Meiosis is a type of cell division that produces haploid gametes (or spores) from diploid parent cells, involving two successive divisions: Meiosis I (reductional division) and Meiosis II (equational division). Meiosis I reduces the chromosome number from diploid (2n) to haploid (n) by separating homologous chromosome pairs. Meiosis II then separates sister chromatids, similar to mitosis. The overall result: one diploid parent cell → four haploid daughter cells (gametes), each genetically unique due to crossing over and independent assortment. Meiosis is fundamental to sexual reproduction and genetic diversity.

Prophase I is the most complex phase of meiosis, significantly longer and more elaborate than mitotic prophase, and is divided into five substages. Leptotene (thin thread): chromosomes begin to condense and become visible as thin threads. Zygotene (joined threads): homologous chromosomes begin pairing through a process called synapsis, facilitated by the synaptonemal complex (a protein scaffold), forming structures called bivalents (or tetrads — 4 chromatids). Pachytene (thick thread): chromosomes are fully synapsed and thickened; crossing over (recombination) occurs, exchanging segments between non-sister chromatids of homologs. Diplotene (double thread): synaptonemal complex dissolves, homologs begin to separate but remain connected at chiasmata (X-shaped structures marking crossover sites); this stage can last years in oocytes (human female). Diakinesis (moving apart): chromosomes reach maximum condensation, nuclear envelope breaks down, spindle begins to form — end of prophase I.

Meiosis I (reductional): parent cell 2n → 2 cells each n. Key features: synapsis, crossing over, bivalent formation, homolog separation in anaphase I. Cell starts and ends with: 2n → n (chromosome number halved). Type: reductional division. Meiosis II (equational): 2 cells n → 4 cells each n. No synapsis, no crossing over. Sister chromatids separate in anaphase II. Cell starts and ends with: n → n (number stays same). Type: equational division (resembles mitosis). Mitosis (equational): 2n → 2 cells each 2n. No synapsis, no crossing over. Sister chromatids separate in anaphase. Type: equational division (identical to meiosis II in principle). Key comparison: Meiosis I ≠ Mitosis (completely different). Meiosis II ≈ Mitosis (very similar, both equational).

Meiosis serves two fundamental biological purposes: halving the chromosome number to maintain species chromosome constancy across generations (if gametes were diploid and two fused at fertilisation, chromosome number would double every generation), and generating genetic variation through crossing over (recombination) and independent assortment of homologous chromosomes. Crossing over shuffles alleles between maternal and paternal chromosomes, creating new combinations. Independent assortment of 23 homologous pairs in humans can produce 2²³ = 8.4 million different chromosome combinations even without crossing over. Together these mechanisms ensure every offspring is genetically unique, providing the raw material for evolution by natural selection.