(a) Why did Meselson and Stahl use isotopes in the sources of nitrogen present in the
culture medium in their experiment ? Explain.
(b) Write the conclusion drawn by them from the experiment. (3 marks)
(a) (i) Meselson and Stahl grew E.coli in a medium containing (15N is the heavy isotope of
nitrogen) as the only nitrogen source for many generations. 15N was incorporated in the
two strands of newly synthesised DNA. This heavy DNA could be seperated from the normal
14N DNA by centrifugation in a cesium chloride (CsCl) density gradient.
(ii) They transferred the cells of E.coli into a medium with normal 14NH4Cl and took
out samples at various intervals of time as the cells multiplied and extracted the DNAs and
centrifuged them to measure their densities.
(iii) The DNA extracted from the cells after one generation of transfer from the 15N medium to
14N medium (after 20m) had an intermediate/hybrid density.
(iv) The DNA extracted after two generations (after 40m) consisted of equal amounts of 'light'
DNA and hybrid DNA.
(b) Conclusion was semiconservative mode of DNA replication. They drew this conclusion on the
basis of the observations given above.