Meselson-Stahl Experiment - Online Tutor, Practice Problems & Exam Prep

The Meselson-Stahl experiment demonstrated that DNA replication follows the semiconservative model, where each new DNA molecule consists of one parental strand and one newly synthesized strand. Using nitrogen isotopes, N₁₅ for old strands and N₁₄ for new strands, they confirmed that the conservative and dispersive models were incorrect. This experiment is crucial for understanding DNA replication mechanisms, highlighting the importance of template strands in synthesizing complementary DNA during cell division.

Meselson-Stahl Experiment

In this video, we're going to introduce the Meselson-Stahl experiment. After the structure of DNA was revealed by James Watson and Francis Crick, scientists worldwide began to wonder how DNA genetic material is actually replicated and passed on from one generation of cells to the next. In 1958, scientists Matthew Meselson and Franklin Stahl demonstrated that a specific strain of bacteria called Escherichia coli, or E. coli, replicates its own DNA via a specific model known as the semi-conservative model. The semi-conservative model of DNA replication states that replicated DNA molecules consist of one old or parental DNA strand and one brand newly built DNA strand. It turns out that the old parental strands in the original DNA molecule, during DNA replication, separate from each other and act as templates for the synthesis of brand new DNA, which is complementary to the old parental strands.

We will discuss the mechanism of semi-conservative DNA replication in more detail later in our course. For now, focusing on the Meselson-Stahl experiment, this experiment confirmed semi-conservative DNA replication. If we look at our image below, on the left-hand side, there is a key. The darker blue strands throughout represent a specific DNA strand labeled using the nitrogen-15 isotope, symbolized as N¹⁵. The N¹⁵ isotope labels the old or parental DNA strands, the original DNA strand. Another isotope, nitrogen-14, symbolized as N¹⁴, represented using a light blue color, labels the brand new DNA strands.

At the time of the experiment, there were three competing models for DNA replication. These models were:

1. The conservative model of DNA replication, shown in the first row.
2. The semi-conservative model of DNA replication, which is the correct model, shown in the middle row.
3. The dispersive model, shown in the last row.

Meselson and Stahl confirmed that the semi-conservative model is the correct one. This is indicated by the green check mark in the image, while the red X’s show that the conservative and dispersive models did not yield the expected results.

In the experiment, initially, the E. coli were grown in a medium containing the N¹⁵ isotope. When these bacteria were transferred to a medium containing N¹⁴, the original strands remained labeled with N¹⁵, and the newly built DNA strands were labeled with N¹⁴. The conservative model suggested that the original DNA molecule would be conserved, which was not supported by the results. The semi-conservative model, where ‘semi’ means partial, showed that during DNA replication, half of the molecule consists of an old strand, and the other half consists of a new strand. These two strands separate, and a new strand is built next to each using it as a template.

The experiment’s results were consistent with the semi-conservative model, proving the other models incorrect. This model is the central focus, affirming that DNA replicates via the semi-conservative model. This concludes our introduction to the Meselson-Stahl experiment and the semi-conservative DNA replication model. I’ll see you all in our next video.

The Meselson-Stahl experiment demonstrated that DNA replication produces new molecules of DNA each containing…

• A. Two old strands of DNA.
• B. Two new strands of DNA.
• C. Two stands composed of variable proportions of old and new DNA.
• D. One old strand of DNA and one new strand of DNA.

The DNA of a phage was injected into the bacterial host, but the protein coat remained outside. The viral DNA directed the host to replicate the new phage viruses. Which scientist(s) are associated with this finding?

• A. Hershey and Chase.
• B. Meselson and Stahl.
• C. Thomas and Walters.
• D. Watson and Crick.
• E. Chargaff.

Here’s what students ask on this topic:

What was the Meselson-Stahl experiment and why is it important?

The Meselson-Stahl experiment, conducted in 1958 by Matthew Meselson and Franklin Stahl, demonstrated that DNA replication follows the semiconservative model. This means that each new DNA molecule consists of one parental strand and one newly synthesized strand. They used nitrogen isotopes, N₁₅ for old strands and N₁₄ for new strands, to label the DNA. This experiment is crucial because it confirmed the mechanism of DNA replication, which is fundamental for understanding genetic inheritance and molecular biology principles.

How did Meselson and Stahl use nitrogen isotopes in their experiment?

Meselson and Stahl used nitrogen isotopes to distinguish between old and new DNA strands. They grew E. coli in a medium containing N₁₅ (a heavy isotope of nitrogen) so that the DNA incorporated this isotope. They then switched the bacteria to a medium containing N₁₄ (a lighter isotope). During DNA replication, the old strands containing N₁₅ served as templates for new strands incorporating N₁₄. This allowed them to track the distribution of old and new DNA strands through density gradient centrifugation, confirming the semiconservative model of DNA replication.

What are the three models of DNA replication that were considered before the Meselson-Stahl experiment?

Before the Meselson-Stahl experiment, three models of DNA replication were considered: the conservative model, the semiconservative model, and the dispersive model. The conservative model proposed that the original DNA molecule remains intact and a completely new molecule is synthesized. The semiconservative model, which was confirmed by Meselson and Stahl, proposed that each new DNA molecule consists of one old (parental) strand and one newly synthesized strand. The dispersive model suggested that the original DNA molecule is broken into pieces and dispersed throughout the new DNA molecules. Meselson and Stahl's experiment disproved the conservative and dispersive models.

What were the key findings of the Meselson-Stahl experiment?

The key findings of the Meselson-Stahl experiment were that DNA replication follows the semiconservative model. This means that each new DNA molecule consists of one parental strand and one newly synthesized strand. By using nitrogen isotopes (N₁₅ for old strands and N₁₄ for new strands) and density gradient centrifugation, they demonstrated that after one round of replication, DNA molecules consisted of hybrid strands (one old and one new). After subsequent rounds, the DNA showed a mix of hybrid and fully new DNA molecules, confirming the semiconservative replication model.

How did the Meselson-Stahl experiment disprove the conservative and dispersive models of DNA replication?

The Meselson-Stahl experiment disproved the conservative and dispersive models of DNA replication through the use of nitrogen isotopes and density gradient centrifugation. The conservative model predicted that after replication, there would be one molecule with entirely old DNA and one with entirely new DNA, which was not observed. The dispersive model predicted that DNA would be a mix of old and new segments within each strand, resulting in intermediate densities after each replication cycle. However, the experiment showed distinct bands corresponding to hybrid DNA (one old and one new strand) and fully new DNA, consistent with the semiconservative model.