Notes On Experiments on DNA Replication - CBSE Class 12 Biology
Multiple experiments were conducted by different scientists to conclude that DNA replicate semi conservatively. Watson and Crick proved that DNA have a double helical structure and the two strands separate and act as a template for the synthesis of two new complementary strands. After the replication process was complete, each DNA molecule would end up with one parental strand and one newly synthesised strand. This replication scheme of DNA is called ‘semiconservative’.

To validate this scheme of DNA replication, experiments were conducted by Matthew Meselson and Franklin Stahl on E. coli, a bacterium that divides every twenty minutes. E. coli was grown in a medium that had 15NH4Cl as the only nitrogen source, where 15N is a heavy isotope of nitrogen but not radioactive. Many generations of E. coli were bred in the same medium, so the newly synthesised DNA had the 15N isotope as a constituent of purines and pyrimidines. This new, heavy DNA molecule showed high density as compared to normal DNA when centrifuged in a caesium chloride (CsCl) density gradient. Later, E. coli cells were transferred into a medium with normal 14NH4Cl. After the cells multiplied, their DNA had both 15N and 14N isotopes and showed intermediate density when centrifuged in CsCl gradient, referred to as ‘hybrid DNA’. The next generation of E coli, which was born after forty minutes, had equal amounts of hybrid DNA and ‘light’ DNA and showed low density when centrifuged in CsCl gradient. Thus, the findings of the Meselson and Stahl experiment validated Watson and Crick’s theory of the ‘semiconservative’ DNA replication scheme.

Similar experiments were further conducted in plants and human cells by Dr J Herbert Taylor and his colleagues in 1958 in root-tip cells of faba beans, using radioactive thymidine to determine the distribution of newly synthesised DNA in chromosomes. This experiment too proved that DNA replication is semiconservative.

Summary

Multiple experiments were conducted by different scientists to conclude that DNA replicate semi conservatively. Watson and Crick proved that DNA have a double helical structure and the two strands separate and act as a template for the synthesis of two new complementary strands. After the replication process was complete, each DNA molecule would end up with one parental strand and one newly synthesised strand. This replication scheme of DNA is called ‘semiconservative’.

To validate this scheme of DNA replication, experiments were conducted by Matthew Meselson and Franklin Stahl on E. coli, a bacterium that divides every twenty minutes. E. coli was grown in a medium that had 15NH4Cl as the only nitrogen source, where 15N is a heavy isotope of nitrogen but not radioactive. Many generations of E. coli were bred in the same medium, so the newly synthesised DNA had the 15N isotope as a constituent of purines and pyrimidines. This new, heavy DNA molecule showed high density as compared to normal DNA when centrifuged in a caesium chloride (CsCl) density gradient. Later, E. coli cells were transferred into a medium with normal 14NH4Cl. After the cells multiplied, their DNA had both 15N and 14N isotopes and showed intermediate density when centrifuged in CsCl gradient, referred to as ‘hybrid DNA’. The next generation of E coli, which was born after forty minutes, had equal amounts of hybrid DNA and ‘light’ DNA and showed low density when centrifuged in CsCl gradient. Thus, the findings of the Meselson and Stahl experiment validated Watson and Crick’s theory of the ‘semiconservative’ DNA replication scheme.

Similar experiments were further conducted in plants and human cells by Dr J Herbert Taylor and his colleagues in 1958 in root-tip cells of faba beans, using radioactive thymidine to determine the distribution of newly synthesised DNA in chromosomes. This experiment too proved that DNA replication is semiconservative.

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