IV. CELLULAR CONTROL OF DNA BIOSYNTHESLS 



167 



As with regenerating liver, l)acteria do not exhibit a gross build up of 

 deoxynuc'lcotides i)rior to synchi-onous DXA synthesis nor are purine 

 deoxyriboside derivatives accumulated in even measurable quantities 

 during any portion of the cell cycle (Lark, 1961). ^Moreover, those slight 

 changes in the dcoxynucleotide pool which are observed have been 

 shown to be unrelated to DNA synthesis since they may be retained as 

 discontinuous changes under conditions leading to continuous DXA 

 synthesis (see Fig. 6). In addition, if a step of DNA synthesis is blocked, 

 no measurable excess of deoxynucleotides is accumulated. 



Again, the sudden onset of DNA synthesis in Salmonella typhi- 

 murium, which results from raising the temperature from 25° to 37°C, 



Minutes 



Fig. 6. Acid-soluble deoxyribotide (DOR) s^iithesis in relation to discontinuous 

 DNA synthesis in sj^nchronized bacteria. Deoxj-ribotides are given as milligram 

 equivalents. Deox5Tibotides are sj^nthesized discontinuously about (5 minutes) 

 before the onset of DNA sj^nthesis (A). The quantitj^ of material synthesized is less 

 than 3% of the intracellular content of DNA. If the discontinuity of DNA sj^nthesis 

 is removed (B) by the addition of deo.xyribosides to the medium during a previous 

 cj'cle of synthesis (see text, p. 196), the pattern of subsequent deo.xyribotide synthesis 

 is unaltered. (From Lark, 1961.) 



