Bacteria that have only heavy DNA (15N) are moved to a medium with light nitrogen (14N) and are…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Bacteria that have only heavy DNA (15N) are moved to a medium with light nitrogen (14N) and are allowed to divide three times. DNA is taken out and spun in a caesium chloride gradient.
- What does the tube show after the first generation?
- After three generations, what per cent of the DNA molecules are hybrid?
- After three generations, what per cent are light?
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.
Given. Three generations in 14N, starting with heavy DNA.
Plan. Count the molecules after three generations, and the two that keep an old strand.
Step 1. (a) After one generation all the molecules have one heavy and one light strand. The tube shows one band, in the middle.
Step 2. (b) After three generations there are molecules for each starting molecule. Two are hybrid, so .
Step 3. (c) The rest are light: .
Answer: one hybrid band; hybrid; light.
Check. The same rule gives hybrid after one generation and after two.
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