188 CELL HEREDITY 



TABLE 7.2 



Chemical Composition of Enzymatically Synthesized DNA, 



Synthesized >vith Different Primers. A, Adenine; 



T, Thymine; G, Guanine; C, Cytosine 



(From KoriilHTH, 1960, Science. 131:1507) 



DNA A T G C 



A + G A+T 

 T + C G + C 



M ycohacterium phlei 



Primer 0.65 0.66 1,35 134 1.01 0.49 



Product 0.66 0.65 134 1,37 0.99 0.48 



E. coli 



Primer 1,00 0.97 0.98 1.05 0.98 0.97 



Product 1.04 1.00 0.97 0.98 1.01 1.02 



C^alf thvmus 



Primer 1.14 1.05 0.90 0.85 1.05 1.25 



Product 1.12 1.08 0.85 0.85 1.02 1.29 



Bacteriophage T2 



Primer 1.31 1.32 0.67 0.70 0.98 1.92 



Product 1,33 1,29 0,69 0,70 1,02 1,90 



A-T copolymer 1.99 1.93 <0.05 <0.05 1,03 40 



primer DNA employed and the new DNA synthesized. As can be seen 

 from the examples cited in Table 7.2, there is a very close relation 

 in base ratio between primer and product, strongly indicating that the 

 primer serves as a template for an enzymatic replication of DNA. 



It seems very likely that this enzymatic reaction represents the princi- 

 pal, perhaps the only, mechanism of DNA synthesis in the cell. The dis- 

 covery of this system, consequently, represents a contribution of unique 

 importance in the analysis of cell heredity. 



Evidence that DNA is replicated by a mechanism which involves the 

 laying down of a new polynucleotide strand upon a template consisting 

 of a pre-existing DNA strand, comes also from an entirely different kind 

 of experiment. A method was developed by Meselson and Stahl to dis- 

 tinguish between old and newly formed DNA by means of isotopic label- 

 ing. These experiments utilize the density difference between DNA 

 molecules containing ordinary N^^ and those in which the heavier iso- 

 tope N ^'^ has been substituted by growing cells with N^^H^Cl as sole 

 nitrogen source. 



It has recently been estabhshed that N'^- and N '''-containing DNA 

 molecules can be well separated by equilibrium centrifugation in a 



