Meselson and Stahl moved heavy bacteria to a light medium…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Meselson and Stahl moved heavy bacteria to a light medium. For each of the three models of replication, say what the tube should show after the first generation and after the second generation: how many bands, and where they lie (heavy, hybrid or light). Then say which results of the experiment ruled out each model.
Need a reminder? The key ideaShow
In semiconservative replication, each new double helix has one old strand and one new strand. After generations in a light medium, of the molecules are hybrid, and the rest are light. After one generation the tube shows one band in the middle. After two generations it shows a hybrid band and a light band in equal amounts.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Conservative. The old molecule stays heavy, and a new molecule is light. After the first generation there are two bands, heavy and light, in equal amounts. After the second, a heavy band and a light band, with three times as much light DNA.
Step 2. Semiconservative. After the first generation, one hybrid band. After the second, a hybrid band and a light band, in equal amounts.
Step 3. Dispersive. After the first generation, one band half way between heavy and light, the same place as the hybrid band. After the second generation, still one band, but lighter, since a quarter of each strand is old.
Step 4. What the results ruled out. After one generation the real tube had one band, which rules out the conservative model. After two generations it had two bands, which rules out the dispersive model.
Answer: the results fit only the semiconservative model.
Watch out. The first generation alone cannot tell the semiconservative model from the dispersive one, because both give one band in the middle. The second generation does.
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