Cell Cycle Question

[bytesyzechic]

Hi,
I'm studying this practice test and it has this question about radioactive thymidine exposure during the cell cycle.
I understand that it takes 1 hour for Radioactive thymidine to be first detected due to the length of G2, which, paired with S phase, is when DNA replication occurs. Therefore, at 2 hours, the first set of cells would have just finished M-phase and the next set of G2 cells would have entered Mitosis. However, I'm not entirely sure how to caculate that 20% of labelled cells are in M phase at 2 hours after thymidine exposure.

any help would GREATTTTTLY be appreciated. I attached the question in a jpg to this post.

[bytesyzechic]

Sorry, I think I figured it out.

It's not so much a cellular bio question, just tricky.

In case anyone's wondering, this is what I think it is. It's that all the cells T = 1 hour have either entered G2 phase or are somewhere in S phase (That's the only way that 100% of the cells can be radioactively labelled at 2 hours). Assuming equal distribution of cells across S phase (i.e. 20% in 1st hour, 20% in 2nd hour, etc...) 20% of cells would be entering G2 phase at T=1hr. Those 20% of cells would then enter M-phase at T=2hr.

Am I right in this assumption?