Class 12 Biology Molecular Basis of Inheritance Meselson-Stahl Experiment

Meselson-Stahl Experiment

This experiment was performed to prove the semi conservative nature of DNA replication. Matthew Meselson & Franklin Stahl experimented with bacteria E.coli in 1958.

Basis of the Experiment

  1. If E.coli was grown in a medium with N-15 (isotope of Nitrogen), the E.coli had DNA with  N-15 isotope.


  1. If E.coli was grown in a medium with N-14 (more abundant isotope of Nitrogen), the E.coli had DNA with N-14 isotope.


It was then observed with Centrifugation that DNA with N-15 is heavier than that of N-14.


Making use of the fact that DNA with N-15 is heavier than DNA with N-14, this experiment was performed.

Step 1.  E.Coli was grown in a medium with N-15 for several generations


Step 2.  E.coli with only N-15 in their DNA were transferred to a medium with N-14


Cells of E.coli were allowed to divide. Sample was taken and DNA was extracted periodically as cell division continued to check what type of DNA is being formed now. One replication in E.coli takes around 20 minutes. So, generation I is formed in 20 minutes.

Therefore samples are taken after 20 minutes, then again after 40 minutes. Densities of DNA from the sample were measured to reach to results & conclusion.


Generation I: DNA was found to have intermediate density after 1 replication


Generation II: Equal amounts of DNA with two different densities were found




  • Presence of a hybrid/ intermediate density excluded Conservative hypothesis. Had it been Conservative hypothesis, Generation 1 would have been either Blue(N-15) or Green(N-14); and not an Intermediate one.
  • Presence of N-14 DNA in Generation II excluded Dispersive hypothesis. If it was Dispersive, each DNA should have had the same density. But, in Generation II, we could see 50% of the DNA have intermediate density, whereas remaining 50% have N-14 density.
  • Semi-conservative hypothesis could explain the entire experimental result. Separation of strands concept could explain the outcomes of Generation I & II.

Thus, it was proved that DNA replication is Semi-conservative in nature.

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