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5.6: The Meselson - Stahl Experiment

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  • DNA Replication is Semiconservative

    The structure of DNA suggested to Watson and Crick the mechanism by which DNA — hence genes — could be copied faithfully. They proposed that when the time came for DNA to be replicated, the two strands of the molecule

    • separated from each other but
    • remained intact as each served as the template for the synthesis of
    • a complementary strand.
    Figure 5.6.1 Meselson - Stahl experiment interpretation

    As this interpretative figure indicates, their results show that DNA molecules are not degraded and reformed from free nucleotides between cell divisions, but instead, each original strand remains intact as it builds a complementary strand from the nucleotides available to it. This is called semiconservative replication because each daughter DNA molecule is one-half "old" and one-half "new".

    Immortal strands. Note that the "old" strand (the red one in the top half of the figure) is immortal because — barring mutations or genetic recombination — it will continue to serve as an unchanging template down through the generations.

    E. coli is a bacterium, but semiconservative replication of DNA also occurs in eukaryotes. And because each DNA molecule in a eukaryote is incorporated in one chromosome, the replication of entire chromosomes is semiconservative as well. This also means that the eukaryotic chromosome contains one "immortal strand" of DNA.

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