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DNA Replication Meselson and Stahl Experiment

In this DNA replication meselson and stahl experiment post we have briefly explained about chromosomal replication, semi conservative, the Meselson and stahl experiment

An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell.

Replication

DNA replication, the basis for biological inheritance, is a fundamental process that occurs in all living organisms that copies their DNA. This process is “replication” in that each strand of the original double-stranded DNA molecule serves as template for the reproduction of the complementary strand. Therefore, following replication, two identical DNA molecules have been produced from a single double-stranded DNA molecule. Cellular proofreading and error toe-checking mechanisms ensure near perfect fidelity for DNA replication.

Semi conservative

Semi conservative replication describes the mechanism by which DNA is replicated in all known cells. This mechanism of replication was one of three models originally proposed for DNA replication. Semiconservative replication would produce two copies that each contained one of the original strands and one new strand.

Conservative replication would leave the two original template DNA strands together in a double helix and would produce a copy composed of two new strands containing all of the new DNA base pairs.

Dispersive replication would produce two copies of the DNA, both containing distinct regions of DNA composed of either both original strands and both new strands.

The Meselson and Stahl Experiment

This is an experiment by Matthew Meselson and Franklin Stahl in 1958 which supported the hypothesis that DNA replication was semi conservative. Semiconservative replication means that when the double stranded DNA helix was replicated, each of the two double stranded DNA helices consisted of one strand coming from the original helix and one newly synthesized. The Meselson and stahl experiment has been called “the most beautiful experiment in biology.

Procedure

Nitrogen is a major constituent of DNA. 14N is by far the most abundant isotope of nitrogen, but DNA with the heavier (but non-radioactive) 15N isotope is also functional.

E. coli were grown for several generations in a medium with 15N. When DNA is extracted from these cells and centrifuged on a salt density gradient, the DNA separates out at the point at which its density equals that of the salt solution.

The DNA of the cells grown in 15N medium had a higher density than cells grown in normal 14N medium. After that, E. coli cells with only 15N in their DNA were transferred to a 14N medium and were allowed to divide; the progress of cell division was monitored by measuring the optical density of the cell suspension.

DNA was extracted periodically and was compared to pure 14N DNA and 15N DNA. After one replication, the DNA was found to have close to the intermediate density.

meselson and stahl experiment

DNA Replication Meselson and Stahl Experiment: Meselson And Stahl Experiment Diagram

Since conservative replication would result in equal amounts of DNA of the higher and lower densities (but no DNA of an intermediate density), conservative replication was excluded. However, this result was consistent with both semiconservative and dispersive replication.

 Semiconservative replication would result in double-stranded DNA with one strand of 15N DNA, and one of 14N DNA, while dispersive replication would result in double-stranded DNA with both strands having mixtures of 15N and 14N DNA, either of which would have appeared as DNA of an intermediate density.

The authors continued to sample cells as replication continued. DNA from cells after two replications had been completed was found to consist of equal amounts of DNA with two different densities, one corresponding to the intermediate density of DNA of cells grown for only one division in 14N medium, the other corresponding to DNA from cells grown exclusively in 14N medium.

This was inconsistent with dispersive replication, which would have resulted in a single density, lower than the intermediate density of the one-generation cells, but still higher than cells grown only in 14N DNA medium, as the original 15N DNA would have been split evenly among all DNA strands. The result was consistent with the semiconservative replication hypothesis.

Further Readings

Reference

  1. https://www.mun.ca/biology/scarr/the meselson and stahl experiment.html
  2. https://byjus.com/biology/dna-replication-experiment/
  3. https://maxanim.com/genetics/the meselson and stahl experiment/