Evidence for Semi-Conservative Replication (Meselson and Stahl)

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Meselson and Stahl's Experiment
Meselson and Stahl conducted and experiment that proved that DNA used semiconservative
replication and not conservative. Their work was based on three facts:
DNA contains nitrogen
Nitrogen occurs in two forms (heavier 15
and lighter B)
Bacteria will incorporate nitrogen from their growing medium into any new DNA molecule
The Experiment
The bacteria are grown on a medium containing 14Nproduced `light'DNA and the bacteria grown
on the Nproduced
heavy DNA.
1. The E. coliwere grown in 15 N were then transferred to a 14 N medium and were left for long
enough for one new generation of bacteria to be produced.
2. Samples were taken at intervals so that the parental , first and second generation DNA
could be analysed.
3. Analysis of the DNA composition through CsCl density gradient centrifugation showed the
heavy DNA
(containing 15N) forming a band lower than the light DNA
(containing 15 N).
4. The bands became visible under UV light.


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