What did the Meselson and Stahl experiment show?

DNA replication was thought to be either conservative (two old strands of DNA stay together), semi-conservative (one old and one new strand of DNA), or dispersive (bit of old and newly replicated DNA in each strand). To identify the method of replication E.coli were grown in heavy nitrogen (15N- an isotope) so that all the DNA contained the heavy nitrogen. Then the bacteria were moved to light nitrogen medium (14N) and allowed to replicate once. Then the DNA was extracted and subjected to density gradient centrifugation so the DNA sedimented out.DNA containing only 15N is heavier (seen lower in the tube) than DNA of only 14N (higher band in the tube), a mixture of the two generates an intermediate band in the tube. After one round of replication only a band at the intermediate position was seen- this ruled out conservative replication as this would produce one heavy and one light band. As more replications occured a band at 14N (light) was seen and grew while the intermediate band got smaller. Therefore replication could not be dispersive as otherwise the a band of purely light nitrogen containing DNA would not be produced.