What marks the transition from metaphase to anaphase?

The transition from metaphase to anaphase is characterized by the separation of sister chromatids. During metaphase, chromosomes are aligned at the cell's equatorial plane, and each chromosome consists of two sister chromatids connected by a centromere. The crucial moment that marks the shift to anaphase is when the proteins that hold the sister chromatids together are cleaved, allowing them to be pulled apart toward opposite poles of the cell. This separation is essential for ensuring that each daughter cell receives an identical set of chromosomes.

The formation of spindle fibers occurs prior to metaphase as the cell prepares for chromosome alignment and separation. Condensation of chromosomes happens in earlier stages of cell division, specifically during prophase, when the chromatin tightens into distinct chromosomes. The reformation of the nuclear envelope occurs later in telophase, after the chromatids have been separated and grouped at the poles. Thus, the accurate identification of sister chromatid separation as the defining event of the transition to anaphase highlights the critical processes involved in proper cell division.