What key event occurs during prophase of cell division?

During prophase of cell division, the key event is the dissolution of the nuclear envelope. This process is crucial as it marks the transition from the interphase, where the cell prepares for division, to the actual mitotic phase.

As prophase progresses, chromatin condenses into visible chromosomes which consist of two sister chromatids. The nucleolus also fades, and the mitotic spindle begins to form from microtubules. The breakdown of the nuclear envelope allows the spindle fibers to interact with the chromosomes, facilitating their movement to opposite poles of the cell in subsequent stages of mitosis. This is essential for the proper segregation of genetic material, ensuring that each daughter cell receives an identical set of chromosomes when the cell divides.

The other events mentioned, such as the alignment of chromosomes at the center, formation of the cell membrane, and DNA replication, occur at different stages in the cell cycle. Chromosomes line up at the cell equator during metaphase, a new cell membrane forms during cytokinesis, and DNA replication occurs during the S phase of interphase before mitosis begins.