CHROMOSOME DUPLICATION AND GENETIC RECOMBINATION 



187 



presence of a small amount of DNA which serves as primer of the reac- 

 tion. Each triphosphate is incorporated into the polynucleotide with the 

 splitting off of pyrophosphate which is found in the reaction mixture in 

 stoichiometric quantities. The reaction can be reversed when pyrophos- 

 phate is present in excess. 



The DNA produced in this reaction has essentially the same physical 

 properties as DNA carefully extracted from biological materials, with 

 respect to molecular weight, viscosity, degradation by DNA-ase, and 

 effects of heating. Of particular interest is the relation between the 



Parent 

 molecule 



First generation 

 (Daughter 

 molecules) 



Second generation 



(Granddaughter 



molecules) 



FIGURE 7.3. Proposed model of DNA replicotion. Parental strands of tfie DNA 

 double helix are shown in black, and newly formed complementary strands are 

 shown in outline. Arrows indicate direction of the strands in the sense that phosphate 

 diester bonds connect the C-3 carbon of one sugar with the C-5 of the next. Com- 

 plementary strands run in opposite directions (from Meselson and Stahl, 1958, C.S.H. 

 Symp. Quant. Biol., 23:10). 



