Meselson and stahls experiment-

Meselson and Stahls experiment- 

(By density gradient centrifugation they did experiment to evaluate the 3 different models of replication.)

                          

  

   Hypothesis- Dna replicates semiconservatively.

• E.coli bacteria cells were cultured (grown ) in a medium with ¹⁵N heavy isotope.  ¹⁵N is a heavy isotope of nitrogen so the DNA synthesized after  Cscl density gradient centrifugation is of heavy density.

• E. coli cells with only ¹⁵N in their DNA were transferred to a ¹⁴N medium . DNA was isolated  after successesive  replication cycles . 

•  Replication cycle 1 (after 20 mins )- DNA was all of intermediate density in the ¹⁴N medium and  form a single band.This rules out the conservative replication model, which predicts that both heavy density DNA and light density DNA will be present, but no intermediate density will be present.

•  Replication cycle  2- (after  40 mins ) - DNA with the semiconservative replication model as half the Dna are of intermediates (¹⁵N - ¹⁴N) and half  are of light ( ¹⁴N ) and this  eliminates the dispersive replication model.

• After two replication cycles, two bands of DNA were seen, one of intermediate density and one of light density. 

•  Result-  Semiconservative model predicts: half with ¹⁵N - ¹⁴N intermediate density DNA and half with ¹⁴N-¹⁴N light density DNA.( No intermediates)

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