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How was a heavy isotope of nitrogen used to provide experimental evidence to semi-conservative mode of DNA replication?

Answer

Messelson and Stahl (1958) cultured bacteria E. coli in a cultural medium containing 15N isotopes 15NH4 Cl (15N is a heavy isotope of nitrogen) of nitrogen. After the replication of DNA of E. coli for many generations in 15N medium, it was found that both strands of DNA contained 15N as the constituent of purines and pyrimidines.
This heavy DNA molecule could be distinguished from the normal DNA by centrifugation in a cesium chloride (CsCl) density gradient. Being 15N not a radioisotopic isotope, it can be separated from 14N only based on densities.

When these bacteria with incorporated 15N were placed in medium containing 14N (14NH4Cl), it was noticed newly formed DNA molecules contain one Strand heavier than the other. DNA such formed was found to be hybrid as one strand was made up of ‘^N (old) and another was made up of 14N (new).

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