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Meselson and Stahl's experiment showed DNA is replicated using the semi-conservative method. Their experiment used two isotopes of nitrogen (DNA contains nitrogen) - heavy nitrogen and light nitrogen
1. Two samples of bacteria were grown for many generations - one in a nutrient broth containing light nitrogen, and one in a broth with heavy nitrogen. As the bacteria reproduced, they took up nitrogen from the broth to help make nucleotides for new DNA. So the nitrogen gradually became part of the bacteria's DNA
2. A sample of DNA was taken from each batch of bacteria and spun in a centrifuge. The DNA from the heavy nitrogen bacteria settle lower down the centrifuge tube than the DNA form the light nitrogen bacteria because it's heavier
3. Then the bacteria grown in the heavy nitrogen broth were taken out and put in a broth containing only light nitrogen. The bacteria were left for one round of DNA replication, and then another DNA sample was taken out and spun in the centrifuge
4. If replication was conservative, the original heavy DNA, which would still be together, would settle eat the bottom and the new light DNA would settle at the top
5. If replication was semi-conservative, the new bacterial DNA molecules would contain one strand of the old DNA containing heavy nitrogen and one strand of new DNA containing light nitrogen. So the DNA would settle out between where the light nitrogen DNA settled and where the heavy nitrogen DNA settled out
6. As it turned out, the DNA settled out in the middle, showing that the DNA molecules contained a mixture of heavy and light nitrogen. The bacterial DNA had replicated semi-conservatively in the light nitrogen