Question

Explain the Messelson and Stahl’s experiment.

Answer 1

They selected E.coli bacteria and radioactive nitrogen 15N for their experiment. They grew some E. coli in a medium containing 15N4Cl 15N having for many generation. The 15N in cororated into two two DNA becomes havier then the normal DNA. This DNA is called heavy DNA. These were transferred to normal NH4Cl medium and allowed to multiple bacterial cells were taken out at definite intervals. There DNA was expected and centrifuse on cesium chloride medium to the study their density. The DNA extracted from the bacteria and generation after the transfer 15N to 15N medium had the hybrid and with intermediate density. DNA extracted from the bacteria of second generation i.e. of 40 minutes was of equal amounts of the hybrid DNA and light DNA . It proved that DNA replication is semiconservative.

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