Figure D: (Left) Ultraviolet absorption photographs showing DNA bands from centrifugation of DNA from E. Coli sampled at various times after the addition of an excess of \(\ce{^{14}N}\) substrates to a growing \(\ce{^{15}N}\) culture. (Right) Densitometer traces of the photographs. The initial sample is all heavy (\(\ce{^{15}N}\) DNA). As time proceeds a second intermediate band begins to appear until at one generation all of the sample is of intermediate mass (Hybrid DNA). At longer times a band of light DNA appears, until at 4.1 generations the sample is almost all lighter DNA. This is exactly what is predicted by the Watson-Crick semiconservative mechanism. From Meselson and Stahl (1958)