Meiosis

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

From: Encyclopedia of Genetics , 2001

Fundamentals of Human Genetics

Myron Yanoff Dr. , in Ophthalmology , 2019

Mitosis and Meiosis

In order for cells to split, the entire Deoxyribonucleic acid sequence must be copied and then that each daughter jail cell tin receive a complete complement of DNA. The growth stage of the prison cell cycle terminates with the separation of the two sister chromatids of each chromosome, and the cell divides during mitosis. Before cell division, the complete Deoxyribonucleic acid sequence is copied by the enzyme DNA polymerase in a procedure called DNA replication. Deoxyribonucleic acid polymerase is an enzyme capable of the synthesis of new strands of Dna using the exact sequence of the original Dna as a template. In one case the Dna is copied, the sometime and new copies of the chromosomes form their respective pairs, and the prison cell divides such that one copy of each chromosome pair belongs to each cell (Fig. 1.1.4). Mitotic jail cell division produces a daughter cell that is an exact replica of the dividing prison cell.

Meiotic prison cell partition is a special type of prison cell division that results in a reduction of the genetic cloth in the daughter cells, which go the reproductive cells—eggs (women) and sperm (men). Meiosis begins with Dna replication, followed by a pairing of the maternal and paternal chromosomes (homologous pairing) and an substitution of genetic textile between chromosomes by recombination ( Fig. 1.1.5). The homologous chromosome pairs line up on the microtubule spindle and divide such that the maternal and paternal copies of the doubled chromosomes are distributed to separate daughter cells. A 2nd cell sectionalisation occurs, and the doubled chromosomes separate, which results in daughter cells that take half the genetic textile of somatic (tissue) cells.

Meiosis

M.D. Griswold , P.A. Chase , in Brenner'due south Encyclopedia of Genetics (2nd Edition), 2013

Abstract

Meiosis is a feature of sexual reproduction that results in the independent assortment of genetic material from 2 individuals, providing greater genetic variety. 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 sectionalisation, 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 amidst species and even between sexes in the aforementioned species (e.g., in humans, the timing of meiosis differs between males and females), overall the procedure is highly conserved.

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Human Genetics and Patterns of Inheritance

Robert Resnik Md , in Creasy and Resnik's Maternal-Fetal Medicine: Principles and Exercise , 2019

Meiosis

Gametes are derived from primordial germ cells specific to the ovary and testes. These primordial germ cells have 2n (46) chromosomes (diploid) merely give rise to gametes, which have one-half that number, n (23) chromosomes (haploid). The process leading to this reduction division is termed meiosis . Meiosis is divided into meiosis I and Two. Ane important distinction is that total Deoxyribonucleic acid goes from 4n to 2n during meiosis I and 2n to northward during meiosis II. The configuration of Deoxyribonucleic acid (e.g., tetrad and sister chromatids) represented by chromosomes is also unique (Table one.1). There are feature phases (east.m., prophase, metaphase, anaphase, and telophase) within meiosis I and II. Prophase of meiosis I has five distinct stages (leptotene, zygotene, pachytene, diplotene, and diakinesis). During zygotene, homologous chromosomes (maternal and paternal chromosomes) align at the synaptonemal complex giving way to a bivalent (two homologous chromosomes) tetrad (each chromosome has two sister chromatids). Homologous recombination occurs during pachytene, when sister chromatids of maternal and paternal homologs substitution segments of Dna resulting in genetic variability amidst offspring from the same parents.

An important distinction between male and female gamete development is the time in life that meiosis is initiated and the time course to completion. In males, this is a brusk process (approximately 64 days), has its onset at puberty, and is continuous throughout a human being's reproductive life. In females, oogenesis begins in utero simply stops during prophase I and is dormant past viii months' gestation (come acrossTable 1.1). This arrested country occurs during diplotene. The dormant stage, dictyotene, occurs merely in oogenesis. Meiosis I resumes at puberty, and each calendar month, another 1 or more than oocytes (a function of follicular recruitment) resumes this reduction partitioning. Meiosis I is completed at the fourth dimension of ovulation (outset polar trunk is formed), and meiosis II begins but is once again halted, this time during metaphase. Meiosis II is completed simply if fertilization occurs (2d polar torso is formed). Fertilization about frequently takes place in the fallopian tube.

Meiosis

In Cell Biology (3rd Edition), 2017

Suppression of DNA Replication Betwixt Meiosis I and Meiosis II

Meiosis is unique in that information technology involves two Thou phases with no intervening S stage. On go out from meiosis I, Cdk1 kinase is reactivated immediately. This blocks assembly of prereplication complexes (come across Fig. 42.8), thereby blocking DNA replication. At to the lowest degree ii 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 past phosphorylation at Tyrxv. The absence of Wee1 in meiosis I was showtime observed in Xenopus laevis but this seems to be a universally conserved way of reactivating Cdk1 without an Southward phase. Ectopic expression of Wee1 in mature Ten. laevis oocytes prevents reactivation of Cdk1 immediately after the meiosis I division. As a result, the oocytes reenter interphase and replicate their Deoxyribonucleic acid. Meiotic cells likewise express a specialized isoform of Cdc25, the phosphatase that counteracts Wee1 (see Fig. 43.1).

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Male Reproductive Physiology

Alan W. Partin Md, PhD , in Campbell-Walsh-Wein Urology , 2021

Meiosis

Somatic cells replicate by mitosis, in which genetically identical daughter cells are formed.Germ cells replicate past meiosis, in which the genetic fabric is halved to allow reproduction. Meiosis generates genetic multifariousness, providing a richer source of material on which natural choice can act. Cell replication by mitosis is a precise, well-orchestrated sequence of events involving duplication of the genetic material (chromosomes), breakdown of the nuclear envelope, and equal partition of the chromosomes and cytoplasm into girl cells.The essential divergence between mitotic and meiotic replication is that a single Deoxyribonucleic acid duplication pace is followed by but 1 cell division in mitosis but two jail cell divisions in meiosis (4 daughter cells). Consequently, daughter cells contain merely half of the chromosome content of parent cells. Thus a diploid (2n) parent prison cell becomes a haploid(n) gamete. Other major differences betwixt mitosis and meiosis are outlined inTabular array 64.ii. Research has shown that small RNA molecules (pocket-sized RNAs), including small interfering RNAs (siRNAs), microRNAs (miRNAs), and piwi-interacting RNAs (piRNAs), are of import regulators of gene germ jail cell expression at the post-transcriptional or translation level (He et al., 2009;Tolia and Joshua-Tor, 2007).

Spermatogenesis begins with blazon B spermatogonia dividing mitotically to form primary spermatocytes within the adluminal compartment.Primary spermatocytes are the first germ cells to undergo meiosis (Kerr and de Kretser, 1981). In this process, a meiotic partitioning is followed by a typical mitotic reduction division, resulting in daughter cells with a haploid chromosome complement. In improver, as a consequence of chromosomal recombination, each daughter prison cell contains different genetic information. The resultant cell is the Sa spermatid (seeFig. 64.11).

Chromosomal recombination, the defining feature of mammalian meiosis, ensures that haploid gametes differ genetically from their adult precursors and is the existent engine of genetic diverseness and evolution. During meiotic prophase, formation of a synaptonemal complex with pairing of homologous (maternal and paternal) chromosomes occurs, along with concrete interaction and exchange of Deoxyribonucleic acid through reciprocal sites of crossing over(chiasmata) between homologs. Recent enquiry has shown that defects in the allegiance of recombination within human male germ cells can crusade azoospermia and male infertility (Walsh et al., 2009).In one written report, x% of nonobstructive azoospermic men had significant defects in recombination compared with men with normal spermatogenesis (Gonsalves et al., 2004). In improver, among men with maturation arrest pattern on testis biopsy, faulty recombination was observed in about half of cases, providing evidence that faulty recombination is linked to poor sperm production (Gonsalves et al., 2004). Variations in recombination likewise take implications for sperm aneuploidy, because alterations in crossover position are risk factors for chromosomal nondisjunction.Indeed, evidence suggests that the correlation of faulty recombination and sperm aneuploidy in azoospermic men is strong plenty to explain the higher charge per unit of chromosomal abnormalities in offspring conceived with in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) (Dominicus et al., 2008).

Meiosis

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

Abstract

Meiosis is the specialized type of jail cell partition by which sexual organisms produce gametes. In most organisms, meiosis produces haploid gametes from diploid forerunner 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. Every bit prerequisites for their segregation, homologs go intimately associated and connected by structures called chiasmata. Homolog pairing and formation of chiasmata are both mediated past the Dna repair procedure called homologous recombination.

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Meiosis

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

Definition

Meiosis is divers as the cellular and nuclear processes that reduce the chromosomal content per nucleus from two sets to one set. In about organisms, 2 sets of chromosomes (diploid) are reduced to one prepare (haploid) (see Chromosome Pairing, Synapsis). When the haploid cell becomes involved in the process of fertilization, information technology is referred to as a 'gamete.' If a cell with i set of chromosomes goes on to proliferate, information technology 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 detail 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 being Genetics∗

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

Meiosis

Meiosis consists of one circular of Deoxyribonucleic acid 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 (i) homologous chromosomes pair at prophase of meiosis I; (2) genetic recombination, called meiotic crossing over, occurs regularly at prophase of meiosis I; and (three) the chromosome number is reduced to half later meiosis I, so that the girl cells resulting from meiosis I are haploid (23 chromosomes) (Fig. 16.11).

Effigy 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 likewise 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 practical to safeguarding human health, fertility, and offspring well-being.

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Differentiation of the Ovary

Anne Grete Byskov , Lars Grabow Westergaard , in Fetal and Neonatal Physiology (Third Edition), 2004

MEIOTIC PROPHASE OF OOCYTES

By the end of the 10th calendar week of gestation, the human ovarian cortex contains numerous mitotically dividing germ cells chosen oogonia . When the oogonia finish dividing and enter meiosis, they are termed oocytes. The first oocytes in the human ovary are seen during the 11th week of fetal life. Transformation of an oogonium into an oocyte and then into an ovum with a haploid number of chromosomes denotes oogenesis.

The number of oogonia increases exponentially past mitotic division from around 20,000 in the 6-calendar week-erstwhile man fetus to around 250,000 in the 10th calendar week, 25 and it reaches a maximum of 7 million in the 20th week (Fig. 190-five). 26 Certain genes and transcription factors are involved in multiplication of the oogonia, such as DAX-one 27 and expression of Kit poly peptide. 28,29 Extensive degeneration, however, reduces the number of germ cells drastically during the rest of fetal life, and around nascency merely ane to ii million oocytes are left. 26,30

Process of Meiosis

Meiosis begins with premeiotic DNA synthesis, followed by the transitory stages of the outset meiotic prophase: preleptotene-leptotene-zygotene-pachytene and diplotene stages. When the diplotene phase is reached, the process is arrested (Fig. 190-half dozen). During this first meiotic prophase, the maternal and paternal genes are exchanged between the homologous chromosomes. The diplotene stage, sometimes named the dictyate stage, is maintained until the oocyte resumes meiosis (just before ovulation) or degenerates and may thus last for the whole fertile life bridge. Oocyte degeneration may occur at any time during evolution. At resumption of meiosis, the first meiotic prophase is completed, and the oocyte gain through the first meiotic division, the reduction sectionalization, in which the homologous chromosomes are separated into the daughter cells, at present each containing 1n chromosomes and 2 c (i.e., copy) DNA. The second meiotic sectionalisation, which takes place by fertilization, is not preceded by the usual Dna synthesis, and the Deoxyribonucleic acid content is therefore halved to i c. In the human, the meiotic divisions result in gametes with 23 chromosomes (1n) and 1 c DNA.

Meiotic Prophase: Morphologic Features and Genetic Control

The different transitory stages of meiosis are recognized by the configuration of the chromosomes. In the preleptotene and leptotene stages, the chromosomes become condensed and are seen equally sparse coiled threads, each thread consisting of identical sister chromosomes. The chromosomes curlicue increasingly during progression through the zygotene to pachytene stages. The synaptonemal complexes begin to form in the zygotene and are completed in the pachytene stage. These structures represent the pairing of homologous chromatids. Information technology seems that proper formation of the synaptonemal complexes ensures normal chromosome segregation in the reduction partition of meiosis. 31 Passing into the diplotene stage, the chromosomes decoil, and chiasmata or crossing-over may be recognized. At this stage, the paternal and maternal chromatids exchange gene material. The corporeality of exchanged gene material tin be evaluated in the metaphase of the second meiotic partition after labeling of premeiotic oogonia in S stage with bromine deoxyuridine. 32 During the diplotene stage, the oocyte and its nucleus enlarge. The chromosomes decondense, thus making the nucleus faintly stainable.

Merely few genes with functions specific for the early on meiotic processes are known. One of these is Dazla, which encodes for a protein with RNA-binding chemical element and is found in human oogonia. 33 Dazla-deficient mice are infertile because the oocytes are lost when passing through meiosis. 34 Other genes are needed for the structural organization of chromosomes during assembly of the synaptonemal complexes and recombination, Scp1-3. 35–37

As discussed earlier, meiosis begins with premeiotic DNA synthesis and ultimately leads to the four c Deoxyribonucleic acid content of the leptotene phase. 38,39 This Dna must last until ovulation ceases, which in the man can occur up to 50 years later. In other mammalian species, information technology has been shown by Deoxyribonucleic acid labeling that no neoformation of oocytes takes place afterwards in life; that is, the entire puddle of oogonia is transformed into oocytes early in life. 40–43 However, in that location seems to be at least 1 exception to this rule, considering DNA synthesis appears to occur in oogonia-similar germ cells in the ovaries of developed prosimians (lower primates). 44,45

In dissimilarity to somatic cells, the Dna synthesis of germ cells is non strictly confined to the South stage. By flow cytometric sorting of rabbit oogonia and oocytes, it was found that Deoxyribonucleic acid synthesis extends into the leptotene stage. 46 Therefore, it seems that the premeiotic DNA synthesis is a role of the early meiotic process itself.

In the early stages of fetal ovarian evolution, oogonia and oocytes are oftentimes interconnected past cytoplasmic bridges 47 forming so-called germ cell cysts. 48 These bridges may be of import for exchange of intercellular signals and may explain why groups of germ cells divide synchronously and enter meiosis in clusters (run into Fig. 190-6). 49

Initiation and Regulation of Meiosis

In the homo ovary, meiosis is initiated in the 11th calendar week of fetal life. 17 The beginning leptotene stages appear in the central part of the ovary, every bit in ovaries of all other mammalian species. Gradually, meiosis proceeds toward the periphery of the cortex. Around the 32nd week of fetal life, all oogonia have been transformed into oocytes, fifty and by the time of birth, about all oocytes are in the diplotene stage. 51

The time of onset of meiosis in the female varies among dissimilar mammalian species but occurs always at an early phase of evolution, oftentimes in fetal life. In the testis, by dissimilarity, initiation of meiosis is postponed until puberty in all species. These discrepancies have led to the hypothesis that an internal clock of the germ cells determines the onset of meiosis, independent of external factors. 52 However, the male germ cells can be induced to enter meiosis prematurely, for case, if the cells are located in the adrenal gland of the man fetus 53 or if they are exposed to a meiosis-inducing substance before the testis is sex activity differentiated. thirteen Thus, information technology seems uncertain whether a germ cell clock governs the switch from mitosis to meiosis or it is triggered past paracrine or other external stimuli (encounter later). Genes that are specifically involved in transformation of the mitotically dividing oogonia into the meiotic direction are non yet identified.

Meiosis-Regulatory Substances

A characteristic feature of the developing mammalian ovary is that the germ cells showtime inbound meiosis are those situated in the central part of the ovary, 54 in close connectedness with the mesonephric-derived rete ovarii. 21 It has therefore been proposed that the mesonephric-derived cells in the ovary trigger the onset of meiosis. 21 In vitro experiments with gonads of fetal mouse 55 and hamster 56 propose that the mesonephros produces a substance that promotes and induces meiosis in male person likewise as in female germ cells. This substance was termed meiosis-inducing substance. 57 Meiosis-inducing substance is produced past gonadal tissues—ovaries and testes—in which meiosis is ongoing, including preovulatory human follicles. 58 In contrast to the developing ovary, meiosis of prespermatogonia in the differentiating testis is prevented when they go enclosed in testicular cords. It was proposed that meiosis-preventing substance within the testicular cords is responsible for meiotic prevention: meiosis-inducing substance and meiosis-preventing substance interact in the control of the meiotic process. The disquisitional factor for determining whether meiosis is induced, resumed, or prevented may exist the ratio between meiosis-inducing substance and meiosis-preventing substance. 13

Meiosis-inducing substance or meiosis-preventing substance activity has been detected in fetal and adult gonadal tissues collected from numerous mammalian species (Table 190-1). Meiosis-inducing substance and meiosis-preventing substance are not species specific, considering meiosis is induced or prevented in fetal mouse germ cells by spent culture media prepared from gonadal tissues of other species. thirteen

The presence of Meiosis-inducing substance in gonads in which meiosis is initiated and its office in this process have been questioned. One report showed that fetal male mouse germ cells are "passively" inbound meiosis when they are mixed and cultured with lung cells. 59 Thus, as mentioned, the control of onset of meiosis may be congenital into the germ cell lines, in females as well as in males, but meiosis is prevented in the male germline (meiosis-preventing substance?) by enclosure in testicular cords.

The chemical structures of meiosis-activating sterols, which are able to induce resumption of meiosis in mammalian oocytes, have been identified. sixty Meiosis-activating sterols were extracted from gonads of dissimilar species in which meiosis is ongoing, such as homo preovulatory follicular fluids and bull testicular tissue. 61 Information technology is uncertain at present whether meiosis-activating sterols are identical to the previously described meiosis-inducing substance.

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