Meiosis is a specialized division in eukaryotic cells that generates haploid gametes—sperm in males and eggs in females. Each gamete carries 23 chromosomes, half the diploid 46 found in somatic cells.
Sexual reproduction preserves genetic diversity, which protects populations against environmental threats. By shuffling DNA from both parents, meiosis ensures each gamete is unique.
Meiosis begins like mitosis: 46 replicated chromosomes (paired sister chromatids) are present. Mitosis completes in one division, producing two identical diploid cells. Meiosis, confined to the gonads, requires two consecutive divisions—Meiosis I and Meiosis II—producing four haploid daughter cells.
After the first division, 92 chromatids become 46; after the second, each cell contains 23 chromosomes. Meiosis I is distinct because it separates homologous chromosomes, while Meiosis II resembles mitosis by separating sister chromatids.
Two critical events characterize Meiosis I:
Prophase I unfolds in five substages:
During crossing over, segments of DNA are exchanged between homologous chromosomes at sites called chiasmata. On average, 2–3 crossovers occur per chromosome pair, dramatically increasing genetic variation.
Bivalents align at the cell’s equator. Each side receives either the maternal or paternal half of every pair, a random process yielding 223 (≈8.4 million) possible gamete combinations. When a gamete fuses with another, the theoretical diversity reaches roughly 70 trillion unique zygotes—over ten thousand times Earth’s current population.
Homologous chromosomes separate and migrate to opposite poles, driven by microtubules. Cohesins that glued chromatids together are degraded, allowing separation.
Chromosomes reach the poles, new nuclei form, and cytokinesis divides the cell into two diploid daughters. Each now contains half the number of chromatids, setting the stage for Meiosis II, which will split sister chromatids to produce the final haploid gametes.
