In the 1950s the heavy isotope of nitrogen (15N) and an ultracentrifuge were used in an experiment to investigate the manner in which DNA replicates. Bacteria were grown in a culture containing 15N nucleotides for several generations to ensure that only heavy 15N would be found in their DNA. These bacteria were then grown in a culture containing normal 14N nucleotides for one generation only. Bacteria from this generation were then removed, their DNA was extracted and centrifuged. DNA composed of 15N nucleotides has a greater density than DNA formed of normal 14N nucleotides. This process allowed DNA molecules with different densities to collect and form in bands in different regions of the centrifuge tube.
How did this experiment demonstrate that the replication of a DNA molecule is semi-conservative?
http://en.wikipedia.org/wiki/Meselson-Stahl_experiment
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