Cell division

Cell division is how a parent cell splits into two daughter cells. This usually happens within a larger cell cycle, where the cell grows and copies its chromosomes before dividing. In eukaryotes, two main types exist: mitosis, a vegetative division that creates genetically identical daughter cells, and meiosis, which produces haploid gametes for sexual reproduction and reduces the chromosome number from two of each type in the diploid parent to one in each daughter cell. Mitosis is part of the cell cycle where replicated chromosomes separate into two new nuclei, maintaining the total chromosome count. It is generally preceded by the S stage of interphase, where DNA replication occurs, and followed by telophase and cytokinesis, which splits the cytoplasm, organelles, and cell membrane roughly equally between the two new cells. The stages of mitosis together form the M phase of an animal cell cycle. DNA damage is detected and repaired at various cell cycle checkpoints, which can halt progression by inhibiting certain cyclin-CDK complexes. Meiosis involves two divisions, yielding four haploid daughter cells. Homologous chromosomes separate in the first division, giving each daughter cell one copy of each chromosome (already replicated with two sister chromatids), and these chromatids separate in the second division. Both cycles are used in sexual reproduction at some life stage and are thought to have been present in the last eukaryotic common ancestor.

Prokaryotes, such as bacteria and archaea, usually undergo binary fission, where genetic material is equally split into two daughter cells, though budding also occurs. The divisome, a protein complex, handles cell division, membrane constriction, and peptidoglycan wall remodeling, with the tubulin-like protein FtsZ forming a contractile ring. All cell divisions, regardless of organism, begin with a single round of DNA replication. For simple unicellular organisms like amoebas, one division equals reproduction—a whole new organism. In multicellular organisms, mitotic division creates progeny from cuttings (as in plants) and allows sexually reproducing organisms to develop from a one-celled zygote, formed by the fusion of two gametes produced by meiosis. After growth from zygote to adult, mitosis enables continual construction and repair; the human body undergoes about 10 quadrillion cell divisions in a lifetime. The primary goal of cell division is preserving the original cell’s genome, which requires replicating chromosomal information and cleanly dividing it between progeny cells, supported by extensive cellular infrastructure.

In bacteria, division occurs through binary fission or budding. The divisome protein complex handles division, constriction of inner and outer membranes, and remodeling of the peptidoglycan wall at the division site, with FtsZ playing a key role in forming a contractile ring.

In eukaryotes, division is more complex. If chromosome number is reduced, it is meiosis (reductional division); if not, it is mitosis (equational division). A primitive form called amitosis also exists, with amitotic or mitotic divisions being more atypical and diverse among protists (like diatoms and dinoflagellates) and fungi. In mitotic metaphase, chromosomes (each with two sister chromatids from S phase) align on the metaphase plate, then split and distribute to two daughter cells. In meiosis I, homologous chromosomes pair before separating into two daughter cells; meiosis II resembles mitosis, with chromatids separating similarly. In humans and many higher animals, this is gametic meiosis, producing four gametes. In other groups, especially plants, meiosis yields spores that germinate into a haploid gametophyte phase—called sporic meiosis.

Phases of eukaryotic cell division include interphase, which occurs before mitosis, meiosis, and cytokinesis. Interphase has three main phases: G1, S, and G2. G1 is a growth period where specialized cellular functions prepare the cell for DNA replication. Checkpoints during interphase, such as the one between G1 and S, assess appropriate cell size and conditions to either advance or halt development.