274 



CHAPTER 20 



a 



b 



EXP. NO. 



GENERATIONS 



0.3 



0.7 



1.0 



I.I 



1.5 



1.9 



2.5 



3.0 



figure 20-8. Test of the 

 "replication after chain sep- 

 aration" hypothesis, using 

 the technique of density 

 gradient centrif ligation. 



DNA was extracted from 

 all-heavy (N-l 5-labeled) 

 bacteria grown for different 

 generation times on all-light (N-14-containing) medium. The extracts were subjected to ultra- 

 centrifugation 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 

 bands in different photographs under a and the height of the peaks in the corresponding densi- 

 tometer tracings under h. The rightmost band in the bottom two frames and the bund 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. I his 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. Mesel- 

 son and F. W. Stalil, Troc. Nat. Acad. Sci., U.S., 44:675, 1958.) 



Oandl.9 

 mixed 



Oand 4.1 

 mixed 



