meiosis

(redirected from Metaphase II)
Also found in: Dictionary, Thesaurus, Encyclopedia.
Related to Metaphase II: Meiosis I

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.
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.

mei·o·sis

(mī-ō'sis), Do not confuse this word with miosis.
A special process of cell division comprising two nuclear divisions in rapid succession that result in four gametocytes, each containing half the number of chromosomes found in somatic cells.
Synonym(s): meiotic division
[G. meiōsis, a lessening]

meiosis

(mī-ō′sĭs)
n. pl. meio·ses (-sēz′)
1. Genetics The process of cell division in sexually reproducing organisms that reduces the number of chromosomes from diploid to haploid, as in the production of gametes.
2. Rhetorical understatement.

mei·ot′ic (-ŏt′ĭk) adj.
mei·ot′i·cal·ly adv.

meiosis

The process of nuclear division whereby cells divide without replicating chromosomes, producing mature eggs and sperm with a haploid number of chromosomes. Meisosis is a type of cell division required for sexual reproduction, which consists of two nuclear divisions:
(1) In the first division, the chromosomes undergo recombination, forming different genetics in each daughter gamete—each of which has a full (diploid) complement of chromosomes—which is essentially what occurs in mitosis;
(2) In the second division, the diploid complement is reduced to a haploid number.

The resulting cells contain one part of each pair of homologous chromosomes, which allows the haploid daughter cell from the mother (ovum) to combine with a haploid daughter cell from the father (sperm).

mei·o·sis

(mī-ō'sis)
A special process of cell division comprising two nuclear divisions in rapid succession that result in four gametocytes, each containing half the number of chromosomes found in somatic cells.
Enlarge picture
MEIOSIS

meiosis

(mi-o'sis) [Gr. diminution]
A process of two successive cell divisions, producing cells, egg or sperm, that contain half the number of chromosomes (haploid) in somatic cells. When fertilization occurs, the nuclei of the sperm and ovum fuse and produce a zygote with the full chromosome complement (diploid).
See: illustration; chromosome; mitosis; oogenesis

meiosis

The process in the formation of the sperms (spermatozoa) and eggs (ova) in which chromosomal material undergoes recombination (meiosis I) and the chromosomes are reduced to a single set of 23 (haploid number) instead of the normal 23 pairs (meiosis II). This allows the restoration of the normal number when the spermatozoon fuses with the ovum. See also MITOSIS.
Meiosisclick for a larger image
Fig. 217 Meiosis . (a) Prezygotic meiosis, eg humans. (b) Postzygotic meiosis, eg fungus.

meiosis

a type of nuclear division associated with sexual reproduction, producing four HAPLOID (1) cells from a single DIPLOID (1) cell, the process involving two cycles of division. Although meiosis is a continuous process it has been divided into numerous stages, given below. Further details of each stage can be obtained by referring to individual entries.

PROPHASE I: homologous chromosomes pair, split into CHROMATIDS, and carry out CROSSING OVER. The nuclear membrane disintegrates.

METAPHASE I: chromosomes migrate to the spindle equator to which they become attached by their CENTROMERES. ANAPHASE I: HOMOLOGOUS CHROMOSOMES separate to opposite poles. TELOPHASE I: new nuclei form, in which there is only one type of each chromosome, although each is divided into two chromatids. PHOPHASE II: nuclear membrane goes. METAPHASE II: chromosomes attach to spindle. ANAPHASE II: chromatids separate to poles. TELOPHASE II: a total of four haploid nuclei is produced, each with one of each type of chromosome.

Meiosis has two major functions:

  1. it halves the number of chromosomes to prevent a doubling in each generation,
  2. it produces a mixing of genetic material in the daughter cells by the process of INDEPENDENT ASSORTMENT and RECOMBINATION. Note that the second point is only true if variability already is present in the parent cell.

Meiosis occurs at different stages of the life cycle in haploid and diploid organisms. See Fig. 217 . See also MITOSIS, GAMETOGENESIS and Figs. 167 and 168 .

mei·o·sis

(mī-ō'sis)
Special process of cell division comprising two nuclear divisions in rapid succession that result in four gametocytes, each containing half the number of chromosomes found in somatic cells.
References in periodicals archive ?
* High dose of co-trimoxazole treatment caused significant structural abnormalities and decreases in ovarian quality, including the number of germinal vesicles and metaphase I and metaphase II of rat oocytes.
Maturation rate, in vitro fertilization (IVF) success, and developmental competence at the blastocyst stage in mouse oocytes IVF GV oocytes PB success Blastocyst Group (N) N (%) N (%) N (%) [alpha]-MEM 264 180 (68.18) 125 (69.44) 64 (51.2) ESCM 603 390 (64.67) 240 (61.53) 84 (35) ESGM 332 107 (32.22) 0 (0.0) - p-value - 0.000 0.000 0.0039 GV: Germinal vesicle oocyte; PB: First polar body; Metaphase II (MII); [alpha]-MEM: [alpha]-minimum essential medium; ESCM: Embryonic stem cell conditioned medium; ESGM: Embryonic stem cell growth medium.
2 additional stimulation treatments in a GnRH-antagonist protocol were performed for oocyte accumulation and with both stimulations, a total of 18 COCs were collected and 17 were metaphase II and vitrified.
Sharma, "Oxidative stress and tumor necrosis factor-a-induced alterations in metaphase II mouse oocyte spindle structure," Fertility and Sterility, vol.
of cycles 211 152 Protocol Luteal phase 148 (70.1) 123 (80.9) Flare 39 (18.5) 16 (10.5) Antagonist 24 (11.4) 13 (8.6) Intracytoplasmic sperm 119 (56.4) 80 (52.6) injection Oocytes retrieved 11.0 [+ or -] 5.2 11.3 [+ or -] 5.3 Metaphase II oocytes retrieved 9.3 [+ or -] 4.6 9.6 [+ or -] 4.5 Day of embryo transfer (b,c) 2 11 (5.5) 9 (6.2) 3 124 (61.4) 81 (55.9) 5 67 (33.2) 55 (37.9) No.
Embryo development of fresh 'versus' vitrified metaphase II oocytes after ICSI: a prospective randomized sibling-oocyte study.
The level of metaphase II was done by staining with 1% aceto-orcein [8].
The results showed that supplementation of GTE resulted in a significantly (p[?]0.05) increasing of oocytes developed to metaphase II (M II) when compared to control group, exception group V that no significant difference compared to group I.
Followed by prophase II, metaphase II, anaphase II (Fig.
The metaphase II spindles are similar to those of first division and may be similarly bent (Fig.
Following statistical analysis it was found that women with higher intakes of total fat had fewer metaphase II (MII) oocytes retrieved than women in the lowest tertile.