Meiosis

Meiosis is defined as the cellular and nuclear processes that reduce the chromosomal content per nucleus from two sets to one set.

From: Encyclopedia of Genetics , 2001

Meiosis

M.D. Griswold , P.A. Hunt , in Brenner's Encyclopedia of Genetics (Second Edition), 2013

Abstract

Meiosis is a feature of sexual reproduction that results in the independent assortment of genetic material from two individuals, providing greater genetic diversity. The initiation of meiosis requires both intrinsic and extrinsic signals. Meiosis is characterized by one round of DNA replication followed by two rounds of cell division, resulting in haploid germ cells. Crossing-over of DNA results in genetic exchange of genes between maternal and paternal DNA. Meiosis occurs in plants and animals and, although the details may differ among species and even between sexes in the same species (e.g., in humans, the timing of meiosis differs between males and females), overall the process is highly conserved.

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Meiosis

In Cell Biology (Third Edition), 2017

Suppression of DNA Replication Between Meiosis I and Meiosis II

Meiosis is unique in that it involves two M phases with no intervening S phase. On exit from meiosis I, Cdk1 kinase is reactivated immediately. This blocks assembly of prereplication complexes (see Fig. 42.8), thereby blocking DNA replication. At least two pathways contribute to reactivation of Cdk1.

The first involves downregulation of translation of Wee1 protein kinase in meiosis. Wee1 is a mitotic inhibitor (see Fig. 43.3) that inactivates Cdk1 by phosphorylation at Tyr15. The absence of Wee1 in meiosis I was first observed in Xenopus laevis but this seems to be a universally conserved way of reactivating Cdk1 without an S phase. Ectopic expression of Wee1 in mature X. laevis oocytes prevents reactivation of Cdk1 immediately after the meiosis I division. As a result, the oocytes reenter interphase and replicate their DNA. Meiotic cells also express a specialized isoform of Cdc25, the phosphatase that counteracts Wee1 (see Fig. 43.1).

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Meiosis

N. Hunter , in Encyclopedia of Biological Chemistry (Second Edition), 2013

Abstract

Meiosis is the specialized type of cell division by which sexual organisms produce gametes. In most organisms, meiosis produces haploid gametes from diploid precursor cells. Meiosis halves the chromosome number via two successive rounds of chromosome segregation that follow a single round of chromosome replication. The first round of meiotic chromosome segregation is unique in that the sister chromatids remain associated while parental homologs (pairs of sisters) are segregated. As prerequisites for their segregation, homologs become intimately associated and connected by structures called chiasmata. Homolog pairing and formation of chiasmata are both mediated by the DNA repair process called homologous recombination.

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Meiosis

P.B. Moens , in Encyclopedia of Genetics, 2001

Definition

Meiosis is defined as the cellular and nuclear processes that reduce the chromosomal content per nucleus from two sets to one set. In most organisms, two sets of chromosomes (diploid) are reduced to one set (haploid) (see Chromosome Pairing, Synapsis). When the haploid cell becomes involved in the process of fertilization, it is referred to as a 'gamete.' If a cell with one set of chromosomes goes on to proliferate, it is called a 'gametophytic generation.' This occurs in many fungi, ferns, and, for a few divisions, in plants. Many variations in the meiotic process have evolved that are of particular adaptive value to specific organisms. The products of meiosis in organisms with three or four sets of chromosomes are usually unbalanced because of difficulties in the segregation and assortment of chromosomes. Some of the mechanics of meiosis are presented in the articles on Chiasma, and Synaptonemal Complex.

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Introduction to Human Genetics∗

Bruce R. Korf , in Clinical and Translational Science (Second Edition), 2017

Meiosis

Meiosis consists of one round of DNA replication and two rounds of chromosome segregation. In meiosis, there are two steps: meiosis I and meiosis II. The differences between meiosis and mitosis are (1) homologous chromosomes pair at prophase of meiosis I; (2) genetic recombination, called meiotic crossing over, occurs regularly at prophase of meiosis I; and (3) the chromosome number is reduced to half after meiosis I, so that the daughter cells resulting from meiosis I are haploid (23 chromosomes) (Fig. 16.11).

Figure 16.11. The process of meiosis.

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Genetics of Meiotic Chromosome Dynamics and Fertility

Travis Kent , ... Mary Ann Handel , in Human Reproductive and Prenatal Genetics, 2019

Abstract

Meiosis defines and dictates much of the success of gametogenesis and is pivotal to our understanding of reproduction and causes of infertility. It is not only a defining event of gametogenesis but also a basic feature of eukaryotic genetics. Meiosis is the engine for genetic diversity, and ensures the genomic integrity and correct number of chromosomes that produce healthy offspring. Genetic analyses have been key to our unfolding knowledge of meiosis, highlighted here in discussions of the intricate chromosomal dynamics of meiotic prophase and the precise reductional and equational segregation of chromosomes in the meiotic division phases. These important aspects of meiosis provide lessons that can be applied to safeguarding human health, fertility, and offspring well-being.

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Genetics and Inheritance

A. Balasubramanian , ... C.A. Reynolds , in Encyclopedia of Infant and Early Childhood Development, 2008

Meiosis

Meiosis describes the process of cell division for reproductive cells that will result in sperm or egg cells with half the number of chromosomes. This is in contrast to the cell division that takes place in nongerm cells called mitosis, which results in daughter cells that are identical. Figure 1 represents meiosis in females and males. Reproductive cells contain 23 pairs of chromosomes prior to meiosis, with one set maternally derived and the other set paternally derived. During the first stage of meiosis, each homologous pair of chromosomes line up (maternal chromosome 1 with paternal chromosome 1, and so on), duplicate themselves, and then between homologous chromosomes an exchange of DNA segments of equal length at the identical location occurs, called crossing-over or recombination. In the second phase of meiosis the reproductive cell divides twice more to produce four germ cells with half the complement of 23 chromosomes. In females, one reproductive cell undergoing meiosis results in one viable egg cell and three cells that are not functional, whereas in males a single reproductive cell undergoing meiosis results in four sperm cells. The resulting chromosomes in the germ cells, egg and sperm, contain DNA from maternally and paternally derived chromosomes due to crossing-over in the first phase of meiosis, ensuring added genetic variation. The segments of DNA that are exchanged during the crossing-over process is random for any meiotic division, thus it is next to impossible to recreate the exact combination of genes in germ cells produced in a single individual.

Figure 1. Meiosis. The formation of germ cells (gametes) in (a) males and (b) females. Reproduced from Hetherington et al. (2003) Child Psycology. McGraw-Hill, with permission from McGraw-Hill.

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Gametogenesis, Fertilization and Early Development

Paula E. Cohen , in Encyclopedia of Reproduction (Second Edition), 2018

Introduction and History of Meiosis

Meiosis is the specialized cell division process that takes a diploid precursor cell and from it generates up to four haploid cells for sexual reproduction in eukaryotes. In mammals, the haploid products of meiosis are referred to as gametes: the spermatozoa of males, and the oocytes (or eggs) of females. Meiosis differs from the mitotic cell division in many respects, most notably in that it is comprised of two divisions preceded by only a single phase of DNA replication. While many other differences exist, much of the generalized mitotic machinery is conserved in meiosis, and this conservation is also apparent across organisms and between sexes.

Meiosis was first observed in sea urchin eggs in 1876 by German biologist, Oscar Hertwig. A decade later, Belgian zoologist, Edouard Van Beneden, described a similar process in the eggs of the roundworm, Ascaris. It was not until the 1890s, however, that August Weismann noted the importance of the two cellular division events for generation of haploid gametes for fertilization. The term meiosis, derived from the Greek word for "lessening" was coined in 1905 by Farmer and Moore to reflect the halving of the chromosomes. These early observations preceded the rediscovery of Mendel's work by Hugo de Vries and others in the 1900s, and culminated in 1911 with the discovery by Thomas Hunt Morgan of crossing over, the exchange of genetic information between homologous chromosomes as a result of recombination, a key and essential feature of meiosis.

In order to derive haploid daughter cells following a single round of DNA replication, meiosis consists of two distinct segregation events: meiosis I and meiosis II. Both meiotic divisions consist of prophase, metaphase, anaphase, and telophase (Fig. 1), but whereas the first division is considered equational (maintaining the number of genome copies, or ploidy, of the daughter cells), the second is reductional (halving the ploidy of the daughter cells), and thus very similar to mitosis. During meiosis I, homologous chromosomes, derived from each parent (maternal and paternal), must first find each other and pair, in order to then be segregated equally into two daughter cells. Like mitosis, meiosis II involves separation of sister chromatids, which are connected through their cohesin rings, the protein complexes that maintain sister chromatid interactions through the cell cycle. These sister chromatids are segregated to opposite poles prior to cytokinesis, resulting in the halving of the genome content. The fact that sister chromatids are already paired makes mitosis and meiosis II fairly easy to orchestrate: a simple separation of the chromatids following dissolution of the cohesin ring. For meiosis I, however, the homologous chromosomes are not usually tethered together, and thus, they must undergo a fascinating, and relatively poorly understood process of homology searching, pairing and then physical tethering, in order to allow them to be segregated equally at the first meiotic division. Thus, it is meiosis I, involving homologous chromosome interactions, that represents the unique and defining phase of meiosis.

Fig. 1

Fig. 1. Overview of mammalian meiosis. A representative cell is shown containing two pairs of homologous chromosomes (red/pink and green/yellow). At interphase, these cells are diploid, containing 2n and 2c content. "n" refers to the number of homologous chromosomes and "c" refers to the number of chromatids and/or the number of copies of each gene locus. After DNA replication, therefore, the cell is still 2n, but is now 4c (four copies of each chromatid, two for each chromosome). After the first meiotic division, when homologous chromosomes have divided, the daughter cells (one secondary spermatocytes or one secondary oocyte) are now 1n, 2c, containing only one homologous chromosome, but two sister chromatids. By the end of the second division, this chromosome content is reduced further to 1n, 1c. Note that only one cell emerges at the end of meiosis in females, the remaining genetic content being expelled in the first and second polar bodies.

 Modified from Chapter 1, Physiology of Reproduction, eds. Knobil and Neil (2013).

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The Molecular Genetics of Oogenesis

Fan Zhai , ... Jie Qiao , in Human Reproductive and Prenatal Genetics, 2019

Meiosis Division

Meiosis is a specialized cell division during which the number of chromosomes is reduced by half, creating genetically distinct haploid cells. Analogous to mitosis, the event preceding mitotic division involves DNA replication during the S-phase of the interphase, which leads to the production of two identical sister chromatids attached at a centromere. The G2 phase is not present in meiosis. Instead, DNA replication is followed by two rounds of cell division, known as meiosis I and meiosis II.

Meiosis I and II entail four stages: prophase, metaphase, anaphase, and telophase. During the prophase of meiosis I, homologous chromosomes pair with each other and undergo genetic recombination (crossovers) when they exchange genetic information and produce unique genetic combinations. Meiotic prophase is subdivided into five stages based on the appearance of chromosomes: leptotene, zygotene, pachytene, diplotene, and diakinesis [3]. The oocyte maturation suspends at the diplotene stage of prophase I and enters a resting stage called dictyate or germinal vesicle (GV) arrest, until meiosis resumption is triggered by the luteinizing hormone (LH) at ovulation [21]. Resumed meiosis I progresses through the metaphase I when homologous chromosome pairs move together along the metaphase plate, followed by segregation of the homologous chromosomes in which a pair of sister chromatids remain together in anaphase I. It ends when the chromosomes arrive at the opposite poles in telophase I. Upon completion of meiosis I, the primary oocyte divides into a larger secondary oocyte and extrudes a smaller polar body to discard half the genetic material. Meiosis I results in two haploid cells, each with a single set of chromosomes (half the number of the original parent cell chromosomes), although each chromosome contains a pair of sister chromatids.

Meiosis II starts after meiosis I without DNA replication. The process is similar to mitosis and involves equational segregation of sister chromatids after degradation of cohesin, a protein complex holding sister chromatids at the centromere. As a result, the number of DNA copy halves while the number of chromosomes remains the same. Following meiosis II, the secondary oocyte divides into an oocyte and a second polar body. Notably, in female gametogenesis, only one cell develops into an oocyte and the other meiotic products are eliminated by the extrusion of polar bodies. However, in males four daughter cells produced by meiotic division form sperm.

Meiosis has important biological implications. Compared to male meiosis, female meiosis is more prone to errors, resulting in aneuploidy, which increases with a woman's age. For instance, in 35-year-old women, aneuploidy is observed in about 20% of oocytes, but reaches 60% around menopause [22]. Although aneuploid oocytes can still be fertilized, the embryo can hardly be viable. Aneuploidy is one of the leading causes of reproductive failure and the most frequent genetic cause of developmental disabilities [23, 24].

In contrast to mitotic division, where incorrect segregation affects only a fraction of the cells resulting in mosaicism, the chromosome missegregation in meiosis could affect all the cells. During meiotic division, cells should equally share the chromosome, but sometimes, the whole pair of chromosomes or bivalent end up in one cell while the other one gets nothing. Incorrect chromosome segregation results in aneuploidy, carrying an abnormal number of chromosomes [24]. The aneuploid oocyte can still function, fertilize, and generate an aneuploid embryo. The one copy of autosomes is lethal in humans, leading to spontaneous abortion. Missing a sex chromosome (monosomy) or the addition of a chromosome copy (trisomy) could be viable, but the offspring's health is compromised, including trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), trisomy 13 (Patau syndrome), and monosomy X (Turner syndrome). Trisomy 21 (47, XX/XY, +   21), one of the most common viable aneuploidies in humans, results from the incorrect segregation of chromosome 21 in meiosis I (65%) or meiosis II (23%) [25]. Nondisjunction during meiosis is the most common cause of the aneuploidies, which can also occasionally result from a chromosomal rearrangement (e.g., Robertsonian translocation or a balanced sex chromosome translocation) [26].

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Gametogenesis, Fertilization and Early Development

Bruno Marques , ... Rui G. Martinho , in Encyclopedia of Reproduction (Second Edition), 2018

Meiosis

Meiosis is a process of cell division where a eukaryotic diploid cell originates one or more haploid cells, being essential for gamete production and maintenance of ploidy after egg fertilization. Meiosis is similarly important for generating genetic diversity through chromosome recombination and segregation. Not surprisingly, meiotic defects often lead to sterility and developmental defects ( Ohkura, 2015).

Diploid cells (2n) contain two sets of chromosomes, being one set usually from the mother and another set from the father. A homologous chromosome set corresponds to a pair of chromosomes in a diploid cell. After each round of DNA replication during S-phase, a diploid cell contains two copies of each chromosome (4n), forming sister chromatids that are hold together by the cohesin complex. As a diploid cell enters meiosis, pairs of sister chromatids from the homologous chromosomes are matched together and genetic material is exchanged by crossing over during prophase of meiosis I (prophase I). After synapsis formation and initiation of crossing over during prophase I, the homologous chromosomes are hold together by chiasmata during metaphase I and until the onset of anaphase I, when they are resolved and the homologous chromosomes separated. Sister chromatids for each homologous chromosome are subsequently separated in meiosis II in a process similar to mitosis (Ohkura, 2015).

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