meiosis
Cell division producing four genetically unique haploid gametes (sperm, eggs) from one diploid cell. The source of genetic diversity in sexually reproducing organisms.
stages
meiosis I (reductional)
- prophase I: homologous chromosomes pair (synapsis), crossing over exchanges genetic material between chromatids
- metaphase I: homologous pairs align at the equator, independent assortment randomizes parental chromosomes
- anaphase I: homologous pairs separate (disjunction), sister chromatids stay together
- telophase I + cytokinesis: two haploid cells formed
meiosis II (equational)
Similar to mitosis: sister chromatids separate, producing four haploid cells total.
genetic diversity
- crossing over: recombination creates new allele combinations on chromosomes
- independent assortment: 2^23 possible chromosome combinations in humans (~8.4 million)
- random fertilization: any sperm can meet any egg, multiplying diversity further
biological significance
Foundation of sexual reproduction and evolution. Errors in meiosis (nondisjunction) cause aneuploidy — Down syndrome (trisomy 21), Turner syndrome (monosomy X).
connections
Distinguished from mitosis by two rounds of division and genetic recombination. Gamete formation depends on transcription of meiosis-specific genes. Chromosome structure maintained by polymerization of tubulin.