Mitosis and Cytokinesis

Mitosis consists of prophase, prometaphase, metaphase, anaphase, and telophase. During prophase, chromosomes condense and the spindle apparatus forms. Prometaphase involves nuclear envelope breakdown and kinetochore attachment. Metaphase aligns chromosomes at the cell's equator. Anaphase separates sister chromatids, and telophase reforms nuclear envelopes around each chromosome set.

Kinetochores are protein structures that form on centromeres during prometaphase. Spindle microtubules attach to kinetochores on opposite poles, tethering each sister chromatid to a different cell pole. During anaphase, kinetochore-attached microtubules shorten, pulling the separated chromatids toward opposite ends of the cell.

The spindle apparatus is a web-like structure made of microtubules that forms between centrosomes during prophase. It attaches to kinetochores, aligns chromosomes at the metaphase plate, and pulls sister chromatids apart during anaphase. Polar and astral microtubules also push spindle poles apart and anchor them to the cell membrane.

In animal cells, actin filaments form a contractile ring that creates a cleavage furrow, pinching the cell into two. In plant cells, vesicles from the Golgi apparatus carrying structural proteins form a cell plate at the former metaphase plate location. This cell plate eventually fuses with plasma membranes, forming a new cell wall that divides the cell.

Chromosomes condense during prophase through the action of proteins called condensins, transforming replicated chromatin into visible x-shaped structures. Condensation allows the cell to organize and efficiently distribute genetic material to daughter cells. Condensed chromosomes remain organized throughout mitosis until they decondense back into chromatin during telophase.

The nuclear envelope dissolves during prometaphase, exposing chromosomes to the cell's contents and allowing spindle microtubules to attach to kinetochores. During telophase, two new nuclear envelopes reassemble around each set of chromosomes at opposite poles. This reformation protects the genetic material in each daughter cell.

Mitosis accounts for approximately one hour of the roughly 24-hour cell cycle in typical human cells. The remaining time is spent in interphase, which includes DNA replication and cell growth phases. This timing ensures cells divide at a controlled rate while maintaining genetic stability.