150 A. N. BELOZERSKY AND A. S. SPIRIN 



sion, was analyzed, it was found that during this period, in particular at 

 the end of the lag-phase, considerable DNA synthesis occurred in the cells, 

 and thus an increase of DNA content per cell was observed. 17 ' 20 ' 23 ' 2528 

 Just prior to cellular division the DNA content in the cells was found to 

 double. 17 ' 20 



The method of synchronous culture has given particularly valuable data 

 on the dynamics of the DNA content during the cellular division in micro- 

 organisms. In Escherichia coli, under the conditions of such synchronous 

 cell division, it was shown that the DNA content per cell remains constant 

 almost during the entire period, with the exception of a relatively short 

 period just prior to division 29 ' 30 or at the very beginning of the latter. 31 

 During this short interval, the DNA content in the cell doubles, so that the 

 subsequent cellular division again leads to the usual DNA content, constant 

 for the given kind of cells. 2931 The same regularities were shown, recently, 

 for the synchronous culture of Corynebacterium diphtheriae. 32 Much earlier, 

 Ogur et a/. 33 studied synchronously budding yeasts. They showed that DNA 

 synthesis proceeds, only, either prior to or during the budding. If the buds 

 are not counted separately, the DNA content per cell doubles at the end 

 of the first budding cycle. If the cells are counted in such a manner that each 

 bud is counted as an independent unit, the DNA content proves to be con- 

 stant. Thus, the constancy of the DNA content per nucleus seems to be 

 completely preserved, whereas the increased DNA content in the budding 

 cell is due to the appearance of an independent nucleus in the bud. 



The alga Chlorella was also studied by means of the method of synchro- 

 nous culture. 34 Here, also, it was shown that the DNA content per nucleus 

 was constant in the course of growth. DNA synthesis proceeded only prior 

 to the cellular division, during the stage of "ripening of light cells." 



Thus, all the data presented show that the DNA content in the cells of 

 microorganisms is constant, with the exception of the period just prior to 

 cellular division. 



There seem to exist some special cases, however, when the DNA content- 

 in the cells of microorganisms can alter considerably. Wacker et al. 3b re- 



25 P. C. Fitz -James, Can. J. Microbiol. 1, 525 (1955). 



26 K. G. Lark and O. Maal0e, Biochim. et Biophys. Acta 21, 448 (1956). 



27 N. A. Eliasson, E. Hammarsten, H. Palmstierna, S. Aqvist, and L. Reio, Acta 

 Chem. Scand. 11, 1381 (1957). 



28 J. H. Stuy, J. Bacteriol. 76, 179 (1958). 



29 H. D. Barner and S. S. Cohen, Federation Proc. 14, 177 (1955). 



30 H. D. Barner and S. S. Cohen, J. Bacteriol. 72, 115 (1956). 



31 Y. Maruyama, J. Bacteriol. 72, 821 (1956). 



32 T. Sail, S. Mudd, and A. Takagi, /. Bacteriol. 76, 640 (1958). 



33 M. Ogur, S. Minckler, and D. O. McClary, J. Bacteriol. 66, 642 (1953). 



34 T. Iwamura, J. Biochem. {Japan) 42, 575 (1955). 



35 A. Wacker, D. Pfahl, and I. Schroder, Z. Naturforsch. 12b, 510 (1957). 



