Mitosis

• 1. Introduction

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  Mitosis is the process by which a single eukaryotic cell splits its replicated chromosomes into two genetically identical nuclei. Every time your skin replaces a worn-out cell, every time a starfish regrows a lost arm, and every time a human embryo adds another layer of complexity, mitosis is the mechanism at work. The word itself comes from the Greek mitos, meaning "warp thread," coined by the biologist Walther Flemming in 1882 to describe the thread-like appearance of chromosomes during division. But how, exactly, does a cell copy its entire genetic library and divide it perfectly in two? And what happens when the process goes wrong? Those two questions sit at the heart of everything that follows.

• 2. A Long Road to Discovery

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  Hugo von Mohl, a German botanist, described cell division in the green algae Cladophora glomerata in 1835, concluding that cells multiply through division rather than forming from scratch. A few years later, in 1838, Matthias Jakob Schleiden proposed a different model, suggesting that new plant cells formed inside existing ones. That view was later overturned in favor of Mohl's, thanks in part to the work of Robert Remak and others.

  In animal cells, the picture came into focus more slowly. Cell division with mitosis was first observed in frog, rabbit, and cat cornea cells in 1873, and the Polish histologist Waclaw Mayzel formally described it for the first time in 1875. Around the same time, the German zoologist Otto Butschli published data from observations on nematodes, and a few years later he independently described mitosis based on those findings.

  Naming disputes accompanied the discovery. The term "karyokinesis," meaning nuclear division, was introduced by Schleicher in 1878. August Weismann proposed "equational division" in 1887. Today, "equational division" is more commonly applied to meiosis II, while Flemming's "mitosis" held its ground as the standard term for the process he described.

• 3. The Phases, Step by Step

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  Before a cell can divide, it must prepare. During interphase, the cell grows through three sub-phases: G1, S, and G2. DNA replication happens exclusively during the S phase, producing pairs of sister chromatids bound together by cohesin proteins at the centromere. The entire interphase is regulated by cyclins, cyclin-dependent kinases, and a series of checkpoints that prevent the cell from advancing if its DNA is damaged or incomplete.

  When mitosis proper begins, chromosomes condense and become visible under a light microscope during prophase; in this stage they appear long, thin, and thread-like. Animal cells deploy centrosomes, each containing a pair of centrioles, which polymerize tubulin to form the spindle apparatus. Plant cells lack centrioles entirely, so microtubules assemble on the surface of the nucleus instead, and the chromosomes themselves help organize the spindle.

  In prometaphase, the nuclear envelope in animal cells phosphorylates and disintegrates into small membrane vesicles, allowing microtubules to invade the nuclear space. Each chromosome's kinetochore, a protein structure at the centromere, captures microtubules and uses a molecular motor powered by ATP to crawl toward the originating centrosome. Fungi and certain protists skip this nuclear-envelope breakdown entirely, undergoing what is called closed mitosis.

  At metaphase, tension from the pulling centrosomes aligns the chromosomes along an imaginary equatorial line called the metaphase plate. A checkpoint at this stage confirms that every kinetochore is properly attached before the cell advances. During anaphase A, the cohesin proteins holding sister chromatids together are cleaved, and the two resulting daughter chromosomes are pulled to opposite ends of the cell. In anaphase B, polar microtubules push against each other, elongating the cell further. Chromosomes also reach their maximum condensation level during late anaphase.

  Telophase reverses many earlier events. A new nuclear envelope reassembles around each set of daughter chromosomes, built from the membrane vesicles of the old envelope. The nucleolus reappears, chromosomes begin to relax, and mitosis is complete. Cytokinesis, a separate process, then divides the cytoplasm: in animal cells a contractile ring pinches the membrane inward along what was the metaphase plate; in plant cells, vesicles from the Golgi apparatus fuse to form a cell plate that grows into a new cell wall.

• 4. Why the Body Needs It

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  Red blood cells have a lifespan of roughly three months, and the body replaces them continuously through mitotic division. The same logic applies to the lining of the digestive tract and the outer layers of skin, where cells are constantly shed and renewed. Without mitosis, these tissues would wear away without replacement.

  Development begins with a single fertilized cell, the zygote, and mitosis is what builds the multicellular body from that starting point. A growing embryo, a healing wound, and a regenerating starfish arm all depend on the same mechanism to add new, genetically faithful cells to the organism.

  Not every cell divides indefinitely. Human heart muscle cells and neurons exit the cycle and enter what is called the G0 phase, where they stop dividing. This can also happen when cells become too crowded, a phenomenon called density-dependent inhibition. Some G0 cells can re-enter the cycle under the right conditions. The cells that cannot are part of what makes heart muscle damage so lasting, and neurodegeneration so difficult to reverse.

  Gametes, sperm and egg cells, are one category excluded from mitosis altogether. They are produced instead by meiosis. Prokaryotes, including bacteria and archaea, also divide by a different mechanism entirely called binary fission, since they lack the true nucleus that mitosis is built around.

• 5. When Mitosis Goes Wrong

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  Nondisjunction is one of the most consequential errors a dividing cell can make. During anaphase, sister chromatids fail to separate, so one daughter cell ends up with three copies of a chromosome, a state called trisomy, while the other receives only one, called monosomy. On occasion, cells that experience nondisjunction also fail to complete cytokinesis, leaving both nuclei inside a single binucleated cell.

  A different failure mode, anaphase lag, occurs when a chromatid cannot keep up during anaphase, usually because the spindle failed to attach properly. That straggling chromatid is excluded from both new nuclei and simply lost. The resulting daughter cell is left monosomic for that chromosome.

  Errors can also push mitosis in the opposite direction. Endoreduplication occurs when chromosomes duplicate but the cell skips division entirely, producing polyploid cells with extra chromosome sets. A variant called endomitosis allows the cell to enter but then prematurely exit mitosis, retaining duplicated chromosomes in the original nucleus. Megakaryocytes, the cells that produce blood platelets, go through endomitosis as a normal part of their development.

  Pathological division into three or more daughter cells, called tripolar or multipolar mitosis, can result in embryos so genetically scrambled they fail to implant. In other circumstances, mitotic errors push cells toward cancer. Histopathologists use the mitotic count, sometimes called the mitotic index, as a diagnostic and prognostic marker in tissue samples. In breast cancer classification, for instance, counting mitoses in the most active region of a tumor is a standard part of the assessment. Lag-type mitosis, identified by non-attached condensed chromatin near the mitotic figure, has been linked specifically to high-risk human papillomavirus infection and cervical cancer.

• 6. Variation Across Life

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  Not all eukaryotes run the same version of mitosis. Animal cells undergo open mitosis, where the nuclear envelope breaks down completely before chromosomes separate. Fungal cells typically keep the envelope intact throughout, a form called closed mitosis. An intermediate called semiopen mitosis, where the envelope degrades only partially, is typical of most Apicomplexa. Each variant has its own geometry: orthomitosis features an approximately symmetric spindle, while pleuromitosis features an eccentric, bilaterally symmetric arrangement.

  Among the most ancient-looking forms is closed intranuclear pleuromitosis, found in Foraminifera, many fungi including chytrids, oomycetes, zygomycetes, and ascomycetes, and some Radiolaria. Researchers consider it the most primitive type because it most closely resembles bacterial division. Unicellular Excavata show exclusively closed mitosis, without exception across that entire group.

  Plant cells carry their own distinctive feature in preprophase, a stage that precedes prophase and does not occur in animals. During preprophase, a band of microtubules and actin filaments forms beneath the plasma membrane, marking the precise plane where the cell will eventually divide. That band disappears before division actually begins, but the position it marked is what the new cell wall will follow.

  The evolutionary origin of mitosis reaches back to the base of the eukaryotic family tree. Prokaryotic homologs of actin and tubulin, two of the core molecules of mitosis, exist in bacteria and archaea, which suggests the deeper molecular toolkit predates the nucleus itself. Because meiosis is more complex than mitosis, researchers have proposed that meiosis evolved after mitosis. However, sexual reproduction involving meiosis is also a primitive eukaryotic trait, leaving open the possibility that the two processes evolved in parallel from ancestral prokaryotic mechanisms.