Cell division

Cell division is the fundamental biological process by which a parent cell divides to form two or more daughter cells. It is essential for reproduction, development, growth, and the maintenance of tissues in living organisms. In all known life forms, cell division follows at least one complete round of DNA replication, ensuring that genetic information is faithfully transmitted to subsequent generations.
In eukaryotes, two major forms of cell division occur: mitosis, which produces genetically identical daughter cells, and meiosis, which generates haploid gametes for sexual reproduction. Prokaryotes divide through simpler mechanisms such as binary fission, though variations including budding also exist.

Overview of Cell Division Types

• Mitosis (equational division)Produces two daughter cells genetically identical to the parent cell. Chromosomal number remains constant. Mitosis typically follows DNA replication in the S phase and is completed through cytokinesis, which divides the cytoplasm and organelles.
• Meiosis (reductional division)Produces four haploid daughter cells. Homologous chromosomes separate in meiosis I, and sister chromatids separate in meiosis II. Meiosis generates the gametes involved in sexual reproduction and introduces genetic variation through recombination events such as crossing over.

Both mechanisms appear to have been present in the last common ancestor of modern eukaryotes.

Cell Division in Prokaryotes

In bacteria and archaea, division predominantly occurs through binary fission, in which DNA replication is followed by equal segregation of chromosomes and cytoplasm. The divisome protein complex coordinates membrane constriction and cell wall remodelling. A tubulin-like protein, FtsZ, forms a contractile ring beneath the cell membrane and drives constriction. Alternative modes such as budding are known in a minority of species.

Cell Cycle and Interphase

Cell division in eukaryotes forms part of a larger cell cycle, which comprises:

• G₁ (Gap 1) – Cell growth and preparation for DNA synthesis
• S (Synthesis) – Replication of the chromosomes
• G₂ (Gap 2) – Additional growth and preparation for mitosis, including spindle formation
• M phase (Mitosis or Meiosis) – Division of the nucleus, followed by cytokinesis

Cells may enter G₀, a non-dividing state, from which they may re-enter the cycle depending on physiological requirements.
Progression is regulated by cyclins and cyclin-dependent kinases (CDKs). Three principal checkpoints control the cycle:

• G₁/S checkpoint – Verifies cell size and DNA integrity
• G₂/M checkpoint – Ensures successful DNA replication
• Metaphase (spindle) checkpoint – Confirms correct attachment of chromosomes to the spindle apparatus

Failure at any checkpoint halts progression, preventing propagation of damaged DNA.

Mitosis

Mitosis comprises a sequence of coordinated stages that achieve accurate chromosomal segregation:

• ProphaseChromatin condenses into visible chromosomes. The nucleolus disappears, the nuclear envelope begins to disassemble, and the mitotic spindle forms from centrosomes. Each chromosome consists of two sister chromatids joined at a centromere.
• PrometaphaseComplete breakdown of the nuclear envelope allows spindle microtubules to attach to kinetochores. Stable attachments ensure that sister chromatids are correctly oriented for separation.
• MetaphaseChromosomes align along the metaphase plate. This arrangement is monitored by the spindle checkpoint.
• AnaphaseSister chromatids separate and move toward opposite poles as spindle fibres shorten.
• Telophase and CytokinesisNuclear envelopes re-form around the chromosome sets, chromatin decondenses, and the cytoplasm divides into two daughter cells with comparable organelle content.

Meiosis

Meiosis involves two successive divisions:

• Meiosis I (reductional division)Homologous chromosomes pair and undergo crossing over, facilitated by the Spo11 protein. Homologues are then separated so each daughter cell receives one member of each pair.
• Meiosis II (equational division)Sister chromatids separate, closely resembling the process of mitosis. The result is four haploid cells.

In animals, meiosis produces gametes. In plants and some protists, meiosis yields spores that germinate into haploid gametophytes, with gametes produced later in the life cycle.

Biological Significance

Cell division underpins essential biological processes:

• Reproduction – In unicellular organisms, division creates new individuals.
• Development – Multicellular organisms develop from a zygote through repeated mitotic cycles.
• Growth and Repair – Replacement of damaged or ageing cells depends on mitosis.
• Genomic Continuity – Accurate chromosome segregation preserves genetic stability.

The human body undergoes an estimated 10¹⁶ cell divisions over a typical lifespan, illustrating the scale and precision of this process.

Variations and Additional Forms

Some eukaryotes exhibit atypical or amitotic divisions, especially among protists such as diatoms and dinoflagellates, and in certain fungi. These processes differ from classical mitosis in spindle organisation or chromosome behaviour but fulfill similar functional roles.
The cytoskeleton, including microtubules and actin filaments, plays a crucial role in ensuring reliable chromosome movement and cytoplasmic partitioning across all forms of cell division.
Cell division thus represents one of the most fundamental and conserved biological mechanisms, enabling the propagation of life across successive generations and contributing to both evolutionary change and organismal integrity.