I. REPLICATION OF DNA IN CELL-FREE SYSTEMS 45 



unusually low density suggesting a very low content of cytosine and 

 guanine. A sample of this material was used to prime a reaction using 

 E. coli polymerase and the product was analyzed for nearest neighbor 

 sequence frequencies (Swartz et al., 1962). Over 95% of the product had 

 a sequence made up of alternating deoxyadenylate and deoxythymidylatc 



TABLE XXI 

 Relationship between Extent of Reaction and Size of Product"'' 



" From Schachman el al. (1960). 



^ Reaction mixtures, 6 ml, were incubated directh' in viscometers; at selected times 

 after the viscosity of the solution had been measured, aliquots were removed, and the 

 enzymatic action terminated by the addition of NaCl-citrate solution to give a concen- 

 tration of 0.2 M NaCl-0.1 M sodium citrate. Absorbancy measurements of these samples 

 as compared to the original gave a measure of the extent of the reaction (hypochromic- 

 ity). The samples were then dialyzed, and the reduced viscosity, Vsp/C, in (gm/100 ml)~' 

 and sedimentation coefficient were measured in 0.02 Af sodium citrate. 



residues. This naturally occurring d-AT polymer differs from the 

 unprimed d-AT polymer produced by E. coli polymerase in that it 

 does have a small but significant content of deoxyguanylate and 

 deoxycy tidy late residues accounting for less than 5% of the total 

 deoxyribonucleotides. 



2. The d-GC Polymer 



Much less infonnation is available about an interesting polymer 

 composed exclusively of deoxycytidylate and deoxyguanylate residues. 

 This material is similar to the d-AT copolymer in that it is composed 

 of only two deoxyribonucleotides and is formed after a considerable lag 

 period in an unprimed reaction catalyzed by purified E. coli polymerase. 

 However, this product is distinctly different in one important respect. 

 Whereas d-AT is a copolymer of alternating residues of deoxyadenylate 

 and deoxythymidylatc, the d-GC polymer is made up of separate strands 

 containing either deoxyguanylate or deoxycytidylate. This was shown 



