The bark of the Pacific yew tree (Taxus brevifolia) was the original source of one of the most effective drugs for treating tumors of the breast, lung, and other sites. Taxol, a chemical extracted from this bark, kills actively replicating cells by inhibiting the depolymerization of microtubules. Why are microtubules good targets for killing cancerous cells?
When actively growing cells are treated with Taxol, they often are unable to complete the cell cycle. Based on what you have learned about cell-cycle checkpoints, which checkpoint likely causes these cells to arrest? Explain your reasoning.
-The first M phase checkpoint. If the sister chromatids all kinetochores are not attached properly to the spindle apparatus, then the sister chromatids do not split and the cells would arrest in M phase.
-The second M phase checkpoint. If microtubule depolymerization were inhibited, the chromosomes would not completely separate in anaphase, and the cells would arrest in M phase.
-G2 checkpoint. If DNA is damaged or if chromosomes are not replicated correctly, the inhibitory phosphate on MPFs Cdk subunit won't removed. As a result, MPF is not turned on, and cells remain in G2 phase.
-G1 checkpoint. If the growth of the cell was blocked, or if DNA was physically damaged and cannot be repaired, then the cell would arrest in G1 phase.
Answer:
-The second M phase checkpoint. If microtubule depolymerization were inhibited, the chromosomes would not completely separate in anaphase, and the cells would arrest in M phase.
Explanation:
Depolymerization of spindle microtubules is required as it shortens them and pulls the separated sister chromatids to the opposite poles during mitotic anaphase. Separation of sister chromatids to the opposite poles during anaphase ensures that the daughter cells obtain an equal number of chromosomes. If depolymerization is inhibited by taxol, the second M phase checkpoint would not allow the cells to proceed through anaphase and the cells will be arrested in the M phase.
