The Cell Cycle, Mitosis and Meiosis

The cell cycle: interphase, then division

A dividing cell's life runs through a repeating cell cycle, split into two broad phases: a long interphase, in which the cell grows and prepares to divide, and a much shorter M phase (mitotic phase), in which division actually occurs. Interphase itself splits into three sub-phases: G1 (Gap 1), a period of active cell growth and protein synthesis; S (Synthesis), the phase in which the cell's DNA is actually replicated, doubling its genetic content, alongside replication of the centrioles; and G2 (Gap 2), a further period of growth and protein synthesis that specifically prepares the cell for the mechanics of division ahead. A cell that has stopped actively dividing, most permanently differentiated cells such as neurons, exits this cycle altogether into a distinct resting state called G0, and a cell can also enter G0 temporarily under unfavourable conditions, only re-entering the active cycle later if conditions improve.

Mitosis: one division, two identical daughter cells

Mitosis, the division that occupies the M phase, produces two daughter cells genetically identical to the parent cell and to each other, the mechanism behind ordinary growth, tissue repair and asexual reproduction in single-celled organisms. It proceeds through four named stages, conventionally remembered in order as prophase, metaphase, anaphase and telophase. In prophase, the replicated chromosomes condense into visible, compact structures and the nuclear envelope begins to break down. In metaphase, the condensed chromosomes align along the cell's central plane (the metaphase plate), attached to spindle fibres from opposite poles. In anaphase, the sister chromatids of each chromosome separate and are pulled toward opposite poles of the cell. In telophase, the separated chromosomes decondense, a nuclear envelope re-forms around each set, and the cell then physically splits into two through cytokinesis, completing the division into two new, genetically identical cells.

Meiosis: two divisions, four genetically varied haploid cells

Meiosis is a fundamentally different kind of division, occurring only in the specialised cells that produce gametes (sperm and egg cells), and it involves two successive divisions rather than one. Meiosis I is the reductional division: homologous chromosome pairs (one inherited from each parent) separate from each other, halving the chromosome number from diploid to haploid. Meiosis II is an equational division, mechanically similar to ordinary mitosis, in which the sister chromatids of each already-halved set separate from one another. Together, the two divisions convert one diploid parent cell into four haploid daughter cells, each carrying only half the parent's original chromosome number.

Meiosis carries a second, equally important function beyond simply halving chromosome number: it is the primary source of genetic variation in sexually reproducing organisms, generated through two distinct mechanisms. Crossing over, occurring during prophase I, is the physical exchange of genetic segments between non-sister chromatids of homologous chromosomes, creating new combinations of alleles on a single chromosome that did not exist in either parent chromosome beforehand. Independent assortment, occurring as homologous pairs align randomly at the metaphase I plate, means each gamete receives an independently randomised mix of maternal and paternal chromosomes, rather than a fixed, predictable set. Meiosis's chromosome-halving role and its variation-generating role work together for the same underlying reproductive purpose: halving the chromosome number in gametes is exactly what allows fertilisation (the fusion of two gametes) to restore the normal diploid chromosome number in the next generation, generation after generation, while the genetic shuffling ensures that generation is never a simple genetic copy of either parent.

Quick revision points

• Cell cycle: interphase (G1, growth; S, DNA replication; G2, further growth, prepares for division) followed by M phase (mitosis). G0 is a resting state cells exit the cycle into, either permanently (differentiated cells like neurons) or temporarily (unfavourable conditions).
• Mitosis: one division, produces two genetically identical diploid daughter cells; stages in order are prophase, metaphase, anaphase, telophase (PMAT), followed by cytokinesis. Basis of growth, repair and asexual reproduction.
• Meiosis: two successive divisions (Meiosis I, reductional, separates homologous chromosomes; Meiosis II, equational, separates sister chromatids like mitosis), producing four haploid daughter cells from one diploid parent cell.
• Meiosis generates genetic variation via crossing over (prophase I, exchange between non-sister chromatids of homologous chromosomes) and independent assortment (random alignment of homologous pairs at metaphase I).
• Meiosis's chromosome-halving is what allows fertilisation to restore the normal diploid number each generation, while its variation-generating role ensures offspring are never simple genetic copies of either parent.