Brief pause between meiosis i and meiosis ii in which no dna replication occurs.
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There are 2 parts to the cell cycle: interphase and mitosis/meiosis. Interphase can be further subdivided into Growth 1 (G1), Synthesis (S), and Growth 2 (G2). During the G phases, the cell grows by producing various proteins, and during the S phase, the DNA is replicated so that each chromosome contains two identical sister chromatids (c). Mitosis contains 4 phases: prophase, metaphase, anaphase, and telophase. Show
Mitosis
The cell can now enter Interphase where it grows and replicates its DNA in preparation for division, yet again. Meiosis goes through all 5 phases of the cell cycle twice, with modified mechanisms that ultimately create haploid cells instead of diploid. In sperm cells, the male gametes, meiosis proceeds in the following manner: Meiosis I
Interkinesis/Interphase II There is a brief pause between each round of meiosis providing time for the cell to replenish proteins; however, there is no S phase. Meiosis II
In egg cells, the female gametes, meiosis follows the same general phases with only a slight variation. During telophase I, the cytoplasm divides unequally, creating a larger daughter cell and a smaller polar body. The polar body and the daughter cell both then enter meiosis II. In telophase II, the cytoplasm of the daughter cell again divides unequally and creates a daughter cell and another polar body. In addition, the polar body from meiosis I divides and forms 2 smaller polar bodies. After meiosis is completed, there is one daughter cell (1n, 1c) and 3 polar bodies (1n 1c). The polar bodies disintegrate as they do not have enough cytoplasm and proteins to survive as gametes. Fundamentals of Human GeneticsMyron Yanoff MD, in Ophthalmology, 2019 Mitosis and MeiosisIn order for cells to divide, the entire DNA sequence must be copied so that each daughter cell can receive a complete complement of DNA. The growth phase of the 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 DNA sequence is copied by the enzyme DNA polymerase in a process called DNA replication. DNA polymerase is an enzyme capable of the synthesis of new strands of DNA using the exact sequence of the original DNA as a template. Once the DNA is copied, the old and new copies of the chromosomes form their respective pairs, and the cell divides such that one copy of each chromosome pair belongs to each cell (Fig. 1.1.4). Mitotic cell division produces a daughter cell that is an exact replica of the dividing cell. Meiotic cell division is a special type of cell division that results in a reduction of the genetic material in the daughter cells, which become 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 exchange of genetic material 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 second cell division occurs, and the doubled chromosomes divide, which results in daughter cells that have half the genetic material of somatic (tissue) cells. View chapter on ClinicalKey Human Genetics and Patterns of InheritanceRobert Resnik MD, in Creasy and Resnik's Maternal-Fetal Medicine: Principles and Practice, 2019 MeiosisGametes are derived from primordial germ cells specific to the ovary and testes. These primordial germ cells have 2n (46) chromosomes (diploid) but give rise to gametes, which have half that number, n (23) chromosomes (haploid). The process leading to this reduction division is termedmeiosis. Meiosis is divided into meiosis I and II. One important distinction is that total DNA goes from 4n to 2n during meiosis I and 2n to n during meiosis II. The configuration of DNA (e.g., tetrad and sister chromatids) represented by chromosomes is also unique (Table 1.1). There are characteristic phases (e.g., 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 exchange segments of DNA resulting in genetic variability among 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 short process (approximately 64 days), has its onset at puberty, and is continuous throughout a man's reproductive life. In females, oogenesis begins in utero but stops during prophase I and is dormant by 8 months' gestation (seeTable 1.1). This arrested state occurs during diplotene. The dormant stage, dictyotene, occurs only in oogenesis. Meiosis I resumes at puberty, and each month, another one or more oocytes (a function of follicular recruitment) resumes this reduction division. Meiosis I is completed at the time of ovulation (first polar body is formed), and meiosis II begins but is once again halted, this time during metaphase. Meiosis II is completed only if fertilization occurs (second polar body is formed). Fertilization most often takes place in the fallopian tube. View chapter on ClinicalKey MeiosisIn Cell Biology (Third Edition), 2017 Suppression of DNA Replication Between Meiosis I and Meiosis IIMeiosis 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). View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780323341264000451 Male Reproductive PhysiologyAlan W. Partin MD, PhD, in Campbell-Walsh-Wein Urology, 2021 MeiosisSomatic cells replicate by mitosis, in which genetically identical daughter cells are formed.Germ cells replicate by meiosis, in which the genetic material is halved to allow reproduction. Meiosis generates genetic diversity, providing a richer source of material on which natural selection 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 division of the chromosomes and cytoplasm into daughter cells.The essential difference between mitotic and meiotic replication is that a single DNA duplication step is followed by only one cell division in mitosis but two cell divisions in meiosis (four daughter cells). Consequently, daughter cells contain only half of the chromosome content of parent cells. Thus a diploid (2n) parent cell becomes a haploid(n) gamete. Other major differences between mitosis and meiosis are outlined inTable 64.2. Research has shown that small RNA molecules (small RNAs), including small interfering RNAs (siRNAs), microRNAs (miRNAs), and piwi-interacting RNAs (piRNAs), are important regulators of gene germ cell expression at the post-transcriptional or translation level (He et al., 2009;Tolia and Joshua-Tor, 2007). Spermatogenesis begins with type 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 division is followed by a typical mitotic reduction division, resulting in daughter cells with a haploid chromosome complement. In addition, as a consequence of chromosomal recombination, each daughter 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 real engine of genetic diversity and evolution. During meiotic prophase, formation of a synaptonemal complex with pairing of homologous (maternal and paternal) chromosomes occurs, along with physical interaction and exchange of DNA through reciprocal sites of crossing over(chiasmata) between homologs. Recent research has shown that defects in the fidelity of recombination within human male germ cells can cause azoospermia and male infertility (Walsh et al., 2009).In one study, 10% of nonobstructive azoospermic men had significant defects in recombination compared with men with normal spermatogenesis (Gonsalves et al., 2004). In addition, 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 also have 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 enough to explain the higher rate of chromosomal abnormalities in offspring conceived with in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) (Sun et al., 2008). View chapter on ClinicalKey MeiosisP.B. Moens, in Encyclopedia of Genetics, 2001 DefinitionMeiosis 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. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B0122270800008077 Differentiation of the OvaryAnne Grete Byskov, Lars Grabow Westergaard, in Fetal and Neonatal Physiology (Third Edition), 2004 MEIOTIC PROPHASE OF OOCYTESBy the end of the 10th week of gestation, the human ovarian cortex contains numerous mitotically dividing germ cells called oogonia. When the oogonia stop 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 by mitotic division from around 20,000 in the 6-week-old human fetus to around 250,000 in the 10th week,25 and it reaches a maximum of 7 million in the 20th week (Fig. 190-5).26 Certain genes and transcription factors are involved in multiplication of the oogonia, such as DAX-127 and expression of Kit protein.28,29 Extensive degeneration, however, reduces the number of germ cells drastically during the rest of fetal life, and around birth only 1 to 2 million oocytes are left.26,30 Process of MeiosisMeiosis begins with premeiotic DNA synthesis, followed by the transitory stages of the first meiotic prophase: preleptotene-leptotene-zygotene-pachytene and diplotene stages. When the diplotene stage is reached, the process is arrested (Fig. 190-6). 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 span. Oocyte degeneration may occur at any time during development. At resumption of meiosis, the first meiotic prophase is completed, and the oocyte proceeds through the first meiotic division, the reduction division, in which the homologous chromosomes are separated into the daughter cells, now each containing 1n chromosomes and 2 c (i.e., copy) DNA. The second meiotic division, which takes place by fertilization, is not preceded by the usual DNA synthesis, and the DNA content is therefore halved to 1 c. In the human, the meiotic divisions result in gametes with 23 chromosomes (1n) and 1 c DNA. Meiotic Prophase: Morphologic Features and Genetic ControlThe 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 as thin coiled threads, each thread consisting of identical sister chromosomes. The chromosomes coil 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. It seems that proper formation of the synaptonemal complexes ensures normal chromosome segregation in the reduction division 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 amount of exchanged gene material can be evaluated in the metaphase of the second meiotic division after labeling of premeiotic oogonia in S phase with bromine deoxyuridine.32 During the diplotene stage, the oocyte and its nucleus enlarge. The chromosomes decondense, thus making the nucleus faintly stainable. Only few genes with functions specific for the early meiotic processes are known. One of these is Dazla, which encodes for a protein with RNA-binding 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 4 c DNA content of the leptotene stage.38,39 This DNA must last until ovulation ceases, which in the human can occur up to 50 years later. In other mammalian species, it has been shown by DNA labeling that no neoformation of oocytes takes place later in life; that is, the entire pool of oogonia is transformed into oocytes early in life.40–43 However, there seems to be at least one exception to this rule, because DNA synthesis appears to occur in oogonia-like germ cells in the ovaries of adult prosimians (lower primates).44,45 In contrast to somatic cells, the DNA synthesis of germ cells is not strictly confined to the S phase. By flow cytometric sorting of rabbit oogonia and oocytes, it was found that DNA synthesis extends into the leptotene stage.46 Therefore, it seems that the premeiotic DNA synthesis is a part of the early meiotic process itself. In the early stages of fetal ovarian development, oogonia and oocytes are often interconnected by cytoplasmic bridges47 forming so-called germ cell cysts.48 These bridges may be important for exchange of intercellular signals and may explain why groups of germ cells divide synchronously and enter meiosis in clusters (see Fig. 190-6).49 Initiation and Regulation of MeiosisIn the human ovary, meiosis is initiated in the 11th week of fetal life.17 The first leptotene stages appear in the central part of the ovary, as 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,50 and by the time of birth, almost all oocytes are in the diplotene stage.51 The time of onset of meiosis in the female varies among different mammalian species but occurs always at an early stage of development, often in fetal life. In the testis, by contrast, 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 example, if the cells are located in the adrenal gland of the human fetus53 or if they are exposed to a meiosis-inducing substance before the testis is sex differentiated.13 Thus, it seems uncertain whether a germ cell clock governs the switch from mitosis to meiosis or it is triggered by paracrine or other external stimuli (see later). Genes that are specifically involved in transformation of the mitotically dividing oogonia into the meiotic direction are not yet identified. Meiosis-Regulatory SubstancesA characteristic feature of the developing mammalian ovary is that the germ cells first entering meiosis are those situated in the central part of the ovary,54 in close connection 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 mouse55 and hamster56 suggest that the mesonephros produces a substance that promotes and induces meiosis in male as well as in female germ cells. This substance was termed meiosis-inducing substance.57 Meiosis-inducing substance is produced by 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 become 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 critical factor for determining whether meiosis is induced, resumed, or prevented may be 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, because meiosis is induced or prevented in fetal mouse germ cells by spent culture media prepared from gonadal tissues of other species.13 The presence of Meiosis-inducing substance in gonads in which meiosis is initiated and its role in this process have been questioned. One study showed that fetal male mouse germ cells are “passively” entering meiosis when they are mixed and cultured with lung cells.59 Thus, as mentioned, the control of onset of meiosis may be built 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.60 Meiosis-activating sterols were extracted from gonads of different species in which meiosis is ongoing, such as human preovulatory follicular fluids and bull testicular tissue.61 It is uncertain at present whether meiosis-activating sterols are identical to the previously described meiosis-inducing substance. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780721696546501934 Why is DNA not replicated between meiosis 1 and 2?The event that occurs between meiosis I and meiosis II is called the second interphase or interphase II. In contrast with interphase I where DNA replication does occur, in interphase II DNA does not replicate. This is primarily because during interphase II no S-phase occurs.
Does DNA replication happen in meiosis II?During meiosis II, DNA replication does not occur again. During the cell cycle, cells go through S phase when they replicate their DNA. Then, the proceed through G2 where they make the final preparations for cell division.
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