What is the main event during anaphase in mitosis or meiosis?

During anaphase, the critical event is that chromosomes move toward opposite ends of the cell. This occurs when the sister chromatids, which have been previously duplicated and aligned at the cell's equatorial plane during metaphase, are pulled apart by the spindle fibers attached to their centromeres. This separation ensures that each daughter cell receives an identical set of chromosomes when the cell ultimately divides.

As the chromatids are pulled apart, the cell prepares for the final stages of division, either through mitosis or meiosis. This movement is essential for ensuring proper chromosomal distribution, which is crucial for genetic stability and the successful formation of new cells. The process is a key aspect of cell division and directly contributes to the overall success of cellular reproduction.

The other options—chromosomes condensing, aligning at the equator, and the re-formation of the nuclear envelope—are associated with different stages of the cell cycle. Condensation and alignment occur before anaphase during prophase and metaphase, respectively, while nuclear envelope re-formation happens in telophase, after the chromosomes have already been separated.