Resolution of 14N DNA and 15N DNA by density-gradient centrifugation. (A) Ultraviolet-absorption photograph of a centrifuged cell showing the two distinct bands of DNA. (B) Densitometric tracing of the absorption photograph.
[Data from M. Meselson and F. W. Stahl. Proc. Natl. Acad. Sci. U. S. A. 44:671–682, 1958.]