THE LOCALIZATION 



OF NUCLEIC ACID SYNTHESIS 



IN ROOT MERISTEMS 



F. A. L. CLOWES 



Botany School, Oxford University, Ejigland 



The classical methods of investigating apical meristems ignore rates of 

 cell division and therefore lead to false conclusions about organization. 

 The use of autoradiography in investigating the synthesis of DNA shows 

 that there is a quiescent centre in root meristems where the cells do not 

 divide at allor divide very rarely. This method dépends upon the incorpora- 

 tion ofprecursorsof DNA being équivalent to net synthesis and net synthe- 

 sis indicating préparation for mitosis. Thèse assumptions appear to be 

 valid for meristems, though not necessarily so for other tissues, and the 

 conclusions made from them can be confirmed by other methods. The 

 degree of quiescence of the cells varies with the size of the apex, and 

 is low in roots with single apicâl cells and in the early embryonic stages 

 oflarger roots. The cells of the quiescent cemtre raay be induced to synthe- 

 sise DNA and enter mitosis by suitable surgical or X-ray treatments. 

 Quantitative results may be obtained about the ploidy of cells by measur- 

 ing the rate of synthesis over short periods. The duration of the mitotic 

 cycle may also be measured for ail the parts of the meristem by counting 

 the proportion of nuclei labelled after varions exposures to the radioactive 

 precursors of DNA. 



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