Meselson-Stahl DNA Replication
Replicate the most beautiful experiment in biology. Centrifuge radioactive nitrogen isotopes (15N vs 14N) across bacterial generations to generate heavy, hybrid, and light DNA bands proving semi-conservative replication.
THE MOST BEAUTIFUL EXPERIMENT IN BIOLOGY
In 1958, Matthew Meselson and Franklin Stahl designed an experiment to determine how DNA replication occurs. They wanted to test three competing hypotheses: 1. **Semiconservative**: Each new double helix contains one original (parental) strand and one new (daughter) strand. 2. **Conservative**: The parental double helix remains intact, and a completely new double helix is formed. 3. **Dispersive**: Each strand of both double helices contains a mixture of old and new DNA.
THE DENSITY CENTRIFUGATION METHOD
The researchers grew E. coli in a medium containing **heavy nitrogen ()** for many generations, then switched them to a medium with **light nitrogen ()**. By spinning the DNA in a centrifuge, they could separate DNA by its density. - After **Generation 1**, all DNA had an intermediate density (half-heavy), which immediately ruled out the conservative model. - After **Generation 2**, they observed both intermediate and light DNA bands, confirming the **semiconservative model**.
HOW TO USE THIS VISUALIZATION
1. **Choose Your Model**: Predict the band pattern for Conservative vs. Semiconservative vs. Dispersive. 2. **Run the Generations**: Watch the DNA molecules duplicate over several rounds of division in the medium. 3. **View the Centrifuge Tubes**: Compare the experimental results with your prediction. **Try This**: Run the simulation to Generation 3. How many 'intermediate' density strands are there compared to 'light' strands? Why will the intermediate band never disappear, even after dozens of generations?
AP EXAM CONNECTION
Unit: Unit 6: Gene Expression and Regulation (Topic 6.2)
Learning Objective: IST-1.L
COMMON MISCONCEPTIONS
- Thinking the nitrogen isotopes are radioactive (they are stable heavy isotopes).
- Confusing 15N with the phosphorus in the DNA backbone.
- Assuming the DNA itself is 'growing' (only the atoms are replaced during replication).
KEY TAKEAWAYS
- DNA replication is semiconservative.', 'Each new helix is 50% parental DNA.
- Isotopes provide a density 'tag'.
- Centrifugation reveals replication patterns.
PRACTICE QUESTIONS
Q1 (CONCEPTUAL): After one generation in light nitrogen, the DNA produced a single band of intermediate density. Which hypothesis was ruled out by this observation?
Show Answer & Explanation
Answer: Conservative replication.
Explanation: The conservative model predicts two distinct bands (one heavy and one light) after one generation. A single hybrid band supports both the semiconservative and dispersive models.
Q2 (CONCEPTUAL): What allows scientists to separate the DNA in this experiment?
Show Answer & Explanation
Answer: Differences in density due to nitrogen isotopes.
Explanation: 15N has an extra neutron compared to 14N, making the DNA molecules slightly heavier and causing them to settle at different positions in a centrifuge.
