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Meselson-Stahl experimental procedure

The Meselson-Stahl experiment in 1958 provided experimental evidence for semi-conservative replication.

Initially, bacteria (E. coli) were grown in an environment containing only the rare nitrogen isotope $$\ce{^{15}N}$$.

DNA containing $$\ce{^{15}N}$$ is functional but heavier than normal DNA.

After several generations of replication, the nitrogen in the bacterial DNA was almost entirely $$\ce{^{15}N}$$.

Then a batch of heavy bacteria was allowed to replicate in an environment of the more common nitrogen isotope ($$\ce{^{14}N}$$).

After each replication, the DNA of a sample from the batch was put into a centrifuge to separate the DNA molecules by their density $$(=\text{mass}\div\text{volume})$$.