Working in conjunction with fertilization, meiosis ensures the number of chromosomes remain the same from one generation to the next. Before meiosis occurs, the parent cell is diploid, and the chromosomes have been duplicated providing for two sister chromatids per individual chromosome.
Meiosis involves two nuclear divisions that result in four haploid daughter cells. Crossing over occurs between non-sister chromatids (meaning that they are not on the same chromosome) during prophase I. During anaphase I, the chromosomes of each homologous pair separate into two different daughter cells, thus, producing two haploid daughter cells. In meiosis II, the separation of chromosomes is similar to the pattern found in mitosis, except that the cells are haploid in meiosis II.
In the animal life cycle, meiosis is a part of gametogenesis (formation of gametes). In plants, meiosis produces spores that develop into a gametophyte generation; and among fungi, meiosis occurring after zygote formation produces spores that develop into a haploid adult.
The human life cycle includes mitosis and meiosis (spermatogenesis in males and oogenesis in females). Only during meiosis do the homologous chromosomes pair and undergo independent separation (formation of a tetrad, crossing over, and the separation of the tetrad), resulting in a haploid number of chromosomes in the gametes. Crossing over and independent separation of homologs contribute to increased genetic variation among the daughter cells.