272 



( II M'TF.R 20 



taneously test the hypotheses for both the 

 double-helix structure of DNA and its repli- 

 cation after strand separation. Remember 

 that every pyrimidine or purine base nor- 

 mally found in DNA contains two or four 

 N atoms, respectively. Ordinarily, these are 

 atoms of N-14, or light nitrogen. It should 

 be possible to grow bacteria in a culture 



medium whose only nitrogen source is in the 

 form o\' a heavier isotope, N-15, or heavy 

 nitrogen. If so, after a number of genera- 

 tions have passed, almost all of the DNA 

 present will have been synthesized using 

 heavy nitrogen. Suppose also that one can 

 synchronize the multiplication of the bacteria 

 containing heavy DNA. What will we ex- 



figure 20-7. Diagrammatic representation of the hypothesis of DNA 

 replication after strand separation. 



