycles. 

 (From Hamburger and Zeuthen, 

 1960.) 



If the washed synchronized cells are exposed to a new temperature 

 shock before 50 to 60 time units, the system becomes resensitized to 

 amino-acid analogues (ethionine, thienylalanine, p-fluorophenylala- 

 nine ) . This is a direct demonstration that heat induces the need for the 

 new synthesis of protein which, at 28° C, conditions a later division. 

 Heat will reduce the rate of incorporation of labeled histidine, showing 

 that it acts by reducing the rate of synthesis without necessarily in- 

 creasing the rate of hydrolysis of proteins. 



In protein synthesis, nucleic acids and low-molecular co-factors are 

 required. RNA'ase ( Sigma ) in high concentrations ( 100 /xg/ml, boiled ) 

 is a strong inhibitor of cell division in the washed cells. There is at 

 present no evidence from experiments with anti-metabolites that the 

 cycling heat produces such damage to or causes such shortage of any 

 of the nucleic acids that repair or new synthesis is required before di- 

 vision can take place: Certain drugs used— azathymine (5 X 10"^ M), 

 5-bromouracil ( 0.3 — 16 X 10"^ M ) , 5-fluoro-2'-desoxyuridine ( 0.25 — 

 3.8 X 10-^ M), a-denopterin (6 — 13 X 10"* M), 5-fluorouracil (20 — 

 4 — 0.8 X 10"* M)— did not have significant delaying eflFects on the first 

 synchronous division when they were added to the washed cells im- 

 mediately after E.H. or later (see Figure 14). Later experiments in 



