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Heavy DNA, light DNA: three models on trial
Topic: DNA replication
In 1958 Meselson and Stahl grew E. coli for many generations with heavy ¹⁵N as the only nitrogen source, so all its DNA was dense. They then moved the cells to ordinary ¹⁴N medium and sampled after one and two rounds of division. Double-stranded DNA from each sample was spun to equilibrium in a caesium chloride (CsCl) density gradient, where each molecule forms a band at the level matching its own density.
Experimental setup
- Question
- Is DNA replication conservative, semiconservative or dispersive?
- Independent variable
- Number of generations of growth in ¹⁴N after the switch from ¹⁵N
- Dependent variable
- Position of DNA bands in the CsCl density gradient
- Kept constant
- Same E. coli strain · Same CsCl gradient conditions · DNA kept double-stranded
- Groups
- Reference: Pure ¹⁵N DNA (heavy band) and pure ¹⁴N DNA (light band)
- Generation 1: One division in ¹⁴N
- Generation 2: Two divisions in ¹⁴N
Before looking at the real result, predict the band pattern each model would give after one and after two generations in ¹⁴N.
Conservative model
After 1 generationAfter 2 generationsSemiconservative model
After 1 generationAfter 2 generationsDispersive model
After 1 generationAfter 2 generations
| Row | After 1 generation | After 2 generations |
|---|---|---|
| Conservative model | ||
| Semiconservative model | ||
| Dispersive model |
Answer every part to submit.