Mitosis

Mitosis is the stage of the eukaryotic cell cycle where a cell’s already-duplicated chromosomes are pulled apart into two separate nuclei. Because it produces genetically identical cells with the same chromosome count, it’s called an equational division. This process guarantees that each new cell gets a complete, identical set of chromosomes, preserving genetic consistency from one cell generation to the next.

Mitosis follows the S phase of interphase, when DNA replication happens, and it ends with telophase and cytokinesis. During cytokinesis, the cytoplasm, organelles, and cell membrane are split roughly evenly between two new cells. Together, these stages make up the mitotic phase (M phase) of the cell cycle. The stages of mitosis itself—preprophase (only in plant cells), prophase, prometaphase, metaphase, anaphase, and telophase—each mark the completion of a specific set of events. In mitosis, the already-copied chromosomes condense and attach to spindle fibers, which then pull one copy of each chromosome to opposite sides of the cell. The result is two genetically identical daughter nuclei, and the rest of the cell can then divide via cytokinesis. These phases can be watched in real time using live cell imaging.

Mistakes in mitosis can lead to three or more daughter cells instead of the usual two—called tripolar or multipolar mitosis. Such errors can cause non-viable embryos that fail to implant. Other mistakes can trigger mitotic catastrophe, apoptosis (programmed cell death), or mutations, and some cancers can arise from these mutations.

Mitosis differs between organisms. Animal cells usually undergo open mitosis, where the nuclear envelope breaks down before chromosomes separate, while fungal cells typically have closed mitosis, with chromosomes dividing inside an intact nucleus. Most animal cells also change shape, rounding up into a near-sphere at the start of mitosis. Most human cells come from mitotic division, except for gametes (sperm and egg cells), which are produced by meiosis. Prokaryotes—bacteria and archaea—lack a true nucleus and divide by binary fission instead.

Descriptions of cell division appeared throughout the 18th and 19th centuries with varying accuracy. In 1835, German botanist Hugo von Mohl described cell division in the green alga *Cladophora glomerata*, stating that cells multiply through division. In 1838, Matthias Jakob Schleiden claimed that new cells form inside existing ones, a view later overturned in favor of Mohl’s model, thanks to contributions from Robert Remak and others. In animal cells, mitosis was discovered in frog, rabbit, and cat cornea cells in 1873, first described by Polish histologist Wacław Mayzel in 1875. Otto Bütschli, Schneider, and Fol may also have claimed the discovery of what is now called mitosis. In 1873, German zoologist Otto Bütschli published observations on nematodes, and a few years later he described mitosis based on them. The term “mitosis” was coined by Walther Flemming in 1882, from the Greek *mitos* (“warp thread”). Alternative names include “karyokinesis” (nuclear division), introduced by Schleicher in 1878, and “equational division,” proposed by August Weismann in 1887. Some authors use “mitosis” broadly to cover both karyokinesis and cytokinesis. Today, “equational division” more often refers to meiosis II, the part of meiosis most like mitosis.

The main outcome of mitosis and cytokinesis is transferring a parent cell’s genome into two daughter cells. The genome consists of chromosomes—tightly coiled DNA complexes carrying genetic information. To ensure daughter cells are genetically identical, the parent cell copies each chromosome before mitosis, during the S phase of interphase. This duplication produces two identical sister chromatids held together by cohesin proteins at the centromere. As mitosis begins, chromosomes condense and become visible. In some eukaryotes, like animals, the nuclear envelope breaks into small vesicles, and the nucleolus disappears. Microtubules extend from opposite ends of the cell, attach to centromeres, and align the chromosomes in the cell’s center. The microtubules then contract, pulling sister chromatids apart—now called daughter chromosomes. As the cell elongates, these daughter chromosomes are pulled toward opposite ends and condense fully in late anaphase. A new nuclear envelope forms around each set, and the chromosomes decondense into interphase nuclei. Typically after anaphase starts, the cell may undergo cytokinesis. In animal cells, the membrane pinches inward between the two developing nuclei to create two new cells. In plant cells, a cell plate forms between them. Cytokinesis doesn’t always happen; coenocytic cells, for example, can have multiple nuclei without dividing.