SYNTHESIS OF DEOXYRIBOSE NUCLEIC ACID 



AND NUCLEAR INCORPORATION OF ^^S AS 



SHOWN BY AUTORADIOGRAPHS 



ALMA HOWARD and S, R. PELC 



AuTORADioGRAPHs Can be used as a cytochemical method 

 if their resolution is sufficient to distinguish radioactivity in 

 individual cells and cell parts, and if, of those compounds 

 which contain the administered isotope, all can be removed 

 from the cell after fixation except the one under consideration. 

 When this is done, the method makes it possible to measure 

 the time of incorporation of the isotope into specific com- 

 pounds. This paper extends work already reported (Howard 

 and Pelc, 1951a) and describes experiments designed to 

 examine the time of synthesis of deoxyribonucleic acid 

 (DNA) in relation to the mitotic cycle, nuclear uptake of ^^S, 

 and the effect of X-rays on these processes. 



The material used was the growing main root of Viciafaba 

 seedlings, cultured in aerated water at 19°C. Gray and 

 Scholes (1951) have shown that under similar conditions of 

 growth the meristematic cells, which occupy a region 2 to 

 3 mm. long at the tip of the root, have a mean intermitotic 

 time of 19 hours, while division, from prophase to the end of 

 telophase, occupies 2^ hours. Thus every 21 J hours all the 

 meristematic cells divide, and of their daughter cells, one 

 half remain meristematic and the other half differentiate. 

 At the upper limit of the meristem the mitotic rate falls 

 sharply, reaching virtually zero in the cortex at about 3 mm. 

 from the tip. In the developing vascular region mitotic 

 activity is present at more proximal levels in the root, and 

 at a distance of 1 to 3 cm. the initials of the lateral root 

 meristems can be seen in these tissues. Thus some at least 

 of the cells in this region must be considered as premitotic. 



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