In an experiment, a mitotic cell is treated with a drug that prevents microtubules from polymerizing. The cell has already completed DNA replication in S phase and entered M phase. Chromosomes condense, but kinetochores cannot attach to spindle microtubules, so the spindle checkpoint remains active. Which outcome is most likely for the treated cell during M phase?
- The cell proceeds into anaphase because DNA replication was completed in S phase
- The cell arrests at metaphase with condensed duplicated chromosomes not segregating (correct answer)
- The cell enters G2 and begins DNA replication again without division
- The cell completes meiosis I, separating homologous chromosomes into two cells
- The cell reforms a nuclear envelope around chromosomes that have already separated
Explanation: This question requires analysis of the cell cycle, specifically how the spindle checkpoint responds to failed kinetochore-microtubule attachments. When microtubules cannot polymerize due to drug treatment, kinetochores cannot attach to spindle fibers, which keeps the spindle checkpoint active and prevents progression from metaphase to anaphase. The cell will arrest at metaphase with condensed duplicated chromosomes that cannot segregate (answer B), as the checkpoint continuously monitors attachment status and blocks anaphase onset until all kinetochores are properly attached. Answer A incorrectly suggests the cell proceeds to anaphase, representing a cause-and-effect error where students think DNA replication completion is sufficient for anaphase, when actually proper spindle attachments are required. The strategy is to recognize that the spindle checkpoint specifically monitors kinetochore attachments, not DNA replication status.