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
N
and lighter B)
Bacteria will incorporate nitrogen from their growing medium into any new DNA molecule
synthesised
The Experiment
The bacteria are grown on a medium containing 14Nproduced `light'DNA and the bacteria grown
15
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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