NUCLEIC ACIDS IN CHROMOSOMES AND MITOTIC DIVISION 197 



on the biochemical stability of DNA by showing that different precursors give dif- 

 ferent rates of incorporation into DNA. Until we know the synthetic pathways to 

 DNA, the turnover studies hitherto made are difficult to interpret. 



There are also indications of a heterogenous metabolism within different frac- 

 tions of DNA. In regenerating liver, the incorporation of formate-C'' into the guanine 

 of that fraction of DNA which is insoluble in cold physiological saline was 25% greater 

 than into the guanine of a DNA-fraction remaining soluble. Adenine showed the 

 reverse."^ The field of metabolically, and probably also functionally, different DNA 

 in the same set of chromosomes is just beginning to be explored, and many interesting 

 findings can be expected in the near future. 



As has been indicated, only the use of a proper precursor can be supposed 

 to disclose the renewal of DNA in relation to the different stages of the cell 

 cycle, but up to the present time P^^ as phosphate has mainly been used for 

 the autoradiographic detection of the uptake in single cells. ^^'^'^^^ Root 

 cells of Viciafaba were treated with P^^^ ^nd 4 hours later about 20 % of the 

 nucleic of meristematic cells after extraction of lipid- and acid-soluble phos- 

 phorus showed autoradiographs, while cells in division showed none. It has 

 been suggested by way of explanation that a cell which is preparing to 

 divide synthesizes DNA during the first part of interphase. This explana- 

 tion also gives a period of about 6 to 8 hours between the end of the syn- 

 thesis and the beginning of visible prophase. During actual cell division 

 little or no synthesis could be detected, and a cell which has completed its 

 last division does not incorporate significant amounts of P^^ in the nucleic 

 acids. 



The question of the stability and retention of DNA-phosphorus in resting cells has 

 led to some interesting suggestions, some of which may be mentioned. For example, 

 Hevesy,''* and later Stevens, Daoust, and Leblond^'^ have calculated from the quan- 

 titative relationship between the uptake of P'^ in the DNA and the increments in 

 mass or in the number of nuclei that about twice as much DNA-phosphorus is pro- 

 duced as can be accounted for by newly formed nuclei. "^-^^^ass por example, in the 

 liver it was calculated that 0.71% of the cells were newly formed per day and 1.2% 

 new DNA-phosphorus, in intestinal mucosa 54% new cells and 95 to 114% new DNA- 

 phosphorus. It was therefore suggested that the cell cycle is associated with a re- 

 placement of each of the mother DNA molecules by two new daughter molecules. 

 The mechanism whereby the structural pattern might be kept intact under such cir- 

 cumstances is not understood, however, and until we know more about the mecha- 

 nism of incorporation of P'* into the DNA molecule the question is open. 



In general terms, it may be said that the extended chromosomes of the 

 interphase or "resting" nucleus probably form the structural basis for the 

 chains of syntheses, both of new chromosomal nucleic acids for the next 



150 A. Howard and S. R. Pelc, Exptl. Cell Research 2, 178 (1951). 



i^i A. Howard and S. R. Pelc, in "Isotopes in Biochemistry," (J. N. Davidson, 



ed.), Churchill, London, 1951. 

 '" C. E. Stevens, R. Daoust, and C. P. Leblond, J. Biol. Chem. 202, 177 (1953). 

 »" R. Daoust, F. D. Bertalanffy, and C. P. Leblond, J. Biol. Chem. 207, 405 (1954). 



