Cesium chloride density gradient pictures of native Nicotiana tabacum - DNA bands (the centre of the rotor is at the right of the picture). a. reference substance: DNA of high density from Micrococcus lysodeiktikus; b. 'dense', 15N-labelled DNA (from Nicotiana tabacum); c. hybrid DNA, consisting of a dense (15N) and a light (14N) strand; d. 'light', unlabelled DNA containing 14N. The frames at top and bottom are synthetic mixtures of reference DNA, 15N-labelled DNA and 14N; About the experiment itself: Cell cultures of Nicotiana tabacum were cultivated in the presence of 15N for several days. Afterwards was the medium containing 15N isotopes washed off and substituted with non-labelled medium. The length of one generation of the cells is two days. DNA was isolated at 24-hr intervals following transfer from 15N DNA and 14N DNA. One day after the washing was only labelled ('dense') DNA isolated from the culture. On the second day was more and more hybrid DNA found and on the third day was only hybrid DNA present. The proportion of 'light' DNA increased steadily from the fifth day on and on the ninth day could only 'light' DNA be detected. (P. FILNER, 1965)