



0-^-0'" 

 I I 

 HoC (T 



K^' 



O-P-O 



^o 



i,.. 



HO H 



C^t: 



o-p4o-^-o-f-o" 



Poly- 

 nucleotide 



HO H 



XTP 



Fig.' 5- Postulated Mechanism for Extending a DNA 

 Chain. 



would not be expected to lead to the net synthesis of DNA, Instead, as might 

 be expected, the destruction of DNA by the extracts of such cells and tissues 

 was by far the predominant process and one had to resort to the use of more 

 subtle devices for detection of such a biosynthetic reaction. We used a C^*- 

 labeled substrate of high specific radioactivity and incubated it with ATP 

 and extracts of Escherichia coli, an organism which reproduces itself every 

 20 minutes. The first positive results represented the conversion of only a 

 very small fraction of the acid-soluble substrate into an acid-insoluble fraction 

 (50 or so counts out of a million added). While this represented only a few 

 jufimoles of reaction, it was something. Through this tiny crack we tried to 

 drive a wedge and the hammer was enzyme purification (13). This has been 

 and still is a major preoccupation. Our best preparations are several thousand- 

 fold enriched with respect to protein over the crude extracts, but there are 

 still contaminating quantities of one or more of the many varieties of nuclease 



s-55 



