méiose

Meiosis is a specialized type of cell division that reduces the chromosome number by half, creating four genetically distinct haploid cells from a single diploid cell. This process is essential for sexual reproduction in eukaryotic organisms, as it produces gametes (sperm and egg cells in animals, or spores in plants and fungi). The reduction in chromosome number ensures that when two gametes fuse during fertilization, the resulting zygote has the correct diploid number of chromosomes.

Meiosis occurs in two successive nuclear divisions, meiosis I and meiosis II, each followed by cytokinesis. Meiosis I, the reductional division, separates homologous chromosomes. It begins with prophase I, where homologous chromosomes pair up and exchange genetic material through crossing over, a key source of genetic variation. Metaphase I sees these homologous pairs align at the metaphase plate. Anaphase I then pulls homologous chromosomes apart to opposite poles of the cell, while sister chromatids remain attached. Telophase I and cytokinesis result in two haploid cells, each containing one chromosome from each homologous pair, but with two sister chromatids still attached.

Meiosis II, the equational division, resembles mitosis. It begins with prophase II, followed by metaphase II where chromosomes align at the metaphase plate. In anaphase II, sister chromatids separate and move to opposite poles. Telophase II and cytokinesis complete the process, yielding a total of four haploid daughter cells. Each of these cells is genetically unique due to crossing over in meiosis I and the independent assortment of homologous chromosomes.