Organization, Replication, and Types of DNA in Vivo 



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ultracentrifuge tube is observed to move from 

 the all-heavy DNA position to a position 

 exactly intermediate between the all-heavy 

 and all-light positions (Figure 34-7). This 

 is exactly what is expected if the DNA is 

 "hybrid" in density after one replication. 



What would you expect to find after one 

 additional DNA replication? In this replica- 

 tion the two chains of the hybrid DNA should 



separate, and light complementary chains 

 should be made by both the light and heavy 

 single chains. So, after a second replication 

 half of the double-chain DNA molecules 

 should be all-light and half should be inter- 

 mediate between all-light and all-heavy (that 

 is, they should be hybrid). And, in fact, in 

 the samples of DNA taken at later intervals, 

 the single band at the intermediate position 



FIGURE 34-6. Diagrammatic representation of the hypothesis of DNA repli- 

 cation after chain separation. 



