Meiosis
Meiosis is the two-stage cell division that produces four sex cells, each with half the chromosome number and a unique mix of genes.
Definition
Meiosis is a special kind of cell division that makes sex cells, sperm in males and eggs in females, with only half the usual number of chromosomes, so that when a sperm and egg join the baby gets a full set. One diploid cell divides twice to produce four haploid gametes: in meiosis I, homologous chromosomes pair up, exchange segments by crossing over, and separate, and in meiosis II the sister chromatids separate, so independent assortment and crossing over give each gamete a different combination of alleles, the source of variation in sexual reproduction. One round of DNA replication precedes the two divisions. In prophase I, homologs synapse along the synaptonemal complex and undergo reciprocal recombination at chiasmata, which both creates new allele combinations and physically links the homologs so they orient correctly on the metaphase I spindle. Meiosis I is reductional (homologs separate while sister chromatids stay joined by centromeric cohesin protected by shugoshin) and meiosis II is equational (chromatids separate); segregation errors, called nondisjunction, yield aneuploid gametes, and the rate of maternal nondisjunction rises with age.
Example
A human body cell has $46$ chromosomes; meiosis makes sperm and egg cells with $23$ each, and when they combine, $23 + 23 = 46$ again. Because each of the $23$ chromosome pairs can line up in two ways, independent assortment alone can produce $2^{23}$, about $8.4$ million, different chromosome combinations in one person's gametes, and crossing over multiplies that further. Human males produce four functional sperm from each meiosis, while females produce one egg plus polar bodies, and the egg is arrested in prophase I from before birth until ovulation, sometimes for $40$ years or more; trisomy 21 (Down syndrome) results from nondisjunction of chromosome 21, usually in meiosis I of the egg.
Key Insight
Meiosis shuffles the genes, so no two sperm or eggs are exactly alike, which is why brothers and sisters look similar but not identical. Mitosis keeps the chromosome number the same and meiosis cuts it in half; without meiosis, chromosome number would double every generation. Meiosis is why sexual reproduction generates variation without changing the DNA sequence itself: recombination and independent assortment reshuffle existing alleles, giving natural selection new combinations to test every generation.