314 



CHAPTER 34 



a 



EXP. NO. 



GENERATIONS 



0.3 



0.7 



I.I 



1.5 



1.9 



2.5 



3.0 



4.1 



Oandl.9 

 mixed 



Oand 4.1 

 mixed 



FIGURE 34-7. Test of the 

 ^'replication after chain sepa- 

 ration''' hypothesis, using the 

 technique of density gradient 

 centrifugation. DMA was 

 extracted from all-heavy {N- 

 15-labeled) bacteria grown 

 for different generation times 

 on all-light {N-14-containing) medium. The extracts were subjected to ultracentrifugation to position 

 the DNA in the centrifuge tube according to its density. {Density increases to the right of the figure.) 

 DNA absorption of ultraviolet light is indicated by the bcmds in different photographs under a and the 

 height of the peaks in the corresponding densitometer tracings under b. The rightmost band in the bot- 

 tom two frames and the band in the top frame represent all-heavy DNA. The leftmost band, seen 

 clearly in all generation times after 1.5 generations, represents all-light DNA. The only other clear 

 band is between the all-heavy and all-light ones. This is the only band present after 1.0 generations, and 

 represents DNA which is hybrid in density. Note that at 1.9 generations, half the DNA is all-light and 

 half is hybrid in density {see row showing and 1.9 mixed). {Courtesy of M. Meselson and F. W. 

 Stahl, Proc. Nat. Acad. Sci., U.S., 44:675. 1958.) 



