Mitosis

Mitosis is the phase of the eukaryotic cell cycle in which duplicated chromosomes are accurately separated into two new nuclei. As an equational division, mitosis preserves the chromosome number and produces daughter cells genetically identical to the parent cell. It is preceded by the S phase of interphase, during which DNA replication takes place, and is usually followed by cytokinesis, which divides the cytoplasm and organelles to form two complete daughter cells. Most somatic cells in animals and plants proliferate through mitosis; exceptions include the gametes, which are produced by meiosis.

Discovery and Terminology

Systematic observations of mitosis began in the nineteenth century. Early descriptions of cell division appeared in studies of algae and animal tissues, with notable contributions from Hugo von Mohl and Robert Remak. The process was first described in detail in animal cells during the 1870s by researchers including Wacław Mayzel and Otto Bütschli. Walther Flemming coined the term mitosis in 1882, deriving it from the Greek mitos, meaning thread, in reference to the appearance of condensed chromosomes. Alternative terms such as karyokinesis (nuclear division) and equational division have been used historically, though mitosis is now standard for the division of the nucleus in somatic cell cycles.

Overview of the Mitotic Process

The central objective of mitosis is the faithful distribution of the genome. Prior to mitosis, each chromosome is replicated to form two identical sister chromatids, which remain connected at the centromere by cohesin complexes. As the cell enters mitosis, the chromatids condense and become visibly distinguishable under a microscope. Microtubule fibres form the mitotic spindle, a structure that orchestrates chromatid alignment and segregation.
In organisms undergoing open mitosis, such as animals, the nuclear envelope disintegrates early in the process. In many fungi and related lineages, mitosis is closed, meaning the nuclear envelope remains intact and spindle formation occurs within the nucleus. Most animal cells adopt a rounded morphology at the onset of mitosis, which facilitates spindle assembly and chromosome movement.
Errors during mitosis can have serious consequences. Faulty spindle attachment, incomplete chromatid separation, or mis-segregation can result in multipolar division, non-viable embryos, or aneuploidy. Cells may respond through mitotic catastrophe or apoptosis. Persistent mitotic errors contribute to tumour formation and genomic instability.

Phases of Mitosis

Mitosis is typically divided into distinct stages that reflect the progression of chromosome and spindle dynamics.
Preprophase (in plant cells)This stage precedes prophase and involves the formation of the preprophase band, a microtubule ring that predicts the future plane of cell division.
ProphaseChromosomes condense into compact structures. The nucleolus disperses, and in animal cells the centrosomes migrate to opposite poles, initiating spindle formation.
PrometaphaseThe nuclear envelope fragments (in open mitosis), allowing spindle microtubules to attach to chromosomes at the kinetochores. Chromosomes undergo active motion as spindle fibres capture them.
MetaphaseChromosomes align along the metaphase plate at the cell’s equator. Proper bipolar attachment is verified by checkpoint mechanisms that ensure each sister chromatid is linked to opposite spindle poles.
AnaphaseCohesin proteins are cleaved, allowing sister chromatids—now daughter chromosomes—to separate and move to opposite poles. The cell elongates as spindle fibres shorten or lengthen appropriately.
TelophaseDaughter chromosomes arrive at the poles and begin to decondense. New nuclear envelopes reassemble around each chromosome set, reconstructing two genetically identical nuclei.
CytokinesisAlthough distinct from nuclear division, cytokinesis typically overlaps with telophase. In animal cells a contractile ring tightens to form a cleavage furrow; in plant cells a cell plate develops between the daughter nuclei. In some organisms cytokinesis does not occur, resulting in multinucleate coenocytic cells.

Interphase and Cell Cycle Regulation

Interphase, consisting of G₁, S, and G₂ phases, is far longer than the mitotic phase and prepares the cell for division. During G₁ and G₂ the cell grows, synthesises proteins, and increases its organelle content. DNA replication occurs exclusively during the S phase. Cell cycle progression is controlled by cyclins, cyclin-dependent kinases, and numerous regulatory proteins that ensure accurate chromosome duplication, DNA integrity, and readiness for mitosis.
Cells that stop dividing may enter the G₀ phase, either temporarily or permanently. This occurs in terminally differentiated tissues such as neurons and cardiac muscle. Cells can re-enter the cycle if conditions permit.
During interphase, DNA repair systems maintain genome integrity. Non-homologous end joining is active throughout interphase, whereas homologous recombination-based repair functions during S and G₂, when sister chromatids are available as templates.

Variations Among Organisms

Mitosis displays evolutionary diversity. While animal cells commonly undergo open mitosis, many fungi retain an intact nuclear envelope throughout the process. Plant cells possess unique structures such as the preprophase band and construct a new cell wall through the cell plate mechanism during cytokinesis. Despite these differences, the fundamental principles of chromosome replication, alignment, and segregation remain conserved.