Question

3.Explain the semiconservative model of DNA replication. Describe the experiment that confirmed the model over the conservative and dispersive models, and why the findings pointed toward the semiconservative model.

Answer 1

According to semi-conservative model of DNA replication, a parental DNA molecule undergoes DNA replication in a semi conservative way to form daughter DNA molecule. The daughter DNA molecule contains one parental strand and a newly synthesized daughter DNA strand.

Other two probable models of DNA replication are conservative model and dispersive model.

According to conservative model, the newly formed daughter DNA molecule contains both newly formed daughter DNA strands. None of the strands is parental in origin.

According to dispersive model the newly formed daughter DNA molecule contains two strand both of which are mixture of parental DNA fragment and newly formed DNA fragments.

The experiment performed by Meselson and Stahl give an unequivocal proof that DNA replication is semiconservative in nature. They grew E coli in media containing heavy nitrogen for indefinite number of generations. Then they transferred one colony into a media containing light nitrogen and allowed it to grow for one generation. This lead to the formation of daughter DNA molecules having intermediate density. This ruled out conservative model of DNA.

Now they allowed it to grow for the second generation and obtained DNA with intermediate and light density. This ruled out dispersive model of DNA replication.

Hence, making a conclusion that DNA replication is a semiconservative process.

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