196 BO THORELL 



further investigation. It has been shown that intact, dividing chick fibro- 

 blasts possess a region between the chromosomes at anaphase which ab- 

 sorbs ultraviolet at 265 mju to a significantly higher extent than does the 

 adjacent cytoplasm. ^^* 



2. Turnover Studies 



Studies on the metabolism of the chromosomal nucleic acids in relation 

 to mitosis have so far been concerned mainly with two questions: Is there 

 a stable state of the nucleic acids in the resting nucleus or not, and, sec- 

 ondly, during which stage of the cell cycle are the nucleic acids synthesized 

 for the next division? 



The PNA-fraction of mammaUan cells and nuclei has in most instances 

 been found to be rapidly and extensively renewed. ^^^-^^ Whether this also 

 applies to the chromosomal PNA is unknown. The question of the bio- 

 chemical stabihty of DNA will be discussed at length in Chapter 26, but a 

 few of the salient features may be briefly summarized here. 



The use of P^^ j^as indicated that there is no significant turnover of the DNA-phos- 

 phorus in resting tissue representing a stable stage of the cell cycle"^''^*'^' but the 

 incorporation of P'^ increases in proportion to the mitotic index of growing tissues. ''*• 

 140,141 The same state of affairs has been found with N'^-labeled adenine, '^^ and in 

 some cases also with N'^-labeled glycine."^ Once formed, the DNA molecule appears 

 to be stable; using regenerating liver and labeled adenine Bendich'''^ calculated a 

 "half-life" of about 50 days for the DNA-fractions as compared with about 8 days 

 for PNA. Using other precursors (formate, glycine, orotic acid, cytidine, and de- 

 oxyribosides), Hammarsten, Reichard, and others'"^"'" have thrown a different light 



" H. G. Davies, Exptl. Cell Research 3, 453 (1952). 



3^ D. Shemin and D. Rittenberg, J. Biol. Chem. 153, 401 (1944). 



" E. Hammarsten and G. Hevesy, Ada Physiol. Scand. 11, 355 (1946). 



" A. Bergstrand, N. A. Eliasson, E. Hammarsten, B. Norberg, P. Reichard, and 



H. von Ubisch, Cold Spring Harbor Symposia Quant. Biol. 13, 22 (1948). 

 " J. N. Davidson, (ed.), in "Isotopes in Biochemistry," p. 175. Churchill, London, 



1951. 

 38 G. Hevesy, "Radioactive Indicators." Interscience, New York, 1948; Advances 



in Biol, and Med. Phys. 1: 409 (1948). 

 "9 A. Marshak, J. Cellular Comp. Physiol. 32, 381 (1948). 

 4" E. Andreasen and J. Ottesen, Acta Physiol. Scand. 5, 237 (1945). 

 " E. E. Osgood, H. Tivey, K. B. Davidson, A. J. Seaman, and J. G. Li, Cancer 5, 



331 (1952). 

 " R. Abrams and J. M. Goldinger, Arch. Biochem. and Biophys. 35, 243 (1952). 

 " E. Hammarsten, B. Thorell, S. E. G. Aqvist, N. Eliasson, and L. Akerman, Exptl. 



Cell Research 5, 404 (1953). 

 " A. Bendich, Exptl. Cell Research Suppl. 2, 181 1952. 



" E. Hammarsten, P. Reichard, and E. Saluste, J. Biol. Chem. 183, 105 (1950). 

 « P. Reichard and B. Estborn, J. Biol. Chem. 188, 839 (1951). 

 " D. Elwyn and D. B. Sprinson, /. Am. Chem. Soc. 72, 3317 (1950). 

 « G. A. LePage and C. Heidelberger, /. Biol. Chem. 188, 593 (1951). 



P. Reichard, Acta Chem. Scand. 3, 422 (1949). 



