Which event is associated with the onset of anaphase during mitosis?

During anaphase in mitosis, the defining event is the separation of sister chromatids, which then begin to move toward opposite poles of the cell. This separation is facilitated by the spindle apparatus, which attaches to the chromatids at their centromere. As anaphase commences, the cohesin proteins that hold the sister chromatids together are cleaved, allowing them to split apart. The pulling force generated by the spindle fibers then guides each chromatid toward opposite ends of the cell, ensuring that when the cell divides, each daughter cell receives an identical set of chromosomes.

The other events mentioned, such as the alignment of chromosomes at the cell's equator, are characteristic of metaphase, which occurs just prior to anaphase. The breakdown of the nuclear membrane is a feature of prophase, preceding the alignment and eventual separation of chromatids. The dissolution of spindle fibers is not an event that initiates anaphase; rather, it occurs later, as the cell prepares to complete the division process after the chromatids have been segregated. Thus, the movement of chromatids to opposite poles is the hallmark event that characterizes the onset of anaphase.