88 METHODS FOR DETERMINING MOLECULAR SIZE AND SHAPE 



Density increases 



Generation number 



1.9 



Band is at position 

 of unlabeled normally 

 dense DNA 



and 1.9 

 mixed 



Band is at position 

 of isotopically labeled 

 dense DNA 



^ f 



Band is at a density 

 intermediate between 

 that of isotopically 

 labeled dense DNA and 

 that of unlabeled 

 normally dense DNA 



Fig. 42. Schematic drawings of photographs of DNA extracted from iso- 

 topically labeled dense organisms at various times after organisms were placed 

 in unlabeled normally dense medium. The DNA was placed in CsCl and banded 

 as explained in Fig. 40. Since the DNA absorbs highly specifically and strongly 

 at 260 mfx, the band positions can be photographed by absorption of this light, 

 as explained in Fig. 35. 



In this sketch, each section represents a photograph of the centrifuge 

 tube taken so as to show materials which absorb ultraviolet light, since 

 nucleic acids absorb that part of the spectrum much more strongly than 

 other biological materials. The single band at generation zero is clue to 

 the dense nucleic acid. After one generation, there are no molecules of 

 the parental density, demonstrating unequivocally that a completely 

 conservative mechanism of replication does not exist. The density of the 

 nucleic acid molecules found after one generation is actually midway 

 between that of the dense parental molecules and that of molecules of 

 cells grown in a normal medium, as can be seen by looking at genera- 

 tions 1.9 or 3.0, in which most of the molecules are from cells which were 

 formed after the transfer to a normal growth medium. It should be noted 

 that at all times there are molecules of this intermediate density. Thus 

 we see evidence that molecules which have obtained half their nucleic 

 acid from the original parents and half from new normal density mate- 



