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Cell division

Cell division is the process by which a parent cell divides into two or more daughter cells, enabling growth, tissue repair, development, and reproduction across all living organisms.^[1] In prokaryotes, such as bacteria, this occurs primarily through binary fission, a rapid and simplified mechanism where the single circular chromosome replicates, the cell elongates, and a septum forms to separate the cytoplasm, producing two genetically identical daughter cells.^[2] This asexual process is the sole means of reproduction for unicellular prokaryotes, allowing rapid population expansion under favorable conditions.^[2] In eukaryotes, cell division is more complex and typically involves the cell cycle, which includes interphase (growth and DNA replication) followed by the mitotic phase (nuclear division and cytokinesis).^[3] There are two main types: mitosis, which produces two genetically identical diploid daughter cells for somatic growth and maintenance, and meiosis, which generates four genetically diverse haploid gametes (sperm and eggs) through two sequential divisions, halving the chromosome number to ensure genetic stability across generations.^[1] Mitosis consists of four phases—prophase, metaphase, anaphase, and telophase—coordinated by regulatory proteins like cyclin-dependent kinases to accurately segregate chromosomes, while cytokinesis physically divides the cytoplasm.^[4] Errors in these processes can lead to uncontrolled division, as seen in cancer, or genetic disorders from improper chromosome distribution.^[1] Overall, cell division maintains genomic integrity and drives multicellular organization, with its mechanisms conserved yet adapted across evolutionary lineages.^[4]

Overview

Definition and types

Cell division is the process by which a single parent cell divides to produce two or more daughter cells, distributing genetic material to ensure continuity across generations while facilitating organismal growth, development, and repair.^[5] This fundamental biological mechanism allows cells to replicate their contents, including DNA, and partition them accurately into the progeny cells.^[6] Cell division occurs in two primary modes based on reproductive strategy: asexual and sexual. Asexual division produces genetically identical daughter cells from a single parent, promoting rapid population growth in unicellular organisms or tissue maintenance in multicellular ones; examples include binary fission in prokaryotes and mitosis in eukaryotes.^[1]^[7] In contrast, sexual reproduction involves meiosis, which generates genetically diverse haploid gametes by halving the chromosome number and introducing variation through crossing over and independent assortment, with subsequent fusion of gametes (fertilization) restoring the diploid state; this is essential for sexual reproduction in eukaryotes.^[1] These modes operate differently in unicellular contexts, where division primarily serves reproduction, versus multicellular organisms, where it supports both reproduction and somatic functions like wound healing.^[1] A key distinction lies between prokaryotic and eukaryotic cell division. Prokaryotic division, such as binary fission, is a simple and rapid process that duplicates a single circular chromosome and splits the cell without a nucleus or complex organelles.^[7] Eukaryotic division, however, is more intricate, involving linear chromosomes within a membrane-bound nucleus and additional structures like the cytoskeleton, often occurring within the broader framework of the cell cycle.^[6]

Biological significance

Cell division plays a pivotal role in the growth and maintenance of multicellular organisms by enabling tissue expansion during development and replacing damaged or senescent cells throughout life. In these organisms, controlled proliferation ensures the renewal of cell populations, such as the continuous replacement of skin and intestinal epithelial cells, which is essential for organismal homeostasis and repair after injury.^[8]^[9] In unicellular organisms, cell division serves as the primary mechanism of reproduction, allowing a single cell to produce genetically identical offspring and thereby sustain population dynamics in response to environmental pressures. This process facilitates rapid propagation, enabling species like bacteria to colonize new habitats and maintain ecological balance. Additionally, in both unicellular and multicellular contexts, cell division underpins asexual reproduction and asexual propagation in some multicellular forms.^[8]^[9] Evolutionarily, cell division represents a highly conserved process across all domains of life, from prokaryotes to eukaryotes, originating over 3 billion years ago as a fundamental mechanism for transmitting genetic material to daughter cells. This universality has allowed organisms to adapt to diverse environments and, through variations like meiosis, promote genetic diversity that drives speciation and evolutionary innovation.^[9]^[10] Dysregulation of cell division can lead to severe pathological conditions; uncontrolled proliferation, often due to mutations in cell cycle regulators like TP53 or RB1, is a hallmark of cancer, resulting in tumor formation and metastasis. Conversely, insufficient or aberrant division contributes to developmental disorders, such as neurodevelopmental conditions linked to disruptions in pathways like PI3K/mTOR and MAPK, which impair proper cell differentiation and tissue formation during embryogenesis.^[11]^[12]

Prokaryotic cell division

Binary fission in bacteria

Binary fission is the primary mechanism of asexual reproduction in most bacteria, resulting in the division of a single parent cell into two genetically identical daughter cells. This process is simpler and more rapid than eukaryotic mitosis, lacking distinct phases or complex checkpoints, and is directly coupled to cellular growth. In model organisms like Escherichia coli, binary fission typically occurs every 20-60 minutes under optimal nutrient-rich conditions, allowing rapid population expansion.^[13] The process begins with the replication of the bacterial chromosome, a circular DNA molecule, initiated at a specific site called the origin of replication, or oriC. The initiator protein DnaA binds to oriC, unwinding the DNA and recruiting helicase and polymerase enzymes to synthesize two identical copies of the chromosome in a bidirectional manner from the origin toward the terminus. This replication occurs concurrently with cell elongation, ensuring that the duplicated DNA is distributed as the cell grows.^[14] Once replication is complete, the newly synthesized chromosomes must be segregated to opposite poles of the cell to prevent unequal distribution. This is mediated by the ParABS partitioning system, consisting of the DNA-binding protein ParB, which loads onto centromere-like parS sites near oriC, and the ATPase ParA, which interacts with ParB to actively transport the chromosomes apart in a DNA-relay mechanism. ParB-ParA complexes generate oscillatory movements that push the origins toward the cell poles, ensuring faithful partitioning even during overlapping replication cycles in fast-growing bacteria.^[15] Septum formation follows chromosome segregation, marking the site of cell division at the midcell. The tubulin homolog FtsZ polymerizes into a contractile ring-like structure, the Z-ring, which anchors to the inner membrane and recruits additional divisome proteins. The Z-ring constricts, guiding the invagination of the cytoplasmic membrane and synthesizing new peptidoglycan cell wall material via enzymes like FtsI and MurG, ultimately splitting the cell into two daughters. This coordinated constriction ensures precise division without disrupting cellular integrity.^[16]

Division in archaea

Archaea, like other prokaryotes, lack a membrane-bound nucleus and undergo division without the complex mitotic apparatus seen in eukaryotes. Cell division in archaea exhibits diversity across phyla, with mechanisms that parallel bacterial processes in some lineages but incorporate unique, eukaryote-like elements in others. While many archaea replicate their circular chromosomes and partition them prior to cytokinesis, the molecular machinery for constriction and scission varies significantly. In euryarchaeota and certain other archaeal groups, cell division relies on an FtsZ-based system analogous to bacterial binary fission, where FtsZ polymerizes into a contractile ring at the division site to drive membrane ingression.^[17] However, in crenarchaeota and the TACK superphylum, division employs the Cdv (cell division) system, a machinery evolutionarily related to the eukaryotic endosomal sorting complexes required for transport (ESCRT-III). This system facilitates membrane constriction through protein polymerization and remodeling, bypassing FtsZ entirely. The core Cdv components include CdvA, a crenarchaea-specific protein that localizes to the division site; CdvB, a homolog of ESCRT-III that assembles into filaments; and CdvC, an AAA+ ATPase akin to Vps4 that disassembles these structures using ATP hydrolysis.^[17] A prominent example is the hyperthermophilic crenarchaeon Sulfolobus acidocaldarius, where the cdvABC operon is expressed during the late stages of the cell cycle, coinciding with nucleoid segregation. In synchronized cultures, Cdv proteins form visible band-like structures between segregating chromosomes, which progressively constrict to complete cytokinesis over approximately 50–75 minutes. This process is tightly regulated, with division inhibited under DNA damage from UV irradiation, suggesting a checkpoint mechanism for genome integrity.^[17] Chromosome segregation in archaea often involves multiple replication origins per genome, enabling rapid duplication in polyploid states. In Sulfolobus species, three origins initiate replication synchronously early in the cell cycle, with forks progressing bidirectionally at 80–110 base pairs per second until asynchronous termination, ensuring complete genome duplication before division.^[18] Partitioning is mediated by dedicated proteins such as SegA, a Walker-type ATPase that forms dynamic filaments, and SegB, a DNA-binding protein that stimulates SegA polymerization at specific centromere-like sites, pulling sister chromosomes apart.^[19] Archaeal division rates are generally slower than in mesophilic bacteria, with Sulfolobus cell cycles lasting around 240 minutes under optimal conditions, reflecting adaptations to extreme environments like high temperatures (up to 80°C) and acidity. The Cdv system's thermal stability supports reliable constriction in such harsh settings, while the multi-origin replication strategy accommodates slower fork speeds without compromising fidelity. Overexpression of segregation factors like SegAB disrupts partitioning, leading to anucleate cells and growth defects, underscoring their essential role.^[19]^[17]

Eukaryotic cell division

The cell cycle

The eukaryotic cell cycle is a highly regulated process that governs the growth and division of cells, ensuring accurate replication and distribution of genetic material. It consists of a series of sequential phases that prepare the cell for division and execute the division itself. This cycle is fundamental to eukaryotic cell division, providing the temporal framework for processes like mitosis.^[20] The cell cycle is divided into four main phases: G1 (gap 1), S (synthesis), G2 (gap 2), and M (mitotic) phase. During the G1 phase, the cell grows and synthesizes proteins and organelles necessary for DNA replication. The S phase involves the duplication of the cell's DNA, resulting in two identical sets of chromosomes. In the G2 phase, the cell continues to grow and prepares for mitosis by checking DNA integrity and assembling the mitotic machinery. The M phase encompasses mitosis, where the replicated chromosomes are segregated, followed by cytokinesis, which divides the cytoplasm into two daughter cells. Additionally, some cells enter a quiescent state known as G0 phase, where they exit the cycle temporarily or permanently, such as differentiated neurons or resting lymphocytes.^[20]^[21] The duration of the cell cycle varies significantly depending on the cell type and organism. In many mammalian cells grown under optimal conditions, the cycle typically lasts about 24 hours, with interphase (G1, S, and G2) occupying the majority of this time. However, the cycle is much shorter in rapidly dividing cells, such as those in early embryonic development, where durations can be as brief as a few hours to facilitate rapid proliferation.^[22]/24%3A_Mitosis/24.03%3A_The_Eukaryotic_Cell_Cycle) Progression through the cell cycle phases is coordinated by cyclin-dependent kinase (CDK) complexes, which are activated by regulatory proteins called cyclins whose levels oscillate throughout the cycle. Specific cyclin-CDK pairs drive transitions between phases by phosphorylating target proteins that control DNA replication, chromosome segregation, and other key events. This oscillatory mechanism ensures orderly progression and prevents errors in division.^[23]^[24]

Mitosis and meiosis overview

Mitosis is a fundamental process of eukaryotic cell division that produces two genetically identical diploid daughter cells from a single diploid parent cell, maintaining the chromosome number across generations. This division occurs primarily in somatic (body) cells and serves essential roles in organismal growth, tissue maintenance, and repair, as well as in asexual reproduction in certain unicellular eukaryotes.^[1]^[25] In contrast, meiosis is a specialized eukaryotic cell division process that generates four genetically distinct haploid gametes (such as sperm and eggs) from one diploid parent cell, halving the chromosome number to ensure stable ploidy upon fertilization during sexual reproduction. Meiosis consists of two sequential divisions—meiosis I and meiosis II—following a single round of DNA replication, and it occurs exclusively in germ cells within reproductive organs like the testes and ovaries.^[1]^[26] The primary differences between mitosis and meiosis lie in their outcomes and mechanisms: mitosis involves a single division without genetic recombination, resulting in two identical diploid cells for clonal expansion and genetic stability; meiosis, however, features two divisions with recombination through crossing over between homologous chromosomes, yielding four non-identical haploid cells that promote genetic diversity and ploidy reduction.^[26]^[27] Both processes share analogous phases of chromosome condensation, alignment, and segregation, but meiosis incorporates unique steps like homologous pairing to facilitate variation.^[25]

Phases of eukaryotic cell division

Interphase

Interphase is the longest phase of the eukaryotic cell cycle, comprising approximately 90% of its duration, during which the cell prepares for division without visible changes to chromosome structure under light microscopy.^[28] This phase encompasses three subphases—G1, S, and G2—characterized by cellular growth, DNA replication, and final preparations for mitosis, respectively. Unlike the mitotic phases that follow, interphase chromosomes remain decondensed and dispersed within the nucleus, facilitating ongoing gene expression and metabolic activities.^[29] The G1 phase, or first gap phase, initiates interphase and focuses on cell growth, during which the cell increases in size and synthesizes organelles such as ribosomes and mitochondria to support future replication demands.^[30] Protein synthesis and metabolic processes dominate, ensuring the cell reaches a sufficient mass before proceeding; this phase can vary significantly in length depending on cell type and environmental signals. A critical checkpoint at the G1/S transition, known as the restriction point, assesses cell size, nutrient availability, and DNA integrity to permit entry into DNA synthesis.^[20] During the S phase, or synthesis phase, the cell replicates its DNA in a semi-conservative manner, where each parental strand serves as a template for a new complementary strand, effectively doubling the genetic material from 2n to 4n.^[31] Concurrently, new histones are synthesized and assembled onto the daughter DNA strands to maintain chromatin structure, ensuring proper packaging of the replicated genome. This process occurs at multiple origins of replication along each chromosome, coordinated to complete within the phase's timeframe.^[32] The G2 phase, or second gap phase, allows for further cell growth, DNA repair of any replication errors from S phase, and preparation of the cytoskeletal components, including microtubule organization via centrosome maturation, for impending mitosis.^[20] A G2 checkpoint verifies complete and accurate DNA replication, halting progression if damage persists to prevent propagation of errors. Following successful G2 completion, the cell transitions into prophase of mitosis.^[20]

Prophase

Prophase is the initial phase of mitosis, initiating the process of nuclear division in eukaryotic cells. It follows the G2 phase of interphase and is characterized by the first visible signs of mitotic activity, including the compaction of chromatin and the reorganization of the cytoskeleton. Throughout prophase, the nuclear envelope remains intact, confining these early events to the nuclear interior.^[33]^[34] A hallmark of prophase is the condensation of chromosomes, where replicated chromatin transitions from a diffuse, thread-like state to more compact, rod-shaped structures. This process is mediated primarily by the condensin II complex, a heterohexameric ATPase belonging to the structural maintenance of chromosomes (SMC) family, which binds to chromatin and promotes the formation of large-scale loops and intra-chromosomal interactions. Condensin II localizes to chromosomes within the nucleus during early prophase, triggered by phosphorylation from cyclin A-CDK complexes, enabling the initial axial shortening and radial thickening of chromatids. In contrast, condensin I remains cytoplasmic during prophase and only engages chromosomes later. This condensation prevents tangling of sister chromatids and facilitates their resolution for segregation.^[33]^[34]^[35] Concurrently, the duplicated centrosomes—each consisting of a pair of centrioles surrounded by pericentriolar material—undergo separation and migration to opposite sides of the nucleus. Centrosome duplication occurs earlier in the S phase, but their active positioning begins in prophase, driven by the kinesin-5 motor protein (KIF11/EG5), which cross-links and slides antiparallel microtubules between the centrosomes. This migration orients the centrosomes along the spindle axis and is tethered to the nuclear envelope via interactions involving dynein and nucleoporin Nup133, ensuring proper bipolarity. As centrosomes mature, they recruit additional γ-tubulin ring complexes (γ-TuRCs), amplifying their capacity to nucleate microtubules by over threefold compared to interphase.^[36]^[37] These centrosomes serve as the primary sites for microtubule nucleation, initiating spindle assembly. Astral microtubules radiate outward from each centrosome toward the cell cortex, aiding in spindle positioning, while interpolar microtubules extend centrally and begin overlapping with those from the opposing centrosome to establish the bipolar framework. Microtubule dynamics shift dramatically, with increased polymerization rates and disassembly of the interphase array, setting up the mitotic spindle.^[37]^[33] Prophase typically occupies the majority of mitosis, lasting 20–60 minutes in mammalian cells, though this varies by cell type and organism; for instance, it can extend longer in larger cells to accommodate extensive chromatin compaction. These coordinated events in prophase prepare the cell for nuclear envelope breakdown and chromosome capture in prometaphase.^[38]^[6]

Prometaphase

Prometaphase follows prophase, during which chromosomes have condensed into distinct structures. This phase is characterized by the disassembly of the nuclear envelope, which allows the mitotic spindle to interact directly with the chromosomes. The nuclear envelope breaks down through phosphorylation of nuclear lamins and pore complexes by kinases such as CDK1, dispersing its components into the cytoplasm and creating an open environment for spindle assembly.^[39] In prometaphase, kinetochores—protein complexes assembled on the centromeres of chromosomes—begin capturing microtubules from the forming bipolar spindle through a process known as "search-and-capture." Microtubules dynamically explore the cytoplasmic space, with their plus ends probing for kinetochores; upon attachment, initial end-on or lateral interactions stabilize via recruitment of stabilizing proteins like the KMN network. Captured chromosomes then exhibit rapid, erratic movements as they are pulled toward the spindle equator by microtubule depolymerization and motor proteins such as dynein and kinesin-7 (CENP-E), congressing bi-oriented sister chromatids. This phase is inherently chaotic, marked by frequent microtubule attachments and detachments, reflecting the stochastic nature of kinetochore-microtubule encounters.^[39]^[40] A critical aspect of prometaphase involves error correction to ensure proper bipolar attachments. The Aurora B kinase, localized at the inner centromere as part of the chromosomal passenger complex, phosphorylates kinetochore substrates like the NDC80 complex when attachments lack tension, reducing microtubule-binding affinity and destabilizing incorrect configurations such as syntelic or merotelic orientations. This tension-dependent mechanism promotes selective stabilization of amphitelic attachments, where sister kinetochores bind microtubules from opposite spindle poles. Inhibition of Aurora B leads to persistent errors and delays in progression.^[39]00848-5) Prometaphase is typically a short but highly dynamic phase in mammalian cells, lasting approximately 20-30 minutes under normal conditions, though it can extend if attachments are suboptimal, activating the spindle assembly checkpoint to prevent premature anaphase onset. Its brevity underscores the efficiency of the attachment and correction machinery in achieving initial bipolarity.

Metaphase

Metaphase is the stage of mitosis in which chromosomes achieve stable alignment at the metaphase plate, ensuring equal distribution to daughter cells. Following the initial microtubule attachments established during prometaphase, chromosomes congress to the spindle equator through a combination of kinetochore motility and polar ejection forces exerted by chromosome arms.^[41] This alignment positions each chromosome such that its sister kinetochores are oriented toward opposite spindle poles, forming an orderly array visible under microscopy.00455-3) Central to this process is the biorientation of sister chromatids, where microtubules from opposite poles capture and stabilize attachments to the kinetochores, generating inter-kinetochore tension that confirms proper orientation. This tension arises from the pulling forces of spindle microtubules balanced across the centromere, stabilizing end-on attachments while destabilizing syntelic or merotelic errors via Aurora B kinase-mediated phosphorylation.^[42] Biorientation not only positions chromosomes at the metaphase plate—an imaginary plane equidistant from the spindle poles—but also serves as the primary signal for checkpoint satisfaction.^[41] The spindle assembly checkpoint (SAC) enforces this alignment by inhibiting progression until all chromosomes are bioriented. Unattached or improperly attached kinetochores recruit Mad2, which, in its open conformation, catalyzes the formation of a mitotic checkpoint complex (MCC) that binds and inhibits the anaphase-promoting complex/cyclosome (APC/C) co-activator Cdc20.^[42] This inhibition prevents APC/C-mediated ubiquitination of securin and cyclin B, delaying anaphase onset and providing time for error correction. Mad2 also actively contributes to biorientation by destabilizing tensionless attachments, thereby enhancing the rate of chromosome reorientation to opposite poles.^[43] Once tension is established across all kinetochores, the SAC signal dissipates, silencing Mad2 recruitment and allowing APC/C activation.^[42]

Anaphase

Anaphase is the stage of mitosis in which sister chromatids separate and migrate toward opposite spindle poles, ensuring equitable distribution of genetic material to daughter cells.^[44] This phase is initiated upon satisfaction of the metaphase-anaphase transition checkpoint, which activates the anaphase-promoting complex (APC/C) to degrade securin, thereby unleashing separase activity.^[45] The separation of sister chromatids is triggered by the cleavage of cohesin complexes by separase, a cysteine protease that targets the kleisin subunit (Scc1 in yeast or Rad21 in humans) of the ring-shaped cohesin structure holding chromatids together.^[46] This proteolytic event dissolves centromeric and arm cohesion almost simultaneously in mitosis, allowing individual chromatids to disengage and respond to spindle forces.^[45] Anaphase proceeds in two overlapping subphases: Anaphase A, characterized by the shortening of kinetochore microtubules (kMTs) that pull chromatids poleward, and Anaphase B, involving spindle pole elongation that further separates the chromatids.^[44] In Anaphase A, kMTs depolymerize primarily at their plus ends near kinetochores (via the "Pac-Man" mechanism) and to a lesser extent at minus ends at the poles (via microtubule flux), with proteins like the Dam1 complex in yeast or Ndc80 in humans facilitating the coupling of kinetochore movement to microtubule disassembly.^[47] During Anaphase B, interpolar microtubules slide apart through motor-driven pushing forces (e.g., mediated by kinesins such as Eg5 and KIF4A), while astral microtubules may contribute pulling forces via cortical dynein, resulting in overall spindle elongation.^[48] Chromatids typically move toward the poles at a speed of approximately 1 μm/min during Anaphase A, though this varies by cell type and species (e.g., slower in yeast at 0.3 μm/min and faster in Drosophila at up to 3.6 μm/min).^[49] The combined actions of these subphases ensure the equal partitioning of sister chromatids to each spindle pole, setting the stage for nuclear reformation in the subsequent telophase.^[44]

Telophase

Telophase follows anaphase, marking the final stage of mitosis in eukaryotic cells where the separated sister chromatids arrive at opposite poles and the nuclear architecture begins to reform.^[50] During telophase, the nuclear envelope reassembles around each set of daughter chromosomes through the fusion of vesicles derived from the breakdown earlier in mitosis; this process is mediated by the dephosphorylation of lamins and integral membrane proteins, which bind to the chromosome surfaces and facilitate the formation of a continuous double membrane.^[51] Simultaneously, the chromosomes decondense as cyclin-dependent kinase 1 (Cdk1, formerly Cdc2) is inactivated, leading to the dephosphorylation of condensin complexes and the reversal of their compaction, allowing the chromatin to relax into a more diffuse, interphase-like state.^[51] The nucleoli also reform at this stage, coinciding with chromosome decondensation and the resumption of ribosomal RNA (rRNA) gene transcription on the acrocentric chromosomes.^[51] The mitotic spindle disassembles as microtubules depolymerize, driven by the inactivation of spindle assembly factors and the action of microtubule-depolymerizing kinesins and catastrophi, ensuring the clearance of the spindle apparatus from the reforming nuclei.^[50] Telophase effectively mirrors the events of prophase in reverse, with the duration typically comparable, though variable across cell types, as it unwinds the structural changes initiated at the start of mitosis.^[52]

Cytokinesis

Cytokinesis is the final stage of cell division in which the cytoplasm physically separates to form two distinct daughter cells, ensuring each receives a complete set of organelles and cytoplasmic components.^[53] This process occurs concurrently with the later stages of mitosis and is tightly coordinated to prevent unequal distribution of cellular contents.^[54] In animal cells, cytokinesis is mediated by the formation of a contractile ring composed primarily of actin filaments and myosin-II motors at the cell's equator.^[55] This actomyosin ring assembles during anaphase and begins constricting in telophase, generating contractile forces that ingress the plasma membrane inward to create a cleavage furrow.^[56] The ring's contraction, driven by myosin-II's ATPase activity sliding actin filaments past one another, continues until the furrow deepens and pinches the cell into two, with the process completing through abscission at the intercellular bridge.^[54] A transient structure called the midbody forms at the bridge's center, composed of bundled microtubules and associated proteins, which serves as a scaffold for the final membrane severing and helps coordinate the timing of separation.^[57] In contrast, plant cells lack a contractile ring due to their rigid cell walls and instead divide via the formation of a cell plate.^[58] Cytokinesis begins with the organization of the phragmoplast, a microtubule array that guides Golgi-derived vesicles carrying cell wall precursors toward the division plane.^[59] These vesicles fuse at the equatorial plane, initially forming a tubular network that expands centrifugally through continued vesicle fusion and maturation into a flattened disc, the cell plate.^[60] As the cell plate reaches the parental cell wall, it fuses with it, depositing new cell wall material and completing cytoplasmic separation.^[58] Cytokinesis overlaps with telophase, during which nuclear envelopes reform around separated chromosomes, ensuring cytoplasmic division aligns with nuclear completion to finalize mitosis.^[53] This coordination is essential for symmetric partitioning and is regulated by conserved signaling pathways that link spindle dynamics to contractile apparatus assembly.^[61]

Variants of cell division

Meiosis

Meiosis is a specialized form of cell division in sexually reproducing eukaryotes that reduces the chromosome number by half, producing four haploid gametes from a single diploid precursor cell.^[62] This process is essential for maintaining a constant chromosome number across generations during sexual reproduction, as it generates cells with half the genetic material of the parent cell.^[26] Unlike mitosis, which produces identical diploid cells, meiosis introduces genetic variation through unique mechanisms, ensuring diversity in offspring.^[62] The meiotic process consists of two successive divisions, Meiosis I and Meiosis II, following a single round of DNA replication in interphase.^[62] In Meiosis I, homologous chromosomes pair and segregate, reducing the diploid (2n) set to haploid (n). During prophase I, homologous chromosomes condense and form synaptonemal complexes, enabling crossing over where non-sister chromatids exchange genetic material at chiasmata, a process mediated by proteins like Spo11 that induce double-strand breaks.^[26] This is followed by metaphase I, where tetrads (paired homologs) align at the equator, and anaphase I, where homologs separate to opposite poles, yielding two haploid cells with replicated chromosomes.^[62] Meiosis II then resembles mitosis: sister chromatids separate in anaphase II, resulting in four haploid daughter cells, each with a single copy of each chromosome.^[26] The outcomes of meiosis are four genetically distinct haploid cells, typically gametes such as sperm or eggs in animals.^[62] Genetic diversity arises primarily from two mechanisms: independent assortment of chromosomes during metaphase I, where maternal and paternal homologs align randomly, potentially yielding over 8 million combinations in humans with 23 chromosome pairs, and crossing over, which shuffles alleles within chromosomes.^[62] These processes ensure that each gamete carries a unique combination of genetic material, promoting evolutionary adaptability.^[26] Regulation of meiosis involves specific checkpoints to ensure accurate chromosome pairing and segregation, preventing errors that could lead to aneuploidy.^[63] A key checkpoint in prophase I monitors synapsis, the pairing of homologous chromosomes via the synaptonemal complex; defects trigger ATM/ATR kinases to activate downstream effectors like CHK2, halting progression until repair or pairing is resolved.^[63] In organisms like yeast and nematodes, proteins such as Mek1 and SUN-1 enforce this surveillance, promoting interhomolog recombination and suppressing improper intersister events.^[63] Meiosis shares some phase nomenclature with mitosis but doubles the divisions to achieve reductional segregation.^[26]

Asymmetric division

Asymmetric cell division is a process in which a parent cell divides to produce two daughter cells with distinct fates, sizes, or compositions, contrasting with symmetric mitosis that yields equivalent daughters. This mechanism is crucial for generating cellular diversity while maintaining progenitor populations, particularly in developmental contexts and under stress conditions.00208-0) Central to asymmetric division are mechanisms that establish cellular polarity and orient the mitotic spindle to ensure unequal partitioning of cellular components. In Drosophila neuroblasts, for instance, the protein Inscuteable localizes to the apical cortex, recruiting factors like Partner of Inscuteable and Discs-large to orient the spindle along the apical-basal axis, thereby directing basal segregation of cell fate determinants such as Prospero and Numb.80142-7) This spindle positioning ensures that one daughter inherits the determinants, promoting differentiation, while the other retains stem-like properties. Unequal segregation of determinants, including proteins, RNAs, and organelles, further reinforces asymmetry by biasing signaling pathways in the daughters.00598-6) A prominent example occurs in stem cell renewal, where asymmetric division balances self-renewal and differentiation by producing one stem cell and one committed progenitor. In mammalian neural stem cells, this is achieved through oriented divisions that asymmetrically distribute factors like Numb, inhibiting Notch signaling in the differentiating daughter to promote neuronal fate.00540-1) Similarly, in bacterial sporulation, such as in Bacillus subtilis, an asymmetric septum forms near one pole after chromosome segregation, yielding a smaller forespore and a larger mother cell; the forespore develops into a dormant spore, while the mother cell nurtures it before lysing.00698-0) The significance of asymmetric division lies in its role in establishing tissue asymmetry and facilitating specialized differentiation, such as in immune responses. In developing tissues, it contributes to patterned architectures, as seen in Drosophila sensory organ precursors where oriented divisions generate diverse cell types for mechanosensation.00246-2) In immune cell differentiation, asymmetric divisions in CD8 T cells regulate effector versus memory fates; strong T cell receptor signaling triggers asymmetry, safeguarding memory cell development by unevenly distributing fate determinants like T-bet.00429-3) This process enhances adaptive immunity by producing long-lived memory cells alongside short-term effectors.^[64]

Regulation of the cell cycle

Checkpoints

Cell cycle checkpoints are surveillance mechanisms that monitor the integrity and fidelity of cellular processes, halting progression if errors are detected to prevent propagation of genomic instability. These checkpoints primarily operate at key transition points, ensuring DNA replication is complete and accurate, chromosomes are properly aligned, and damage is addressed before division proceeds. In eukaryotic cells, the main checkpoints include the G1/S, G2/M, and spindle assembly checkpoints, each tailored to specific risks during the cell cycle.^[65] The G1/S checkpoint assesses DNA integrity prior to replication initiation, primarily in response to DNA damage such as double-strand breaks. Activation of this checkpoint stabilizes the tumor suppressor protein p53, which transcriptionally induces cyclin-dependent kinase inhibitor p21, thereby inhibiting cyclin E-CDK2 complexes and arresting the cell cycle to allow repair. ATM and ATR kinases play central roles in signaling; ATM is rapidly activated by double-strand breaks to phosphorylate p53, while ATR responds to single-stranded DNA intermediates, amplifying the response through Chk1 and Chk2 kinases. This checkpoint is crucial in mammalian cells, where its dysfunction, often via p53 mutations, allows damaged cells to enter S phase, contributing to oncogenesis.^[66]^[67]^[67] The G2/M checkpoint verifies completion of DNA replication and absence of damage before mitosis entry, preventing segregation of unreplicated or broken chromosomes. ATM and ATR kinases again dominate signaling: ATM detects residual double-strand breaks from S phase, while ATR monitors replication fork stalling and unfinished synthesis, leading to phosphorylation of Cdc25 phosphatases and inhibition of cyclin B-CDK1 activation. If replication errors persist, cells arrest in G2 to facilitate resolution, with ATR's role being particularly vital during replication stress induced by agents like hydroxyurea. This checkpoint ensures genomic stability by coupling replication fidelity to mitotic commitment.^[68]^[68]^[67] The spindle assembly checkpoint (SAC), active during metaphase, monitors kinetochore-microtubule attachments to ensure bipolar chromosome alignment on the mitotic spindle. Unattached kinetochores generate a diffusible "wait-anaphase" signal via Mad2 and BubR1 proteins, which inhibit the anaphase-promoting complex/cyclosome (APC/C), blocking securin degradation and sister chromatid separation. Satisfaction of attachments silences the SAC, allowing progression to anaphase; defects lead to aneuploidy, a hallmark of cancer. Unlike DNA-focused checkpoints, the SAC operates independently but integrates with overall cell cycle control through cyclin-CDK modulation. Failure to satisfy any checkpoint typically triggers sustained cell cycle arrest, enabling repair, but persistent unresolved issues activate apoptotic pathways to eliminate compromised cells. For instance, prolonged p53 activation at G1/S can shift from arrest to transcription of pro-apoptotic genes like PUMA and BAX, inducing mitochondrial outer membrane permeabilization. Similarly, unchecked SAC activation or G2/M failure can culminate in caspase-mediated apoptosis, safeguarding organismal integrity against mutagenesis. These consequences underscore the checkpoints' dual role in transient halts versus terminal elimination.^[67]^[66]

Protein degradation mechanisms

Protein degradation plays a pivotal role in cell division by ensuring the timely removal of regulatory proteins that drive transitions between cell cycle phases. The primary pathway for this regulated degradation is the ubiquitin-proteasome system (UPS), where target proteins are marked for destruction through covalent attachment of ubiquitin chains, followed by proteasomal breakdown. This mechanism allows cells to precisely control the levels of cyclins, securin, and other key factors, preventing aberrant progression and maintaining genomic stability during mitosis.^[69] Central to the UPS in the cell cycle are E3 ubiquitin ligases, which confer specificity to the ubiquitination process by recognizing and tagging substrates. The anaphase-promoting complex/cyclosome (APC/C) is a multi-subunit E3 ligase that predominantly functions in mitosis, targeting securin and mitotic cyclins such as cyclin B for degradation. APC/C activity is modulated by co-activators like CDC20 during prometaphase to metaphase and CDH1 post-anaphase, ensuring sequential ubiquitination events. In parallel, the SCF (Skp1-Cullin-F-box) complex, another cullin-RING E3 ligase, operates mainly in G1/S and S phases, ubiquitinating substrates like cyclin E and CDK inhibitors (e.g., p27) to facilitate entry into DNA replication. These ligases work in concert to orchestrate protein turnover, with APC/C and SCF representing the major players in mitotic regulation.^[70]^[71]^[69] A critical example of UPS timing occurs at the onset of anaphase, where APC/C^{CDC20} ubiquitinates securin, leading to its proteasomal degradation and subsequent activation of separase for sister chromatid separation. Concurrently, cyclin B is targeted by APC/C, resulting in its rapid degradation, which inactivates CDK1 and promotes mitotic exit through chromosome decondensation and nuclear envelope reformation. This cyclin B destruction is essential for timely progression, as its persistence would sustain high CDK1 activity and halt division. SCF complements this by degrading early cyclins like cyclin A in prometaphase, preventing premature APC/C activation. Overall, these degradation events are briefly referenced in checkpoint regulation to ensure they occur only after proper spindle assembly.^[70]^[69]^[71]

DNA damage response

Detection and signaling

Cells detect DNA damage during division through specialized sensor kinases that recognize specific lesions and initiate signaling cascades to halt progression and maintain genomic integrity. The ataxia-telangiectasia mutated (ATM) kinase primarily senses double-strand breaks (DSBs), which can arise from ionizing radiation, reactive oxygen species, or replication fork collapse during S phase or mitosis.^[72] Upon DSB detection, ATM undergoes autophosphorylation at serine 1981, leading to its monomerization and activation, which allows it to phosphorylate numerous downstream targets.^[72] In parallel, the ataxia-telangiectasia and Rad3-related (ATR) kinase serves as the primary sensor for replication stress, including single-stranded DNA (ssDNA) regions generated by stalled replication forks or UV-induced damage, which are common during the S phase of the cell cycle.^[73] ATR activation involves recruitment to RPA-coated ssDNA via the ATR-interacting protein (ATRIP), followed by autophosphorylation at threonine 1989, which is recognized by TOPBP1 to stimulate ATR activation.^[73] These sensors propagate signals through phosphorylation cascades that activate effector kinases Chk1 and Chk2. ATM predominantly phosphorylates Chk2 at threonine 68, promoting its dimerization and full activation, while ATR phosphorylates Chk1 at serine 345 and serine 317, often in conjunction with other modifiers.^[74] Activated Chk1 and Chk2 then phosphorylate p53 at multiple sites, stabilizing it and enhancing its transcriptional activity to induce the cyclin-dependent kinase inhibitor p21 (also known as CDKN1A).^[74] Elevated p21 levels inhibit cyclin E/CDK2 and cyclin A/CDK2 complexes, enforcing G1/S arrest, or cyclin B/CDK1 for G2/M arrest, thereby preventing propagation of damaged DNA into daughter cells. These detection pathways briefly trigger broader checkpoint responses and repair initiation to safeguard division fidelity.^[74]

Repair during the cell cycle

DNA repair pathways are tightly integrated with the cell cycle to ensure genomic integrity, with specific mechanisms activated during distinct phases to address different types of lesions. Non-homologous end joining (NHEJ) predominates in the G1 phase, where it directly ligates double-strand breaks (DSBs) without requiring a homologous template, making it suitable for the pre-replicative state when sister chromatids are absent.^[75] In contrast, homologous recombination (HR) is favored in the S and G2 phases, utilizing the newly synthesized sister chromatid as a template for accurate repair of DSBs, which minimizes error-prone outcomes post-replication.^[76] These pathways are activated by upstream damage signaling to coordinate repair with cell cycle progression.^[77] Base excision repair (BER) operates throughout the cell cycle, addressing small base lesions such as those caused by oxidation or alkylation by removing the damaged base and replacing it via short-patch or long-patch synthesis.^[78] Mismatch repair (MMR), which corrects replication errors like base-base mismatches and insertion/deletion loops, is most active during S phase, where it couples with DNA synthesis to excise and resynthesize the erroneous strand, thereby preventing fixed mutations.^[79] This phase-specific integration ensures that HR and MMR leverage the availability of undamaged templates during or immediately after replication, while NHEJ and BER provide flexible, error-tolerant options in non-replicative phases. Failure to repair DNA damage before key transitions can result in persistent lesions that lead to mutations, such as base substitutions from unrepaired single-strand lesions, or chromosomal aberrations causing aneuploidy from unresolved DSBs.^[80] For instance, unrepaired DSBs progressing into mitosis may trigger chromosome missegregation, contributing to aneuploidy and genomic instability.^[81]

History

Early observations

The foundations of understanding cell division were laid in the 19th century through microscopic observations that established the cell theory. In 1838, Matthias Jakob Schleiden proposed that all plant tissues are composed of cells, viewing them as the fundamental units of life. Theodor Schwann extended this idea in 1839 to animal tissues, asserting that cells are the basic building blocks of both plant and animal structures. This culminated in Rudolf Virchow's 1855 declaration, "omnis cellula e cellula," emphasizing that all cells arise from pre-existing cells, thereby rejecting spontaneous generation and highlighting division as the mechanism of cellular reproduction.^[82]^[83] Advancements in microscopy enabled detailed visualization of the division process. In 1879, Walther Flemming observed thread-like structures in the epithelial cells of salamander larvae, which he termed "mitosis" from the Greek word for thread, describing the equitable distribution of these structures to daughter cells during division. Flemming's staining techniques allowed him to track the continuity of these threads—later identified as chromosomes—across cell generations, providing the first clear evidence of organized nuclear division.^[84]^[85] Concurrent observations illuminated reproductive cell division. In 1876, Oscar Hertwig studied sea urchin eggs and noted that fertilization involves the fusion of a single sperm nucleus with the egg nucleus, forming a diploid zygote nucleus that subsequently undergoes division. This discovery underscored the role of nuclear fusion in sexual reproduction and implied mechanisms for halving chromosome numbers in gamete formation, though the full process of meiosis was not yet delineated.^[86]^[87] These early microscopic insights into cell division paved the way for subsequent molecular investigations into its mechanisms.

Molecular and modern discoveries

The elucidation of DNA's double-helix structure in 1953 by James Watson and Francis Crick provided a foundational molecular framework for understanding DNA replication during cell division, revealing how genetic information is precisely duplicated and distributed to daughter cells.^[88] This discovery built upon earlier microscopic observations by integrating biochemical and structural insights, emphasizing base pairing and helical unwinding as key mechanisms in S phase.^[88] In the 1970s, Leland Hartwell's genetic screens in budding yeast (Saccharomyces cerevisiae) identified cell division cycle (CDC) genes, uncovering essential checkpoints that ensure orderly progression through the cell cycle and prevent errors in division.^[89] Building on this, the 1980s saw the discovery of cyclins—proteins that oscillate in concentration to drive phase transitions—by Tim Hunt and colleagues using sea urchin embryos, where they observed a protein synthesized from maternal mRNA that accumulated and was degraded at each cleavage.90420-8) Concurrently, Paul Nurse identified cyclin-dependent kinases (CDKs), such as Cdc2 in fission yeast (Schizosaccharomyces pombe), as the enzymatic partners of cyclins that phosphorylate targets to trigger mitosis entry. These findings established the cyclin-CDK oscillator as the core engine of eukaryotic cell division regulation.^[89] The 2001 Nobel Prize in Physiology or Medicine recognized Hartwell, Hunt, and Nurse for their pivotal contributions to cell cycle control, highlighting how disruptions in these mechanisms underlie diseases like cancer.^[89] Parallel advances in imaging technology, particularly the adaptation of green fluorescent protein (GFP) as a genetically encoded tag in the mid-1990s, enabled real-time visualization of dynamic processes in living cells, such as cyclin localization and chromosome movements during mitosis. For instance, early GFP fusions to histones allowed high-resolution tracking of chromatin condensation and segregation without perturbing cellular function.^[90] These tools revealed spatiotemporal coordination of molecular events, transforming the study of cell division from static snapshots to dynamic molecular narratives.

Recent advances

3D genome dynamics

Recent advances in imaging techniques have revealed that the three-dimensional (3D) structure of the genome does not fully disassemble during mitosis, as previously assumed. A 2025 study from MIT researchers utilized Region-Capture Micro-C (RC-MC), a method providing 100 to 1,000 times higher resolution than traditional Hi-C, to map chromatin interactions in dividing human cells. This approach demonstrated that small-scale 3D loops, forming regulatory microcompartments between enhancers and promoters, persist throughout mitosis.^[91] These persistent loops challenge the long-held view of a complete 3D genome reset during cell division, where chromosomes were thought to lose all spatial organization to facilitate equal partitioning. Instead, the microcompartments strengthen during mitotic chromosome condensation, preserving key regulatory contacts that help maintain epigenetic memory across generations of daughter cells. This continuity ensures more faithful gene expression patterns post-division, influencing cellular identity and function.^[91] Further insights into 2025 chromosome compaction dynamics indicate that the iconic X-shaped mitotic chromosomes form through progressive shortening and thickening without requiring a total unfolding of the interphase chromatin architecture. This process allows for stable structural organization within minutes of mitosis entry, supporting efficient segregation. These findings relate briefly to the precise alignment of chromosomes at the metaphase plate, enhancing division accuracy.^[92]

Novel protein and gene roles

Recent research from 2023 to 2025 has uncovered novel roles for proteins and genes in cell division, expanding understanding of molecular mechanisms beyond the foundational cyclin-dependent kinase (CDK) pathways. A 2025 study demonstrated that evolutionarily recent transcription factors, particularly Krüppel-associated box zinc finger proteins (KZFPs) such as ZNF519, play a critical role in regulating rhythmic expression of cell cycle genes in humans. These factors, which emerged relatively late in primate evolution, influence progression through G1/S and G2/M phases; perturbing them disrupts timely cell cycle advancement, highlighting their integration into core oscillatory networks.^[93] In spindle assembly, centromere-associated protein E (CENP-E) was found to have an unexpected stabilizing function rather than acting solely as a motor for chromosome congression during metaphase. High-resolution imaging in human cells such as RPE-1 and HeLa revealed that CENP-E primarily reinforces initial microtubule attachments to kinetochores, preventing misalignment that could lead to aneuploidy—a hallmark of cancer. This discovery challenges prior models emphasizing CENP-E's kinesin-like motility and suggests targeted inhibition could exploit segregation errors in tumors.^[94] Work from the National Cancer Institute in 2023 showed that mammalian cells can reversibly exit the division process and return to a quiescent G0 state, even after entering S phase, if growth signals via CDK4/6 are withdrawn. Approximately 15% of cells in early S/G2 phases reversed course by downregulating replication factors and reactivating quiescence markers, overturning the assumption of an irreversible commitment post-R point. This plasticity, observed in non-transformed fibroblasts, implies potential therapeutic windows to halt aberrant proliferation in cancers.^[95] Advancements in cellular reprogramming introduced mitomeiosis in 2025, an engineered reductive division process that halves chromosome ploidy in polyploid somatic cells without DNA replication. Applied to human skin fibroblasts reprogrammed toward oocytes via somatic cell nuclear transfer, mitomeiosis synchronized chromosome segregation to produce haploid gamete-like cells capable of fertilization and embryonic development in vitro. This technique addresses a key barrier in generating functional gametes from induced pluripotent stem cells, with implications for fertility restoration.^[96] Researchers at the University of Manchester in 2025 upended conventional views of mitotic spindle mechanics by demonstrating that in vivo tissue stretch anisotropically modulates spindle orientation through localized NuMA (nuclear mitotic apparatus protein) recruitment. In developing epithelial tissues of Xenopus embryos, uniaxial mechanical forces from stretching altered astral microtubule pulling and cortical dynein clustering, dynamically fine-tuning division axis without altering intrinsic spindle length or polarity. This mechanical feedback loop ensures asymmetric divisions align with tissue morphogenesis, revealing environment-driven adaptability in spindle positioning.^[97]

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Wilhelm August Oscar Hertwig (1849-1922)
Nov 1, 2007 · Most importantly, however, Hertwig found that only one spermatozoon was required to fertilize an egg, and that once one spermatozoon entered ...Missing: 1876 | Show results with:1876
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A Structure for Deoxyribose Nucleic Acid - Nature
The determination in 1953 of the structure of deoxyribonucleic acid (DNA), with its two entwined helices and paired organic bases, was a tour de force in ...
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The Nobel Prize in Physiology or Medicine 2001 - Press release
Timothy Hunt (born 1943), Imperial Cancer Research Fund, London, is awarded for his discovery of cyclins, proteins that regulate the CDK function. He showed ...
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Histone–GFP fusion protein enables sensitive analysis of ...
Mar 26, 1998 · Using confocal microscopy, H2B–GFP allowed high-resolution imaging of both mitotic chromosomes and interphase chromatin, and the latter revealed ...
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Dynamics of microcompartment formation at the mitosis-to-G1 ...
Oct 17, 2025 · Integrating 3D polymer modeling, we show how an interplay of affinity, extrusion activity and chromosome compaction can explain these structural ...
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https://www.nature.com/articles/171737a0