Mitosis and Cytokinesis

Before mitosis starts, S phase has already copied every chromosome, so each one exists as two identical sister chromatids joined at a centromere. Mitosis then runs through four stages. In prophase, chromosomes condense and become visible, the nuclear envelope breaks down, and spindle fibers start growing from centrosomes at opposite poles. In metaphase, those spindle fibers attach to a protein structure on the centromere called the kinetochore, and this tug-of-war lines every chromosome up along the cell's equator. In anaphase, the connection holding sister chromatids together snaps, and spindle fibers shorten, hauling one chromatid of each pair to opposite poles. In telophase, two new nuclear envelopes form around each full chromosome set and the chromosomes relax back out.

The careful lineup at metaphase matters because it guarantees that when chromatids separate, each pole gets exactly one copy of every chromosome, not a random assortment. Skip that checkpoint-driven alignment and a cell risks sending two copies of one chromosome to one side and none to the other. Cytokinesis finishes the job by physically splitting the cytoplasm. Animal cells pinch inward using a contractile ring of actin filaments that tightens like a drawstring, forming a cleavage furrow. Plant cells, boxed in by a rigid cell wall, instead build a new wall from the inside out: vesicles from the Golgi collect at the center and fuse into a cell plate that grows outward until it meets the existing wall.

Because replication already doubled the DNA before any of this starts, mitosis itself never halves or shuffles genetic material, it only distributes identical copies, which is why daughter cells match the parent and each other. This sits right after the cell cycle checkpoints that decide whether a cell is cleared to divide at all, and sets up the next topic: what happens when that control breaks down.