meiosis /mei·o·sis/ (mi-o´sis) cell division occurring in maturation of sex cells, wherein, over two successive cell divisions, each daughter nucleus receives half the number of chromosomes typical of the somatic cells of the species, so that the gametes are haploid.meiot´ic 
meiosis
meiosis [mi-o´sis]
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Meiosis (only two of the 23 human chromosome pairs are shown, the chromosomes from one parent in blue, those from the other parent in pink).
mei·o·sis (m -  s s)n. pl. mei·o·ses (-s  z ) The special process of cell division in sexually reproducing organisms that results in the formation of gametes, consisting of two nuclear divisions in rapid succession that in turn result in the formation of four gametocytes, each containing half the number of chromosomes that is found in somatic cells. mei·ot ic (- tk) adj.mei·ot i·cal·ly adv. |  light and dark chromosomes indicate chromosomes from two different individuals |
meiosis
[mī·ō′sis]
Etymology: Gk, becoming smaller
the division of a sex cell as it matures into two and then four haploid cells. The nucleus of each receives one half of the number of chromosomes present in the somatic cells of the species. Also called reduction division. Compare mitosis. See also anaphase, metaphase, oogenesis, prophase, spermatogenesis, telophase. meiotic [mī·ot′ik] , adj.
meiosis [mi-o´sis]
the process of cell division by which reproductive cells (gametes) are formed. There are two successive divisions, meiosis I and meiosis II, in which four daughter cells that have the haploid chromosome number (23 in humans) are formed. As in mitosis (somatic cell division), meiosis I and II are each divided into four phases: prophase, metaphase, anaphase, and telophase. adj., adj meiot´ic.
The first meiotic prophase is a complex process separated into five stages. During leptotene the chromosomes coil and contract; each consists of two chromatids joined along their length. During zygotene pairs of homologous chromosomes come into point-to-point contact along their length. This process is called synapsis and the structure formed is called a bivalent. The X and Y chromosomes synapse only at the ends of the short arms. During pachytene the chromosomes thicken, and the chromatids of each chromosome separate except at the centromeres. The bivalent is now a tetrad of four chromatids. During this stage crossing over occurs, in which the chromatids of homologous chromosomes break and rejoin, resulting in chromatids that contain sections derived from both the mother and the father. During diplotene the two chromosomes of each bivalent separate except for X-shaped chiasmata where crossover has occurred. In the female, this stage (called dictyotene) is prolonged; the oocyte remains in this stage from late fetal life until the time of ovulation. In the last stage, diakinesis, the chiasmata move to the ends of the chromosomes.
The other phases of meiosis I and II resemble those of mitosis, except that in meiosis I the two chromosomes of each bivalent separate and move to opposite poles. Thus, each daughter cell receives the haploid number of chromosomes, each with two chromatids. The assortment is random; either the maternal or the paternal chromosome can go to a daughter cell. Meiosis II then follows immediately without DNA replication. Both daughter cells formed by meiosis I divide again and the two chromatids of each chromosome separate and go to separate daughter cells. This produces four haploid daughter cells with chromosomes composed of single chromatids.
The first meiotic prophase is a complex process separated into five stages. During leptotene the chromosomes coil and contract; each consists of two chromatids joined along their length. During zygotene pairs of homologous chromosomes come into point-to-point contact along their length. This process is called synapsis and the structure formed is called a bivalent. The X and Y chromosomes synapse only at the ends of the short arms. During pachytene the chromosomes thicken, and the chromatids of each chromosome separate except at the centromeres. The bivalent is now a tetrad of four chromatids. During this stage crossing over occurs, in which the chromatids of homologous chromosomes break and rejoin, resulting in chromatids that contain sections derived from both the mother and the father. During diplotene the two chromosomes of each bivalent separate except for X-shaped chiasmata where crossover has occurred. In the female, this stage (called dictyotene) is prolonged; the oocyte remains in this stage from late fetal life until the time of ovulation. In the last stage, diakinesis, the chiasmata move to the ends of the chromosomes.
The other phases of meiosis I and II resemble those of mitosis, except that in meiosis I the two chromosomes of each bivalent separate and move to opposite poles. Thus, each daughter cell receives the haploid number of chromosomes, each with two chromatids. The assortment is random; either the maternal or the paternal chromosome can go to a daughter cell. Meiosis II then follows immediately without DNA replication. Both daughter cells formed by meiosis I divide again and the two chromatids of each chromosome separate and go to separate daughter cells. This produces four haploid daughter cells with chromosomes composed of single chromatids.
Meiosis (only two of the 23 human chromosome pairs are shown, the chromosomes from one parent in black, those from the other parent in outline). From Dorland's, 2000.
meiosis (mio´sis),
n a type of cell division of maturing sex cells that ensures that each daughter cell contains the necessary complement of chromosomes for future embryonic development.
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Second meiotic division.
meiosis
the process of cell division by which reproductive cells (gametes) are formed. There are two successive divisions, meiosis I and meiosis II, in which four daughter cells that have the haploid chromosome number are formed. As in mitosis (somatic cell division), meiosis I and II are each divided into four phases: prophase, metaphase, anaphase and telophase.
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