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Meselson and Stahl Experiment

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Meselson and Stahl Experiment: Introduction

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Replication is the process of generating a duplicate of anything. Regarding the replication model of DNA, or how DNA replicates, numerous hypotheses have been put forth by various scientists. The semiconservative replication of DNA was proposed by Watson and Crick. This model states that the DNA's two strands split apart and for the synthesis of a new strand, each strand serves as a template. Based on complementary base pairing with the template, the new strand is created.

The Meselson and Stahl Experiment was carried out to demonstrate the semi-conservative character of DNA replication. In 1958, Franklin Stahl and Matthew Meselson conducted research on E. coli bacteria.

Meselson and Stahl Experiment

Meselson and Stahl used the E.coli bacterium as a model system in their well-known investigations into DNA replication. They started by raising E. coli in nutritional broth or a medium that contained the "heavy" isotope of nitrogen, i.e., 15N (an isotope of an element contains different number of neutrons in its nucleus). The bacteria absorbed the nitrogen when cultivated on a medium containing heavy 15N and utilised it to create new biological molecules, including DNA.

The DNA of the bacteria's nitrogenous bases was all marked with heavy 15N after many generations of growth in the 15N. The bacteria were then transferred to a medium containing a "light" 14N isotope and left to continue growing for a number of generations. 14N, the sole nitrogen that would have been available for DNA synthesis following the transition, would have had to make up all of the DNA.

Meselson and Stahl were able to gather small samples in each generation, extract the DNA, and purify it because they knew how frequently E. coli cells divided. Then, using density gradient centrifugation, they calculated the DNA's density (and, indirectly, its 15N and 14N concentration).

By spinning molecules like DNA at high speeds while another molecule, such as caesium chloride, generates a density gradient from top to bottom of the spinning tube, this technique divides molecules like DNA into bands. Minute changes, like those between 15N and 14N labelled DNA, can be detected using density gradient centrifugation.

Results of the Experiment

Generation 0

After centrifugation, DNA recovered from cells at the beginning of the experiment ("generation 0," right before switching to 14N medium) formed a single band. The DNA should have had only a heavy 15N at that time. Therefore, this finding was reasonable.

Generation 1

When centrifuged, DNA that had undergone one generation (one cycle of DNA replication) also created a single band. The density of this band, which was higher and between the heavy 15N and light 14N DNA, was intermediate.

From the intermediate band, Meselson and Stahl learned that the DNA molecules created during the initial round of replication were a mix of light and heavy DNA. The dispersive and semi-conservative models, but not the conservative model, agreed with this result.

Two different bands in this generation would have been predicted by the conservative model (a band for the heavy original molecule and a band for the light, newly made molecule).

Generation 2

Two bands emerged after centrifuging second-generation DNA. One was higher (seemed to be designated merely with 14N), while the second was in the same place as the intermediate band from the first generation. Meselson and Stahl were informed by this finding that the DNA was replicated semi-conservatively.

The pattern of two separate bands—one at a hybrid molecule's place and the other at a light molecule's position—is exactly what we would anticipate for semi-conservative replication. A "purely light" molecule cannot be produced in dispersive replication since every molecule should contain both old and new DNA fragments.

Generation 3 and 4

Each hybrid DNA molecule from the second generation should, according to the semi-conservative model, result in both a hybrid molecule and a light molecule in the third generation, but each light DNA molecule should only produce more light molecules.

Therefore, over the third and fourth generations, we would anticipate the light band to get stronger and the hybrid band to get fainter (as it would represent a smaller portion of the total DNA because it would represent a larger fraction).

Meselson and Stahl experiment Diagram

Meselson and Stahl Experiment Diagram


In semi-conservative replication, each of the two parental DNA strands acts as a template for new DNA strands to be synthesised. However, after replication, each parental DNA strand base pairs with the complementary newly synthesised strand, and both double-stranded DNAs include one parental or "old" strand and one daughter or "new" strand. 

The word semi-conservative refers to the fact that the parental helix is half preserved and each parental single strand stays intact. 

Conclusion of Semi-Conservative Replication of DNA:

Meselson and Stahl experiment proved that DNA replicates semi-conservatively, which means that each of its strands acts as a template for the synthesis of a new, complementary strand.

Despite the fact that Meselson and Stahl conducted their research using the bacterium E. coli, we now understand that semi-conservative DNA replication is a universal process that all life forms on Earth share. The cells are replicating their DNA in a semi-conservative manner.

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FAQs on Meselson and Stahl Experiment

1.  Why does semi-conservative replication occur?

The process of semiconservative replication takes place in every individual cell so that the genetic make-up of the parent cells can be maintained. We only resemble one another on a species level and are genetically related to our parents because of this; otherwise, if the semi-conservative mode of replication had not been used, events would have been extremely random. In other words, we are only related to one another on a species level. Due to the semi-conservative nature of DNA replication, every species manages to maintain its genetic purity.

2. What is the biochemical nature of the transforming principle?

To find the transforming principle, bacteriologists did a number of experiments.

  • Alcohol precipitated the transforming principle. This demonstrated that it wasn't a carbohydrate.

  • Proteases were unable to eliminate the transforming principle. So, the protein was not the cause.

  • The lipases were unable to remove the transforming principle. This demonstrated that it wasn't a lipid.

  • Ribonuclease could not inactivate the transforming principle. Hence, RNA was not effective.

  • Deoxyribonuclease may be used to inactivate the transforming principle.

  • DNA was the transforming principle. As a result, DNA was the genetic material.

3. What are the conservative and semi-conservative methods of replication of DNA?

The characteristics of the conservative model, the semi-conservative model and the dispersive model are as follows.

  1. A single molecule containing both the parental copy of DNA and another copy including both strands of newly synthesised DNA is produced as a result of DNA replication, according to the conservative model of DNA replication.

  2. Two parental strands unwind in the semi-conservative model and each serves as a template for the synthesis of its complementary strand, resulting in the formation of two molecules of DNA, each with a parent strand and a daughter strand.

  3. According to the dispersive model of DNA replication, the parental DNA strands are randomly inserted into both daughter DNA molecules to produce hybrid DNA strands.

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