Answer» - Meselson and Stahl, in their experiments, grew E.coli in a medium containing 15NH4Cl as the only source of nitrogen for a number of generations.
- As a consequence, the heavy isotope of nitrogen, 15N, was incorporated into the DNA as well as other nitrogenous compounds.
- This heavy DNA molecule, i.e., 15N/15N DNA, can be distinguished from the normal 14N/14N DNA, by cerium chloride density gradient centrifugation based on their densities.
- Then the cells were transferred to a medium containing 14NH4Cl (normal nitrogen) and samples were taken at definite time intervals.
- They isolated the DNA and the samples were independently centrifuged on CsCl density gradient to measure the densities of the DNA.
- The DNA isolated after one generation (after 20 minutes) of transfer to 14N medium, had a hybrid or intermedciate density; it is because of semiconservative replication.
- The DNA isolated after two generations (after 40 minutes) has equal amounts of hybrid density DNA and light (14N/14N) DNA, showing semiconservative replication.
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