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The method developed by Meselson and Stahl to separate heavy DNA with N15 from DNA with N14 for providing evidence for semi-conservative replication of DNA is

Option: 1

Ion exchange chromatography


Option: 2

Density gradient centrifugation


Option: 3

Isopycnic centrifugation


Option: 4

Gel filtration


Answers (1)

  • Matthew Meselson and Franklin Stahl invented the “density gradient centrifugation” method, which uses N15 to separate heavy DNA from DNA and provide evidence for semi-conservative DNA replication.
  • They used the E. coli bacteria as a model system for their experiment.
  • They use a medium with a "heavy" nitrogen isotope (N15) to grow E. coli.
  • The bacteria take in the nitrogen and use it to make new biological molecules, like DNA, from it.
  • The bacteria are then transferred to a medium containing a "light" isotope of nitrogen (N14) and allowed to grow for several generations after growing for many generations in the N15 medium.
  • After the switch, the DNA would need to be comprised of N15 on the grounds that this was the main nitrogen accessible for DNA union.
  • Every generation, they took small samples and extracted and purified the DNA. They then used density gradient centrifugation to measure the DNA's density.
  • Density gradient centrifugation spins the DNA bands at high speeds in the presence of caesium chloride to separate them.

The following findings were discovered following the analysis of the first four generations of E. coli:

  • After one generation, a single band of DNA is produced when DNA is centrifuged. It has a density that is somewhere in between the light N14 and heavy N15 DNA. The DNA of this generation is a cross between light and heavy.
  • After the second generation, two bands of DNA are produced when the centrifuge is spun. The first band is in the same spot as the intermediate band of the first generation, while the second band is slightly higher up at the light molecule's position. DNA's semi-conservative replication is demonstrated here.
  • In the third and fourth generations of centrifugation, it has been observed that light molecules will produce lighter molecules, and hybrid DNA molecules will give rise to hybrid ones.
  • Following their experiment, Mendelson and Stahl came to the conclusion that every strand in a DNA molecule serves as a template for the production of a new, complementary strand.

Thus, the correct option is 2).

Posted by

Sumit Saini

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