Meeselson and Stahl

Meeselson and Stahl devised and carried out an experiment to investigate the way in which DNA replicates or copies its self. Watson and Crick came up with the hypothesis that DNA would separate by a semi conservative mechanism basically meaning that half of one DNA would come from the original strand and the other half would of being built by adding new nucleotides.
The procedure carried out by Meeselson and Stahl saw the following occur:
•Bacteria cells in an isotope of nitrogen were grown.
•The nitrogen isotope had more neutrons then a normal atom of nitrogen (meaning heavier).
•The bacteria cells DNA soon had heavy nitrogen in it on both strands.
•The bacteria cells were then switched to a solution with lighter nitrogen with in it and let it reproduce two more times.
•At the beginning and then again after each generation they spun some cells in a centrifuge.

DNA with heavy isotopes on both strands formed a layer low in the centrifuge tube, because it was heavy. After the first replication of DNA in light nitrogen there was one medium weight band of DNA in the middle of the centrifuge tube. After the second generation there were two bands of DNA in the centrifuge – one medium and one light. There were no heavy bands. This pattern followed that expected for semi conservative replication.