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What is meant by semi conservative replication? How did Meselson and Stahl prove it experimentally?

Class XII
Molecular basis of inheritance



Semi-Conservative DNA Replication: During replication the 2 strands of DNA would separate and act as template for the synthesis of new complementary strands. At the end of replication, each daughter DNA molecule would have one old strand or parental strand and one newly synthesized DNA strand. This proves a semi-conservative DNA replication

Semiconservative DNA Replication was proved by the work of MATHEW MESSELSON and FRANKLIN STAHL(1958). They grew E.Coli in a medium containing 15NH4Cl(15N is the heavy isotope of Nitrogen) as the only nitrogen source for many generations. The result was that 15N was incorporated into newly synthesized DNA(as well as other Nitrogen containing compounds). This heavy DNA molecule could be distinguished from the normal DNA by centrifugation in a cesium chloride(CsCl) density gradient.

1) They transferred the cells into a medium with normal 15NH4Cl and took samples at various definite time intervals, as the cells multiplied, and the extracted DNA remained as, double stranded helices. The various samples were separated independently as CsCl gradients to measure the densities of DNA.

2) Thus, the DNA that was extracted from the culture one generation after the transfer of 15N to 14N medium(i.e. after 20 minutes) had a hybrid density. DNA extracted from the culture another generation(i.e. after 40 minutes, 2 generation) was composed of equal amounts of hybrid DNA and of light DNA. As it did not contain any heavy nitrogen or N15. It  confirmed the semi-conservative mode of replication.

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