Cell division is the process by which a parent cell produces daughter cells, distributing its genetic material and cellular contents between them. It enables reproduction in many unicellular organisms and supports growth, development, and tissue replacement in multicellular organisms. Division normally requires duplication of DNA, separation of the resulting genetic copies, and physical partitioning of the cell. Its mechanisms differ among organisms: prokaryotes commonly divide by binary fission, whereas eukaryotes generally coordinate nuclear division with division of the cytoplasm. (openstax.org)
Genetic material and preparation
Successful division requires each daughter cell to receive the genetic information needed for its subsequent activities. DNA is organized into chromosomes, which carry genes. In eukaryotes, nuclear chromosomes consist of DNA associated with proteins, forming chromatin. Chromatin becomes highly compacted during nuclear division, making individual chromosomes readily distinguishable. This organization helps long DNA molecules separate without becoming entangled or damaged. (openstax.org)
Before a typical mitotic division, DNA replication produces two copies of each chromosome. These copies, called sister chromatids, remain connected until the appropriate stage of division. Replication doubles the amount of DNA but does not immediately double the chromosome number under the usual convention of counting chromosomes by their centromeres. After sister chromatids separate, each becomes an individual chromosome in a daughter nucleus. (openstax.org)
The eukaryotic cell cycle
The cell cycle is the ordered sequence of preparation and division. Its conventional stages are G₁, S, G₂, and M. G₁, S, and G₂ collectively form interphase, an active period rather than a resting interval. During G₁, the cell grows and produces cellular components. DNA replication occurs in S phase. G₂ provides further preparation before entry into M phase, when chromosome segregation and usually physical division occur. (ncbi.nlm.nih.gov)
Not every cell continuously follows this sequence. Some leave the proliferative cycle and enter G₀, a nondividing state that may be temporary or long-lasting. Cell-cycle duration differs substantially among organisms and cell types. Nuclear division and cytoplasmic division can also become uncoupled: repeated nuclear divisions without cellular separation produce cells containing multiple nuclei. Consequently, an increase in nuclear number does not necessarily represent an increase in cell number. (openstax.org)
Mitosis and chromosome segregation
Mitosis distributes replicated nuclear chromosomes into two daughter nuclei, normally preserving the parental chromosome complement. Its stages are commonly described as prophase, prometaphase, metaphase, anaphase, and telophase. These names identify recognizable transitions within a continuous process rather than sharply separated events. (openstax.org)
During prophase, chromosomes condense and the mitotic spindle begins to assemble. The spindle consists of microtubules and associated proteins belonging to the cytoskeleton. In many eukaryotes, the nuclear envelope breaks down during prometaphase, allowing spindle microtubules to attach to chromosome-associated structures called kinetochores. At metaphase, chromosomes align between the spindle poles. During anaphase, sister chromatids separate and move toward opposite poles. Telophase establishes the daughter nuclei as chromosomes begin to decondense. (openstax.org)
Mitosis is therefore nuclear division, not a synonym for the entire process of cell division. Under normal conditions it gives daughter nuclei equivalent chromosome sets, although replication errors or segregation failures can produce genetic differences. (ncbi.nlm.nih.gov)
Cytokinesis
Cytokinesis physically partitions the cytoplasm and completes separation into daughter cells. In typical animal cells, an actin–myosin contractile ring forms beneath the cell membrane. Its contraction creates a cleavage furrow that deepens until the cells separate. Cytokinesis often begins before nuclear division has fully finished. (ncbi.nlm.nih.gov)
In plant cells, the existing rigid wall prevents division by the same inward-pinching mechanism. Membrane vesicles instead assemble a cell plate between the daughter nuclei. The plate expands outward and contributes to the new membranes and wall separating the daughter cells. Thus, chromosome segregation and cellular separation solve distinct mechanical problems, even when they are closely coordinated. (openstax.org)
Meiosis and sexual life cycles
Meiosis is a specialized division sequence that reduces the chromosome complement from diploid to haploid. One round of DNA replication is followed by two nuclear divisions. Homologous chromosomes separate during meiosis I, whereas sister chromatids separate during meiosis II. There is no intervening round of chromosome replication. (ncbi.nlm.nih.gov)
Chromosomal crossing-over exchanges DNA between homologous chromosomes, while their independent assortment generates additional combinations of inherited material. Meiotic products therefore differ genetically rather than being straightforward copies of the parent. In animals, meiosis participates in gamete formation; in plants, it produces spores within alternation of generations. Fusion of haploid cells at fertilization restores the diploid complement. (ncbi.nlm.nih.gov)
Prokaryotic division
Many bacteria reproduce through binary fission. Their chromosomes replicate, the copies segregate as the cell grows, and a division septum forms between them. Because these cells lack a membrane-bound nucleus, they do not undergo mitosis. In many bacteria, FtsZ forms a ring that organizes proteins involved in building the division site. (openstax.org)
Archaea use diverse division machinery. Some employ FtsZ-based systems, whereas others use proteins related to the eukaryotic ESCRT membrane-remodeling system. Prokaryotic division therefore comprises multiple molecular mechanisms rather than one universally identical process. (pmc.ncbi.nlm.nih.gov)
Regulation and division errors
Cell-cycle progression is controlled by cyclins and cyclin-dependent kinases, together with inhibitory regulators. Checkpoints can delay progression when DNA is damaged, replication is incomplete, or chromosomes are improperly attached to the spindle. Such delays permit DNA repair or correction of other problems; some damage responses can initiate apoptosis. (openstax.org)
Failures in these controls can allow abnormal cells to continue multiplying. Changes affecting growth-regulating and DNA-repair genes contribute to cancer, although cancer also involves properties beyond division, including invasion and, in many cases, spread to distant tissues. Normal cell multiplication, by contrast, remains coordinated with the requirements of the organism. (cancer.gov)