Mitosis and Cytokinesis

During prophase, replicated chromosomes condense into visible threadlike structures with the aid of condensin proteins. Simultaneously, centrosomes migrate to opposite cell poles and begin forming the mitotic spindle apparatus from microtubules. The nucleolus disappears, signaling the impending breakdown of the nuclear envelope.

Kinetochores are protein structures on centromeres where spindle microtubules attach during prometaphase. During anaphase, kinetochore-attached microtubules shorten, pulling sister chromatids apart toward opposite cell poles. This separation is enabled by the breakdown of cohesion proteins holding the chromatid pairs together.

The metaphase plate is the cell's equatorial region where spindle microtubules align all condensed sister chromatid pairs in a fixed row during metaphase. This alignment ensures that each daughter cell receives an identical set of chromosomes. The metaphase plate marks the transition point before sister chromatids separate during anaphase.

In animal cells, actin filaments form a contractile ring that pinches the cell into two, creating a cleavage furrow. In plant cells, Golgi vesicles carrying structural proteins form a cell plate at the former metaphase plate location, which fuses with plasma membranes to create a new cell wall dividing the cell.

During telophase, chromosomes decondense into chromatin at opposite cell poles, and spindle microtubules depolymerize into tubulin monomers for reuse in daughter cells. Nuclear envelopes reassemble around each chromosome set, restoring two distinct nuclei. These events prepare the cell for cytokinesis and the formation of two genetically identical daughter cells.

The spindle apparatus is a web-like structure of microtubules that extends from centrosomes at opposite cell poles. Kinetochore microtubules attach to chromosomes and shorten during anaphase, pulling sister chromatids apart. Polar and astral microtubules push spindle poles apart, coordinating chromosome segregation and cell elongation throughout mitosis.

Mitosis accounts for approximately one hour of the 24-hour cell cycle in most human cells. The complete cell cycle includes DNA replication during S phase and gap periods for cell preparation. This timing ensures coordinated, regulated progression through all mitotic stages from prophase through cytokinesis.