Mitosis

Mitosis is the process of nuclear division in eukaryotic organisms through which duplicated chromosomes are separated into two daughter nuclei. It normally gives each nucleus an equivalent set of genetic information and preserves the chromosome number of the parent nucleus. Mitosis is commonly accompanied by division of the cell, but nuclear division and division of the cell’s contents are distinct processes. Together, they support growth, tissue renewal, and reproduction in many organisms. (genome.gov)

Position in the cell cycle

Mitosis belongs to the M phase of the cell cycle. In a typical proliferating cell, it follows interphase, which comprises G₁, S, and G₂ phases. During G₁, the cell grows and performs its usual functions; during S phase, DNA replication duplicates the nuclear genetic material; and during G₂, the cell prepares for division. Replication therefore occurs before mitosis, rather than being a stage of mitosis itself. (genome.gov)

After replication, each chromosome consists of two sister chromatids, each containing a DNA molecule. These copies remain associated until their separation during mitosis. The distinction between chromosome number and DNA quantity is important: replication doubles DNA content without immediately doubling the conventional chromosome count. Once sister chromatids separate, each is considered an individual daughter chromosome. (genome.gov)

In a typical human somatic cell, the two daughter nuclei each receive 46 chromosomes. Mitosis does not inherently require two chromosome sets, however; its central function is to distribute the duplicated complement faithfully, rather than to reduce that complement by half. (genome.gov)

Chromosome organization and the spindle

Before segregation, chromatin becomes compact, producing chromosomes that can be distinguished microscopically. Their movement depends on the mitotic spindle, a bipolar structure composed of microtubules and associated proteins. Microtubules are components of the cytoskeleton, and their assembly, disassembly, and interactions with motor proteins contribute to chromosome movement. (ncbi.nlm.nih.gov)

Spindle microtubules attach to kinetochores, specialized protein assemblies formed at chromosome centromeres. For accurate segregation, the kinetochores of the two sister chromatids normally attach to microtubules extending from opposite spindle poles. This arrangement, called biorientation, allows the copies to move toward different poles when their linkage is released. (ncbi.nlm.nih.gov)

Stages of mitosis

Mitosis is continuous, but it is conventionally divided into stages according to chromosome appearance, spindle organization, and nuclear-envelope behavior. Some classifications include prometaphase within prophase; others recognize it separately. (ncbi.nlm.nih.gov)

Prophase. Chromosomes condense, and the spindle begins to assemble. Each replicated chromosome retains its two sister chromatids. The cell reorganizes its internal structures in preparation for chromosome segregation. (ncbi.nlm.nih.gov)

Prometaphase. In open mitosis, the nuclear envelope breaks down, allowing spindle microtubules to reach the chromosomes. Kinetochores establish spindle attachments, and chromosomes undergo movements as those attachments develop. (ncbi.nlm.nih.gov)

Metaphase. Chromosomes align near the spindle equator, forming the metaphase plate. This plate is a spatial arrangement, not a membrane or other physical partition. Sister kinetochores are normally connected to opposite poles. (ncbi.nlm.nih.gov)

Anaphase. The linkage between sister chromatids is released. Daughter chromosomes move toward opposite spindle poles, while separation of the poles can further increase the distance between the chromosome groups. (ncbi.nlm.nih.gov)

Telophase. The segregated chromosomes decondense, and daughter nuclei are established. In cells undergoing open mitosis, nuclear envelopes reassemble around the chromosome groups, restoring separate nuclear compartments. (ncbi.nlm.nih.gov)

Regulation and checkpoints

Mitotic entry is controlled by cyclin–cyclin-dependent kinase complexes. Their activation changes the phosphorylation of cellular proteins, coordinating chromosome condensation, spindle formation, and other mitotic events. Their subsequent inactivation helps reverse these changes as cells leave mitosis. (ncbi.nlm.nih.gov)

The spindle assembly checkpoint delays anaphase when kinetochores remain unattached. It restrains activation of the anaphase-promoting complex/cyclosome, or APC/C. Once checkpoint inhibition is relieved, APC/C promotes destruction of securin, releasing separase. Separase cleaves a component of cohesin, the complex maintaining sister-chromatid association, enabling segregation. APC/C also promotes destruction of mitotic cyclins, helping coordinate mitotic exit. (ncbi.nlm.nih.gov)

These controls improve segregation fidelity but do not guarantee error-free division. Failures in cell-cycle regulation can compromise genome integrity; genomic instability is associated with cancer development. (ncbi.nlm.nih.gov)

Cytokinesis and organismal variation

Cytokinesis divides the cytoplasm and completes physical cell division. It often begins during anaphase, overlapping with mitosis. In animal cells, an actin–myosin contractile ring draws the cell membrane inward to form a cleavage furrow. In higher plant cells, a cell plate develops internally and contributes to the new wall separating the daughter cells. (ncbi.nlm.nih.gov)

Mitosis without cytokinesis can produce a cell containing multiple nuclei. Conversely, coordinated nuclear and cytoplasmic division need not produce daughters of identical size or cellular contents. Equivalent chromosome inheritance should therefore not be confused with complete equivalence in daughter-cell structure or function. (ncbi.nlm.nih.gov)

Nuclear-envelope behavior also varies. Animals and many plants undergo open mitosis, whereas yeasts and some other unicellular eukaryotes undergo closed mitosis, retaining the envelope while chromosomes segregate within the nucleus. (ncbi.nlm.nih.gov)

Distinction from meiosis

Unlike mitosis, meiosis involves two successive nuclear divisions after one round of DNA replication and normally reduces the chromosome complement by half. Pairing of homologous chromosomes and crossing over during meiosis generate new combinations of inherited variants. Mitosis instead normally distributes sister copies in a single nuclear division, maintaining the parental chromosome complement. (genome.gov)