meiosis (reduction division) A type of nuclear division that occurs at some stage in the life cycle of sexually reproducing organisms. It is a mechanism whereby the number of chromosomes is halved to prevent doubling in each generation. Genetic material can be exchanged between homologous chromosomes. Two successive divisions of the nucleus occur, with corresponding cell divisions, following a single chromosomal duplication. This produces gametes or sexual spores that have one half of the genetic material or chromosome number of the original cell. This halving of the chromosome number (2n to n) compensates for its doubling when the gametes (n + n) unite to form a zygote (2n) during sexual reproduction. The process occurs during gamete or spore formation (some plants produce spores asexually); in many fungi, and in green algae, it occurs immediately after fertilization or on germination of the zygote. Chromosomes first appear in the first stage of prophase I (leptotene) of meiosis, as single threads. The 2 homologous members of each chromosome pair associate side by side with corresponding loci adhering together: this is called pairing. It occurs during the zygotene stage, each resulting pair being called a bivalent. Thus the apparent number of chromosome threads is half what it was before, being the number of bivalents rather than the number of single chromosomes. During the pachytene stage, each bivalent separates into 2 sister chromatids (except at the region of the centromere), with some localized breakage and crossing-over of genetic material of both maternal and paternal origin. There are now n groups of 4 chromatids lying parallel to each other and forming a tetrad. During the diplotene stage, 1 pair of sister chromatids in each of the tetrads begins to separate from the other pair except at the sites where exchanges have taken place. In these regions the overlapping chromatids form a cross shaped structure called a chiasma (pl. chiasmata) and these chiasmata slip towards the ends of the chromatids so that their position no longer coincides with that of the original cross-overs. This process continues until, during diakinesis, all the chiasmata reach the ends of the tetrads and the homologues can separate during anaphase. At diakinesis the chromosomes coil tightly, so shortening and thickening to form a group of compact tetrads which are well spaced out in the nucleus, and the nucleolus disappears. This ends the prophase stage of meiosis. During the first division (metaphase through to telophase), the nuclear envelope disappears, with the tetrads arranged at the equator of the spindle. The chromatids of a tetrad separate in such a way that maternal chromosomal material is kept distinct from paternal material except at regions distal to the points of crossing-over. This first division produces 2 daughter nuclei containing dyads (a dyad is half a tetrad), each of which becomes surrounded by a nuclear envelope. In many plants there is no telophase, cell-wall formation, or interphase, and the cell passes directly from anaphase I into prophase of the second meiotic division. In the second division the nuclear membrane disappears once more and the dyads arrange themselves upon the metaphase plate, the chromatids of each dyad being equivalent to one another (except for those regions distal to points of crossing-over). The centromere divides and so allows each chromosome to pass to a separate cell and the process is complete. Compare MITOSIS.