Meioszis

Meioszis, known in English as meiosis, is a specialized cell division that reduces the chromosome number by half, producing haploid gametes from a diploid germ cell. It is essential for sexual reproduction and genetic variation. Meioszis occurs in the gonads of sexually reproducing organisms and comprises two consecutive divisions, meiosis I and meiosis II, following an initial DNA replication during interphase.

Meiosis I: Prophase I features condensation of chromosomes and pairing of homologous chromosomes in a process called synapsis, with crossing over between non-sister chromatids that creates recombinant chromatids. This genetic recombination, visible as chiasmata, contributes to variation. In metaphase I, homologous pairs align along the cell equator in random orientations. Anaphase I then separates homologous chromosomes to opposite poles, while sister chromatids stay together. Telophase I and cytokinesis produce two haploid cells, each with replicated chromosomes.

Meiosis II: The two haploid cells quickly enter meiosis II, which resembles mitosis. Prophase II, metaphase II, anaphase II, and telophase II result in the division of sister chromatids, yielding a total of four haploid gametes. No DNA replication occurs between meiosis I and II in most organisms. The final gametes have half the original chromosome number and differ genetically due to recombination and independent assortment in meiosis I.

Errors in meiosis, such as nondisjunction, can produce cells with abnormal chromosome numbers, leading to conditions like aneuploidy in offspring. Meiosis thereby balances chromosome number across generations and fuels genetic diversity.