Experiment of Meselson and Stahl to prove the semiconservative nature of DNA

• DNA replication is semi conservative in nature. It was first observed in E.Coli by Mathew Meselson and Franklin Stahl through an experiment in 1958.

• DNA replication is semi conservative in nature. It means that when DNA duplicates or It makes its own copy, in this copy of DNA, One strand of DNA is old and another strand is newly synthesised.

• Meselson and stahl  grew E.Coli. in a medium containing ammonium chloride in which 15N was used.

• 15N can be differentiated from 14N on the basis of density through cesium chloride density gradient centrifugation.

Read more about the Experiment of Griffith

• The E.Coli that was grown in ammonium chloride in which 15N was used, had 15N in its DNA . And  it had been observed that after 20 minutes by replication, 15N was present in both copies of DNA.

👌👌Remember E.Coli takes 20 minutes to produce its copy. 

Generation -First 

• When E.Coli was transferred in a medium of normal ammonium chloride containing 14N then it had been observed that 14N was present in one of the strands of DNA along with a strand with 15N.

• On the other hand, In a newly synthesised DNA through replication, one strand was old or paternal with 15N and the other strand was new with 14N.

• This was the first generation that was regarded by the Meselson and stahl.

Generation - Second

• In the Second generation after 40 minutes when DNA with one strand 15N and another strand with 14N was replicated then it produced two molecules of DNA.

• In the first molecule of DNA, one stand was with 15N and the other strand with 14N whereas in the second molecule of DNA, Both strands of DNA were with 14N.  

Read more about the Experiment of Hershey and Chase

• When DNA with both strands containing 14N was replicated then it  also produced two molecules of DNA.

• In the first molecule of DNA, both strands were 14N whereas in the second molecule of DNA, Both strands of DNA were  also with 14N. 

 

Conclusion -

• In Both generations, the hybrid or newly synthesised  DNA contained equal density of both 15N and 14N.

• In both generations, cesium chloride density gradient centrifugation was used to analyze the presence of 15N and 14N.

• This experiment of Meselson and stahl proved the semi conservative replication of DNA .