194 BO THORELL 



of Tetrahymena, have recently induced 85 % of the cells to divide simul- 

 taneously. During the period of heat treatment growth was not inhibited; 

 for example, the DNA content per cell increased up to four times. This 

 material thus seems excellent for the study of the division processes as 

 several milligrams of dry substance can easily be obtained. 



The increase in the nucleic acid content of individual cells during the 

 cycle has been studied by different cytochemical techniques. From ultra- 

 violet absorption measurements at different stages in the spermatogenesis 

 of Gomphocerus, Caspersson^'^ calculated an increase in the nuclear nucleic 

 acids from early prophase to metaphase from about 16 to 26 X 10~^ mg. 

 Similar results are reported by Ris.^^ From the different shapes of the ab- 

 sorption curves, Caspersson calculated a change in the ratio of nucleic acids 

 to protein from 1 : 20 in the early prophase chromosomes to 1 : 3 in the 

 metaphase chromosomes.*^^'^^'^* During telophase-interphase the changes 

 were reversed. As a result of the nature of the method and the materials 

 used, however, the values mentioned above may be regarded more as an 

 expression of general changes in density than as nucleic acid and protein 

 changes. 



Particularly suitable material has been studied by Walker and 

 Yates^-"^^"^'^^*^ in the form of actively dividing tissue culture cells which 

 were investigated directly under the ultraviolet microscope. Taking into 

 consideration the sensitivity towards ultraviolet irradiation, the cells were 

 observed by phase-contrast microscopy and the ultraviolet absorption was 

 measured only at certain defined stages of the cycle. In spite of the relative 

 unspecificity of the ultraviolet-absorption microtechnique, a fair agree- 

 ment was found in certain materials between the amount of DNA calcu- 

 lated from the absorption at 265 m/x and values from the literature for total 

 DNA-phosphorus in known numbers of nuclei. No sudden increase in ultra- 

 violet-absorbing material could be demonstrated during prophase, but a 

 continuous augmentation was observed during the interphase. When the 

 ultraviolet absorption and the Feulgen-staining intensity were compared, 

 the authors concluded that the actual increase in DNA occurred during 

 interphase. The difference observed between the total nuclear ultraviolet 

 absorption and the Feulgen color was intei'preted as being due partly to 

 the presence of precursors of nucleic acids. 



Indirect evidence of DNA synthesis during interphase in growing cells 

 has been put forward by Swift and others,-^'^^^'^-^ on the basis of micro- 

 photometric estimations on individual nuclei stained by the Feulgen reac- 

 ts T. Caspersson, Chromosoma 1, 147 (1939). 

 "9 P. Walker, Discussions Faraday Soc. 9, 497 (1950). 

 '20 P. Walker, Heredity Suppl. 6, 275 (1952). 

 1" H. Swift, Anat. Record 105, 497 (1949). 

 122 H. Swift, Proc. Natl. Acad. Sci. U. S. 36, 643 (1950). 



