A flask holds bacteria that contain 4 heavy DNA molecules in all (15N in both strands of each)…
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A flask holds bacteria that contain 4 heavy DNA molecules in all (15N in both strands of each). The bacteria are moved to a medium with 14N, and they grow without any loss until they have divided 5 times. Count the DNA molecules, and ignore the other chromosomes of the cells.
- How many DNA molecules are there in all?
- How many of them are hybrid?
- How many of them are light?
- What per cent of the molecules are hybrid?
Need a reminder? The key ideaShow
The worked solution
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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. heavy molecules, generations.
Plan. Double the number five times. Find the hybrids from the number of old strands.
Step 1. (a) molecules.
Step 2. (b) The starting molecules have old strands. Each old strand is in a different molecule, so there are hybrid molecules.
Step 3. (c) light molecules.
Step 4. (d) .
Answer: ; ; ; .
Check. The share is the same, because the number of starting molecules cancels.
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