40 MAURICE J. BESSMAN 



extensive synthesis (6-fokl) was calculated and c()m])arc(l to the experi- 

 mental findings. Again there is good agreement between the observed 

 and predicted values. The calculation was based on the assumption that 

 both strands replicate equally and this appears to be borne out by the 

 data. The results were extended by subjecting the products of "limited" 

 and "extensive" synthesis to nearest neighbor analysis. Based on the 

 assumption that both strands replicate equally well, values were calcu- 

 lated for nearest neighbor frequencies in the DNA from "extensive" 

 synthesis and the data in Table XIX support this contention very well. 

 These elegant experiments further support the proposed scheme for 

 replication (Fig. 10) in which both strands of the primer DNA act as 

 templates through which specific base-pairing is accomplished. 



A few of the applications of nearest neighbor analysis have been 

 mentioned here. It is clear that until a new technique is devised for 

 determining longer sequences in the DNA molecule, the nearest neighbor 

 approach will provide the most revealing information about the fine 

 structure of DNA. 



G. SYNTHESIS OF DEOXYADENYLATE-DEOXYTHYMIDYLATE AND 

 DEOXYGUANYLATE-DEOXYCYTIDYLATE POLYMERS 



(Schachman et al., 1960) 



We have already seen an exception to our generalization that all four 

 deoxyribonucleoside triphosphates are required for DNA synthesis in 

 that single deoxyribonucleotides can add to the ends of DNA chains. 

 In this section, two reactions catalyzed by E. coli polymerase, w^hich not 

 only do not require all four triphosphates, but in addition may proceed 

 in the absence of primer, will be discussed. 



1. The d-AT Polymer 



When E. coli polymerase is incubated with all four triphosphates in 

 the absence of a primer there is an extensive lag period, 3 to 4 hours, 

 in which no detectable reaction occurs. This period of inactivity is 

 followed by a rapid synthesis of a high molecular weight polymer of 

 deoxyadenylate and deoxythymidylate. The kinetics of the reaction are 

 shown in Fig. 12, where three independent techniques were used to follow 

 the reaction. In Expt. 1, plots of the rate of incorporation of P^- from 

 dATP''- and the rate of increase in viscosity are superimposable. In 

 Expt. 2, the hypochromicity curve is the mirror image of the P^- in- 

 corporation. In Fig. 13, the kinetics of DNA synthesis in the complete 

 system is compared to the synthesis of the d-AT polymer. The complete 

 system contained, beside polymerase and calf thymus DNA, all 4 tri- 

 phosj)hates. Polymer formation as measured bv viscositv increase starts 



