Meios

Meios, the term used in some languages for the process known in English as meiosis, is the biological process by which eukaryotic organisms produce haploid gametes or spores from diploid germ cells. It reduces chromosome number by half, ensuring that when gametes fuse during fertilization, the offspring retains the species-specific diploid complement. Meios comprises two consecutive cell divisions, meiosis I and meiosis II, with only one round of DNA replication in between.

During meiosis I, homologous chromosomes pair in prophase I (synapsis) and crossing over occurs, exchanging genetic material. This recombination contributes to genetic variation. In metaphase I, paired homologs align along the equator; in anaphase I, they are pulled to opposite poles, separating homologs, while sister chromatids stay together. Telophase I and cytokinesis produce two haploid cells with duplicated chromosomes.

Meiosis II resembles mitosis: in prophase II, the spindle forms; in metaphase II, sister chromatids align; in anaphase II, centromeres split and sister chromatids migrate to opposite poles; telophase II and cytokinesis yield four genetically distinct haploid daughter cells. In animals, these become gametes; in plants and fungi, they may develop into spores.

Meiosis contributes to genetic diversity via crossing over, independent assortment, and, in some organisms, random fertilization. Errors such as nondisjunction can lead to aneuploidies or polyploidies. Overall, meiosis is a central mechanism that maintains chromosome number across generations and drives evolutionary variation.