What is the Meselson-Stahl experiment?

Answer 1

the reason for this experiment was to clarify how DNA replicates.There were 3 suggested models :the conservative.the semiconservative and the dispersive model. The experiment proved that DNA replicates in a semiconservative way i.e each of the parental strands serves as a template for the composition of a new strand.

Answer 2

The Meselson-Stahl experiment showed that in DNA duplication one stand remained the same and the second was the duplicated. This is called semi-conservative replication.

Answer 3

The Meselson-Stahl experiment showed that DNA replication is semiconservative; that a daughter cell's DNA is composed of one strand from the parent cell and one completely new strand.

Previously, there existed three alternative models of DNA replication - conservative, semiconservative, and dispersive.

The conservative model stated that the two strands of the parent cell would separate, act as templates for the new strands, then reassociate.

The semiconservative model stated that the two parent strands would separate, act as templates for the new complimentary strand to their own.

The dispersive model stated that each strand of both daughter cells would contain a mixture of new and old DNA.

In the 1950's Matthew Meselson and Franklin Stahl created a very clever and now classic experiment to decide, once and for all, which model it was.

Their experimental organism was Escherichia coli (E. coli), which they cultured for several generations in a medium with a heavy isotope of nitrogen, 15N. They then transferred the bacteria into a new medium with a lighter isotope of nitrogen, 14N. Two DNA samples were subsequently taken from the new medium with the bacteria; one after the first replication at 20 minutes and one after the second replication at 40 minutes. They then separately centrifuged the first sample (because the DNA would have different densities after being transferred and replicating in the lighter isotope).

If the conservative model was correct, they would see a clear contrast of less dense DNA (daughter) at the top of the test tube and the denser DNA (parental) at the bottom.

If the semiconservative model was correct, they would see all of the DNA with a consistent density because they would all have one heavy strand and one light strand.

If the dispersive model was correct, they would still see all of the DNA with a consistent density because the DNA strands would be mixtures of the old (heavy) and new (light) DNA.

They saw that the DNA from the first replication was of hybrid (14N and 15N) DNA with a consistent density. This meant that the conservative model must be wrong, but there were still two models - that was one too many.

So Meselson and Stahl centrifuged the second sample.

If the semiconservative model was correct, they would see some DNA at the same density as it was before (the hybrid DNA) and some DNA even less dense than before (14N).

If the dispersive model was correct, they would see all of the DNA with a lighter consistent density than before because the DNA strands would be mixtures of the old (hybrid) and new (light) DNA.

They saw some DNA at the same density as it was before and some DNA even less dense than before.

The semiconservative model was proven to be correct! This was used as proof for Crick and Watson's semiconservative model of DNA replication.