IV. CELLULAR CONTROL OF DNA BIORYNTHESLS 163 



in DNA synthesis, to what extent does such precursor production control 

 DNA biosynthesis during the division cycle of the normal cell? 



To date, experiments to answer this question have utilized synchro- 

 nized cell systems of various types. Two types of experimentation may 

 1)0 considered: 



A. THE DIRECT MEASUREMENT OF DEOXYNUCLEOTIDES OR PRECURSORS 

 THEREOF AND THEIR RELATION TO THE DNA CYCLE 



The most striking example of a control mechanism which may rest 

 on precursor production is that of DNA synthesis in the microspores 

 of the anthers of Lilium longijorum. and Trillium erectum. The lily 

 system has proved useful because of the fact that the length of the 

 flower bud enclosing the anthers is correlated with the synchronized 

 activities of the microspore (Erikson, 1948). In Trillium, microspore 

 development is also synchronized with respect to time. By sampling 

 anthers from a given bud at different times, or by selecting buds of the 

 proper length, it is possible to examine large samples of material at 

 known stages of the division cycle and to study the synthesis of DNA 

 by the microspores (Taylor and McMaster, 1954; Foster and Stem, 

 1958). When this is done, it can be shown that the deoxyribosides of 

 guanine, adenine, cytosine, and thymine appear in the fluid surrounding 

 the microspores shortly before DNA synthesis and mitosis (Stem, 

 1960a). These disappear abruptly as DNA is made (Fig. 4). It has been 

 shown that such material originates from the destruction of a polydeoxy- 

 ribotide in extra sporageneous tissue (Stem, 1961) and that the deoxy- 

 nucleosides thus formed are rapidly converted in the microspores or the 

 microspore walls into triphosphates, the "half-life" of which is very 

 short (Stern, 1960b; Hotta and Stern, 1961a). Recent evidence has 

 demonstrated that the ability of microspores to phosphorylate thymidine 

 increases immediately following the appearance of deoxyribosides in the 

 extracellular fluid. It therefore ajipears that these phosphoiylating 

 enzymes (necessary to produce deoxynucleotide triphosphates) may be 

 induced by deoxyribosides (Hotta and Stern, 1961b). Although it has 

 not been possible to demonstrate directly the conversion of the extra- 

 cellular deoxyriboside material into DNA, the data at present are 

 consistent with a mechanism by which the presence of deoxyribosides 

 initiate the synthesis of DNA, presumably by making a pool of pre- 

 cursor material available. 



One difficulty, however, is that two cycles of deoxyriboside production 

 and disappearance occur for each step of DNA synthesis, a pattern 

 which is most striking in experiments with Lilium. This may arise from 

 a slight shift in the phase of synchrony among anthers from a single 



