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Page 89 of the Study Guide for Campbell Biology, 11th Edition has the following question:

  1. A plant cell has 12 chromosomes at the end of mitosis. How many chromosomes would it have in the G2 phase of its next cell cycle?

    a. 6

    b. 12

    c. 24

    d. 48

Page 449 states that the answer is b.

As far as I understand, the number of chromosomes a cell has in G2 phase is double the number of chromosomes as it has after mitosis (which would imply the answer is c).

For (b) to be true, the plant cell must have had 6 chromosomes at the end of mitosis, but the question states it had 12. The only way I can imagine this to be true is that the question assumes that "a plant cell [...] at the end of mitosis" represents both daughter plant cells before the cell plate & cell wall has fully split the two daughter cells up, but doesn't "the end of mitosis" imply that the cell wall has completely formed and there are two cells (each with 12 chromosomes)?

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I think I know what is confusing you. A chromosome can have one or two chromatids. If you have two chromatids, you don't necessarily have two chromosomes. This is what happens during S-phase, during replication. Before the S-phase each chromosome had one chromatid, after the replication, it has two. So the number of chromosomes stays the same.

In the karyogram (the picture with all chromosomes), each object is one chromosome. Usually those will be double-chromatid chromosomes, and you will see a nick (like here). After these chromosomes divide, you get two separate objects → two identical chromosomes.

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