What occurs during prometaphase in mitosis?

During prometaphase in mitosis, the key event is the attachment of chromatids to the spindle apparatus. This phase follows prophase, where the chromatin condenses into visible chromosomes. In prometaphase, the nuclear envelope completely breaks down, allowing the spindle microtubules to interact with the chromosomes. Each chromosome, now consisting of two sister chromatids, is captured by the spindle fibers at specialized structures called kinetochores, which are located at the centromeres of the chromosomes.

This attachment of chromatids to the spindle is crucial as it prepares the chromosomes for the subsequent alignment during metaphase and eventual separation during anaphase. Thus, monitoring this attachment is vital for the correct distribution of genetic material to the daughter cells.

The other choices highlight processes that occur at different stages in mitosis. For instance, alignment on the metaphase plate occurs next during the metaphase stage, while the reformation of the nucleus and movement of sister chromatids to the opposite poles happen later, specifically in telophase and anaphase, respectively. Hence, the primary event of prometaphase focuses on the connection of chromatids to the spindle apparatus.