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CHAPTER 34 



can specify exactly the composition of the 

 other strand. Thus, if one strand had the 

 base sequence ATTCGAC, the other strand 

 would have to contain TAAGCTG in the 

 corresponding regions. 



If DNA is genetic material, we should 

 expect it to be capable of being replicated just 

 as precisely as is the genetic material. Since 

 the sequence on one chain is complementary 

 to the sequence on the other, this immediately 

 suggests a simple way that the double helix 

 might replicate itself.^ What we would re- 

 quire is that the two chains first separate, 

 after which each chain would build its comple- 

 ment. This can be called the chain separation 

 hypothesis of DNA replication. This can be 

 visualized as occurring by having each strand 

 serve as a mold or template. We know that 

 complex surfaces (like statues) can be copied 

 exactly by making a mold, which can be used 

 in turn to make a second mold. The second 

 mold is then an exact copy of the original 

 configuration. In the present case the two 

 complementary strands of DNA can be viewed 

 as molds, or templates, for each other. Either 

 strand could act as a mold on which the 

 complementary strand could be synthesized. 

 Figure 34-6 shows one possible sequence of 

 events. At the top of this Figure, the two 

 strands start to come apart. At the center the 

 two single chains exist in the presence of single 

 nucleotides or their precursors. When the 

 complementary free nucleotide approaches 

 the single chain, its base would be H-bonded. 

 Then, after several nucleotides have bonded 

 to the single chain, perhaps an enzyme would 

 Hnk them to start the new complementary 

 chain. The bottom shows sections of the 

 complementary chains whose synthesis is 

 already completed. 



It is possible to design an experiment - 

 which tests, simultaneously, the hypotheses 



^ Based upon the hypothesis of J. D. Watson and 

 F. H.C. Crick (1953b,c). 



^ Based upon the experiments of M. Meselson and 

 F. W. Stahl. 



both for the double helix structure of DNA 

 and for its replication by chain separation. 

 You remember that every pyrimidine or 

 purine base, usually found in DNA, contains 

 two or four N atoms, respectively. These 

 atoms normally are what we can call light 

 nitrogen {N-14). It ought to be possible to 

 grow bacteria in a culture medium whose only 

 nitrogen is in the form of a heavier isotope, 

 N-15, which we can call heavy nitrogen. If 

 so, after a number of generations have passed, 

 almost all of the DNA present will have been 

 synthesized utilizing heavy nitrogen. Sup- 

 pose also that it has been possible to synchro- 

 nize the multiplication of the bacteria con- 

 taining heavy DNA. What would you expect 

 to happen if these bacteria are quickly washed 

 with, and then placed in, culture medium 

 containing only light nitrogen, and are per- 

 mitted to continue their synchronous multi- 

 plication? The DNA should replicate each 

 time the bacteria undergo cell division. 

 During the first replication of DNA the two 

 chains containing heavy N should separate, 

 and each should synthesize a complementary 

 chain containing only light nitrogen. Thus, 

 after one DNA replication, the density of the 

 DNA molecules should be exactly inter- 

 mediate between completely light and com- 

 pletely heavy DNA. 



To test whether or not this expectation is 

 actually observed, the DNA is extracted from 

 "all-heavy" bacteria and also from "all-light" 

 bacteria. These extracts, serving as controls, 

 are ultracentrifuged, first separately, and then 

 together, in a fluid medium of appropriate 

 density (containing cesium chloride). After 

 about 20 hours, the position of the DNA in 

 the medium can be detected by its absorbence 

 of ultraviolet light of 2600 A wave length. 

 Two separate bands of DNA are found in the 

 medium, corresponding to the all-heavy and 

 all-light DNA. When DNA is extracted at 

 various time intervals, after the originally all- 

 heavy bacteria have been placed in the all- 

 light nitrogen medium, the DNA band in the 



