Meselson and Stahl’s Experiment

Before 1958, scientists knew that DNA carried genetic information, but how DNA made exact copies of itself remained a mystery. Three competing models attempted to explain this mechanism. 

About the Scientists:

• Matthew Meselson and Franklin Stahl conducted the experiment at the California Institute of Technology (Caltech), Pasadena, USA​
• Period of experiment: October 1957 to January 1958; published in 1958
• Organism used: Escherichia coli (E. coli) - chosen because it divides every ~20 minutes, making generational tracking feasible

Aim of the Experiment:

To determine the mechanism of DNA replication by tracking nitrogen isotope-labelled DNA strands across generations of E. coli using density gradient centrifugation.

Materials Used:

Principle of the Technique:

CsCl Density Gradient Centrifugation

When a mixture of DNA and cesium chloride (CsCl₂) solution is spun at extremely high speeds in an ultracentrifuge, the CsCl ions form a density gradient within the tube - lighter at the top, denser at the bottom. DNA molecules migrate and form bands at the level matching their own buoyant density.

• Heavy DNA (¹⁵N/¹⁵N): settles lower in the tube (higher density)
• Light DNA (¹⁴N/¹⁴N): settles higher in the tube (lower density)
• Hybrid DNA (¹⁵N/¹⁴N): settles at an intermediate position

Meselson and Stahl’s Experiment

Observations:

Band Pattern Across Generations

Inference and Conclusion:

The experimental results led to the following critical inferences:

1. After Generation 1: Only ONE hybrid band appeared - this eliminated the Conservative model (which would have produced two separate bands: one heavy, one light)​
2. After Generation 2: TWO bands appeared (one hybrid + one light) - this eliminated the Dispersive model (which would have produced a single, progressively lighter band in each generation)
3. Both observations together confirmed that DNA replication is SEMI-CONSERVATIVE - each new DNA molecule retains one original (parental) strand and synthesizes one new strand

• The experiment was performed by Meselson and Stahl in 1958 using E. coli, which divides every 20 minutes and is easy to track across generations.
• Bacteria were grown in heavy nitrogen (¹⁵N) medium, then shifted to light nitrogen (¹⁴N) medium, and their DNA was separated by CsCl density gradient centrifugation.
• After the first replication, a single hybrid band appeared, which ruled out the conservative model.
• After the second replication, one hybrid and one light band appeared, which ruled out the dispersive model.
• These results proved that DNA replication is semi-conservative, where each new DNA molecule has one old strand and one new strand.