Root-tip cells of the broad bean, Vicia faba, were grown for one cell cycle in radioactive…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Root-tip cells of the broad bean, Vicia faba, were grown for one cell cycle in radioactive thymidine, so that the DNA made in that S phase is labelled. The cells were then moved to ordinary medium and allowed to divide twice. Before each metaphase, a chromosome has two chromatids, and each chromatid has one DNA molecule.
- At the first metaphase after the labelled S phase, which chromatids of a chromosome are labelled?
- At the next metaphase, which chromatids are labelled?
- What would the first metaphase show if replication were conservative?
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.
Step 1. (a) In the labelled S phase, each chromosome copies its DNA. Each chromatid has one old, unlabelled strand and one new, labelled strand. So both chromatids are labelled.
Step 2. (b) In the second S phase, there is no label. The chromatid that had a labelled strand makes a molecule with that strand and a new unlabelled strand, and it is labelled. The other chromatid is made of an unlabelled old strand and an unlabelled new strand. So one chromatid is labelled and the other is not.
Step 3. (c) In a conservative copy, one daughter molecule is all new, and so fully labelled, and the other is all old and unlabelled. At the first metaphase only one chromatid would be labelled.
Answer: (a) both; (b) one of the two; (c) only one chromatid at the first metaphase.
Check. The real result, both labelled at the first metaphase, fits the semiconservative model.
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