Mitosis and Chromosome Dynamics (2C) Practice Test

Human epithelial cells were engineered to express a fluorescent kinetochore marker and imaged during mitosis. Investigators added a small molecule that reduces microtubule dynamics without fully depolymerizing the spindle. Compared with vehicle-treated cells, treated cells spent longer between nuclear envelope breakdown and anaphase onset, and many cells displayed chromosomes remaining near one spindle pole rather than forming a tight metaphase plate. Immunostaining showed that these mispositioned chromosomes frequently retained spindle-assembly checkpoint (SAC) proteins at their kinetochores.

Based on the study, which conclusion about chromosome alignment is most consistent with the data?

In a study of chromosome alignment, researchers cultured cells in low-dose nocodazole that partially suppresses microtubule polymerization but does not eliminate spindles. Many cells formed bipolar spindles with several chromosomes failing to align at the metaphase plate. These cells showed persistent SAC signaling and delayed anaphase onset. When nocodazole was washed out, chromosomes rapidly aligned and anaphase proceeded.

Which event would be expected during metaphase in the control (untreated) cells but is least likely in the nocodazole-treated cells?