Summary

The cell cycle is the ordered, repeating sequence of growth, DNA replication and division through which a cell passes, divided into a long interphase (itself made up of the G1, S and G2 sub-phases, occupying roughly 95 percent of the cycle) followed by the much shorter M phase, during which the cell actually divides.

Mitosis, the form of nuclear division (karyokinesis) that occurs in somatic cells, proceeds through prophase (chromatin condenses into visible chromosomes, the spindle forms, the nuclear envelope breaks down), metaphase (chromosomes align at the equatorial plate), anaphase (sister chromatids separate and move to opposite poles) and telophase (nuclear envelopes re-form around the two new sets of chromosomes), followed by cytokinesis, which divides the cytoplasm. Because it produces two daughter cells that are genetically identical to the parent cell, mitosis underlies growth, the repair and replacement of damaged or worn-out cells, wound healing, and asexual reproduction in many organisms, while keeping the chromosome number constant across somatic cell generations.

Meiosis, restricted to germ cells, consists of two successive divisions without an intervening round of DNA replication. Meiosis I, the reductional division, is distinguished by an elaborate prophase I divided into leptotene, zygotene (synapsis, forming bivalents), pachytene (crossing over between non-sister chromatids), diplotene (chiasmata become visible as homologues separate) and diakinesis (maximal condensation, terminalisation of chiasmata), after which whole homologous chromosomes -- not sister chromatids -- separate at anaphase I, halving the chromosome number. Meiosis II then proceeds much like an ordinary mitotic division, separating sister chromatids, so that one diploid parent cell ultimately gives rise to four haploid daughter cells. …