Biological Sciences
Meiosis II
Meiosis II is the second stage of meiosis, a type of cell division that produces gametes. It follows meiosis I and consists of a single round of cell division, resulting in the separation of sister chromatids. This process ultimately produces four haploid daughter cells, each with half the number of chromosomes as the original parent cell.
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11 Key excerpts on "Meiosis II"
eBook - PDF- Julianne Zedalis, John Eggebrecht(Authors)
- 2018(Publication Date)
- Openstax(Publisher)
meiosis I is a reduction division all the cells of a multicellular organism except the gametes or reproductive cells haploid cell that can produce a haploid multicellular organism or can fuse with another spore to form a diploid cell a multicellular diploid life-cycle stage that produces haploid spores by meiosis formation of a close association between homologous chromosomes during prophase I protein lattice that forms between homologous chromosomes during prophase I, supporting crossover two duplicated homologous chromosomes (four chromatids) bound together by chiasmata during prophase I CHAPTER SUMMARY 11.1 The Process of Meiosis Sexual reproduction requires that diploid organisms produce haploid cells that can fuse during fertilization to form diploid offspring. As with mitosis, DNA replication occurs prior to meiosis during the S-phase of the cell cycle. Meiosis is a series of events that arrange and separate chromosomes and chromatids into daughter cells. During the interphases of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell. The first separates homologs, and the second—like mitosis—separates chromatids into individual chromosomes. During meiosis, variation in the daughter 462 Chapter 11 | Meiosis and Sexual Reproduction This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 nuclei is introduced because of crossover in prophase I and random alignment of tetrads at metaphase I. The cells that are produced by meiosis are genetically unique. Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic divisions are single nuclear divisions that produce daughter nuclei that are genetically identical and have the same number of chromosome sets as the original cell.
- Cecie Starr, Christine Evers, Lisa Starr, , Cecie Starr, Cecie Starr, Christine Evers, Lisa Starr(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
● ● Offspring of sexual reproducers inherit new combinations of parental alleles, so they differ from one another and from their parents. 9.6 Meiosis in Sexual Reproduction LEARNING OBJECTIVES ●●● ● Describe the relationship between germ cells and gametes. ●●● ● Explain why sexual reproduction requires meiosis. ●●● ● Describe crossing over and how it fosters diversity among the offspring of sexual reproducers. Most body cells are diploid (2n), with two copies of each chromosome—one inherited from each of two parents. A reproductive cell such as an egg or sperm is haploid (n), which means it has one copy of each chromosome. Meiosis is a nuclear division mechanism that halves the chromosome number, and it is essential to the formation of haploid cells. Stages of Meiosis The process of meiosis is similar to mitosis in several ways. A cell replicates its DNA before either nuclear division process begins, so each chromosome consists of two sister chromatids. As in mitosis, a spindle forms, and its microtubules move the chromosomes. However, meiosis sorts the chromosomes into new nuclei not once but twice. The two consecutive nuclear divisions are called meiosis I and Meiosis II. Figure 9.16 (next page) shows the stages of meiosis in a diploid (2n) cell. Meiosis I The first stage of meiosis I is prophase I. During this stage, the chromo- somes pack tightly, and homologous chromosomes align tightly and swap segments haploid Having one of each type of chromosome. meiosis Nuclear division process that halves the chromosome number. Basis of sexual reproduction. sexual reproduction Reproductive mode by which offspring arise from two parents and inherit genes from both. Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
eBook - PDF- Mary Ann Clark, Jung Choi, Matthew Douglas(Authors)
- 2018(Publication Date)
- Openstax(Publisher)
Therefore, sexual reproduction requires a nuclear division that reduces the number of chromosome sets by half. Most animals and plants and many unicellular organisms are diploid and therefore have two sets of chromosomes. In each somatic cell of the organism (all cells of a multicellular organism except the gametes or reproductive cells), the nucleus contains two copies of each chromosome, called homologous chromosomes. Homologous chromosomes are matched pairs containing the same genes in identical locations along their lengths. Diploid organisms inherit one copy of each homologous chromosome from each parent. Meiosis is the nuclear division that forms haploid cells from diploid cells, and it employs many of the same cellular mechanisms as mitosis. However, as you have learned, mitosis produces daughter cells whose nuclei are genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are at the same “ploidy level”—diploid in the case of most multicellular most animals. Plants use mitosis to grow as sporophytes, and to grow and produce eggs and sperm as gametophytes; so they use mitosis for both haploid and diploid cells (as well as for all other ploidies). In meiosis, the starting nucleus is always diploid and the daughter nuclei that result are haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome replication followed by two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Likewise, Meiosis II (during which the second round of meiotic division takes place) includes prophase II, prometaphase II, and so on.
eBook - ePub- Rene Fester Kratz(Author)
- 2017(Publication Date)
- For Dummies(Publisher)
Telophase I: This is when the cell takes a step back (or forward, depending on your perspective) to an interphase-like condition by reversing the events of prophase I. Specifically, the nuclear membrane reforms, the chromosomes uncoil and spread throughout the nucleus, the nucleoli reform, and the spindles break down.Meiosis II
During Meiosis II, both daughter cells produced by meiosis I continue their dance of division so that — in most cases — four gametes are the end result. The phases of Meiosis II look very similar to the phases of meiosis I with one big exception: The cells start out with half the number of chromosomes as the original parent cell.Meiosis II separates the sister chromatids of each replicated chromosome and sends them to opposite sides of the cell. Cells going from meiosis I to Meiosis II don’t go through interphase again (been there, done that).- Prophase II: Once again, the nuclear membrane disintegrates, the nucleoli disappear, and the spindles form and attach to the chromosomes.
- Metaphase II: Nothing too exciting here, folks. Just as in any old metaphase, the chromosomes line up at the equatorial plane. But remember that the number of chromosomes that lines up is half the number of the original parent cell (and half the number you’d see in mitosis).
- Anaphase II: The sister chromatids of each replicated chromosome move away from each other to opposite sides of the cell.
- Telophase II: The nuclear membrane and nucleoli reappear, the chromosomes stretch out for the briefest of rests, and the spindles disappear.
How Sexual Reproduction Creates Genetic Variation
Sexual reproduction increases genetic variation in offspring, which in turn increases the genetic variability in species. You can see the effects of this genetic variability if you look at the children in a large family and note how each person is unique. Imagine this kind of variability expanded to include all the families you know (not to mention all the families of all the sexually reproducing organisms on Earth), and you begin to get a feel for the dramatic genetic impact of sexual reproduction.
eBook - ePub- Laurie Ann Callihan, David Callihan(Authors)
- 2013(Publication Date)
- Research & Education Association(Publisher)
Cytokinesis differs somewhat in plants and animals. In animal cells, a ring made of the protein actin surrounds the center of the cell and contracts. As the actin ring contracts, it pinches the cytoplasm into two separate compartments. Each cell’s plasma membrane seals, making two distinct daughter cells. In plant cells, a cell plate forms across the center of the cell and extends out towards the edges of the cell. When this plate reaches the edges, a cell wall forms on either side of the plate, and the original cell then splits into two.Mitosis, then, produces two nearly identical daughter cells. (Cells may differ in distribution of mitochondria or because of DNA replication errors, for example.) Organisms (such as bacteria) that reproduce asexually, do so through the process of mitosis.MeiosisMeiosis is the process of producing four daughter cells, each with single unduplicated chromosomes (haploid) . The parent cell is diploid , that is, it has a normal set of paired chromosomes. Meiosis goes through a two-stage process resulting in four new cells, rather than two (as in mitosis). Each cell has half the chromosomes of the parent. Meiosis occurs in reproductive organs, and the resultant four haploid cells are called gametes (egg and sperm). When two haploid gametes fuse during the process of fertilization, the resultant cell has one chromosome set from each parent, and is diploid. This process allows for the huge genetic diversity available among species.Two distinct nuclear divisions occur during meiosis, reduction (or meiosis 1, steps 1 to 5 in Fig. 3-14 ), and division (or meiosis 2, steps 6 to 10 ). Reduction affects the ploidy (referring to haploid or diploid) level, reducing it from 2n to n (i.e., diploid to haploid). Division then distributes the remaining set of chromosomes in a mitosis-like process.Fig. 3-14 Meiosis. See explanations of numbered steps in text.The phases of meiosis 1 are similar to the phases of mitosis, with some notable differences. As in mitosis, chromosome replication (1 ) occurs before prophase; then during prophase 1 (2 ), homologous chromosomes pair up and join at a point called a synapse (this happens only in meiosis). The attached chromosomes are now termed a tetrad a dense four-stranded structure composed of the four chromatids from the original chromosomes. At this point, some portions of the chromatid may break off and reattach to another chromatid in the tetrad. This process, known as crossing over
eBook - PDF- D. Peter Snustad, Michael J. Simmons(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
In some species, chromosome decondensation is incomplete, the daughter nuclei do not form, and the daughter cells proceed immediately into the second meiotic division. The cells produced by meiosis I contain the haploid number of chromosomes; however, each chromosome still consists of two sister chromatids, which may not be genetically identical because they might have exchanged material with their pairing partners during prophase I. Meiosis II AND THE OUTCOMES OF MEIOSIS During Meiosis II, the chromosomes condense and become attached to a new spindle apparatus (prophase II). They then move to posi- tions in the equatorial plane of the cell (metaphase II), and their centromeres split to allow the constituent sister chromatids to move to opposite poles (anaphase II), a phenomenon called chromatid dis- junction. During telophase II, the separated chromatids—now called chromosomes—gather at the poles and daughter nuclei form around them. Each daughter nucleus contains a haploid set of chromosomes. Mechanistically, Meiosis II is therefore much like mitosis. However, its products are haploid, and unlike the products of mitosis, the cells that emerge from Meiosis II are not genetically identical. One reason these cells differ is that homologous chromosomes pair and disjoin from each other during meiosis I. Within each pair of chromosomes, one homologue was inherited from the organism’s mother, and the other was inherited from its father. During meiosis I, the maternally and paternally inherited homologues come together and synapse. They are positioned on the meiotic spindle and become oriented ran- domly with respect to the spindle’s poles. Then they disjoin. For each pair of chro- mosomes, half the daughter cells produced by the first meiotic division receive the maternally inherited homologue, and the other half receive the paternally inherited homologue. Thus, from the end of the first meiotic division, the products of meiosis are destined to be different.
eBook - PDF- Karen Sermon, Stéphane Viville(Authors)
- 2014(Publication Date)
- Cambridge University Press(Publisher)
Chapter 3 Meiosis How to get a good start in life Ursula Eichenlaub-Ritter Introduction Meiosis comprises fundamental processes that permit sexual reproduction and species evolution. Indeed, in addition to producing haploid gametes, it provides a stochastic distribution of maternally and paternally inherited chromosomes, which undergo allelic recom- bination. Thus it generates diversity within the pop- ulation, and is essential for the formation of euploid germ cells that will contribute to a euploid, healthy embryo after fertilization. Meiosis is therefore the basis for maintaining genomic integrity, high developmen- tal potential, and health of the embryo and offspring, and normal fertility in males and females [1]. This chapter will introduce the principles underlying the meiotic process including pairing and recombination, chromosome segregation at meiosis I and Meiosis II, and formation of oocytes and sperm that will form a euploid embryo after fertilization. Since many of the genes in meiosis are highly conserved between species, the chapter will also give a brief overview on exper- imental studies. Sexual dimorphism that contributes to gender-specific predisposition to disturbances is briefly discussed. This may increase risks for spe- cific chromosomal aberrations in oogenesis or sper- matogenesis (for instance, aneuploidy, predivision, or genomic instability), as well as meiotic delay, arrest, or cell death that cause subfertility or infertility. Principles of chromosome segregation at meiosis During mitosis of a diploid cell there is typically one round of replication of all chromosomes during the S phase. The separation of chromatids at mitotic divi- sion results therefore in the formation of two diploid daughter cells, each containing a set of originally maternally and paternally derived chromosomes (see Chapter 1 and Fig. 3.1A).
eBook - PDF- Rene Fester Kratz(Author)
- 2022(Publication Date)
- For Dummies(Publisher)
Just remember that the key difference between the phases of mitosis and meiosis is what happens to the number of chromosomes. Table 5-1 can help you sort out the important differences at a glance. Q. A cell that has a diploid number of four chromosomes is undergoing cell division, as shown in Figure 5-10. Based on the information in the figure, which phase or phases of mitosis or meiosis are possible? A. The figure shows sister chromatids from four chromosomes moving away from each other. The diploid number of chromosomes for this cell is four, so the cell in Figure 5-10 has the full number of chromosomes. This means you can rule out Meiosis II entirely (cells in Meiosis II would already have half the diploid number of chromosomes). If a diploid cell with four chromosomes were in meiosis I, then you’d expect that the four chromosomes would have formed two pairs of homologous chromosomes and that Table 5-1 A Comparison of Mitosis and Meiosis Mitosis Meiosis One division is all that’s necessary to complete the process. Two separate divisions are necessary to complete the process. Chromosomes don’t get together in pairs (synapse). Homologous chromosomes synapse in prophase I. Homologous chromosomes don’t cross over. Crossing-over is an important part of meiosis and one that leads to genetic variation. Sister chromatids separate in anaphase. Sister chromatids separate only in anaphase II, not anaphase I. (Homologous chromosomes separate in anaphase I.) Daughter cells have the same number of chromosomes as their parent cells, meaning they’re diploid. Daughter cells have half the number of chromosomes as their parent cells, meaning they’re haploid. Daughter cells have genetic information that’s identical to that of their parent cells. Daughter cells are genetically different from their parent cells. The function of mitosis is asexual reproduction in some organisms. In many organisms, mitosis functions as a means of growth, replacement of dead cells, and damage repair.
eBook - PDF- Peter Moens(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
IV Chemistry of Meiosis This page intentionally left blank 10 The Biochemistry of Meiosis HERBERT STERN AND YASUO HOTTA Department of Biology University of California, San Diego La Jolla, California 92093 I. INTRODUCTION Biochemical or molecular events directly relevant to meiosis begin no later than the premeiotic S phase and terminate on completion of the second meiotic division. Many molecular events that occur during meiosis are not relevant to the process but are essential to gametogenesis. This is particularly prominent in animal meiocytes, and much less so in those of plants where meiosis is followed by a haploid generation in which gametogenesis occurs. A major difficulty in identifying those metabolic events in a prophase spermatocyte that are concerned with meiosis is the large fraction of events that function in spermiogenesis. Although discussions of gametogenesis almost always include the meiotic pro- cess, it must be understood that the only necessary relation between them is that meiosis must occur at some time following zygote formation and preceding gametogenesis. In some organisms—yeast, for example—many haploid cell generations may intervene between meiosis and the conversion of cells into the equivalent of gametes. Meiosis is primarily and necessarily involved in the reductional division of chromosome sets. Closely, but not necessarily, associated with it is a limited crossing-over between homologous chromosomes. Underlying both events is a pairing of homologous chromosomes. Such pairing is indispensable to a normal reductional division and it is also highly important for the occurrence of recom- bination. Pairing without crossing-over, however, may and does occur, for ex- ample, in male Drosophila and female Bombyx. In meiocytes, where crossing- over is a normal event of division, failure of any bivalent to undergo at least one 303 MEIOSIS Copyright © 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.
eBook - PDF- William V Dashek, Marcia Harrison(Authors)
- 2010(Publication Date)
- CRC Press(Publisher)
CHAPTER 9 Meiosis in Plants Renata Sniezko INTRODUCTION: MEIOSIS IN LIFE CYCLE OF PLANTS WITH DIFFERENT PHYLOGENETIC POSITIONS Alternation of Generations During billions of years of evolution plants have developed a specific mode of living as autotrophic organisms distributed in differ- ent environments in both water and on land. They have changed their morphologies from unicellular algae to multicellular flow- ering organisms and modified their reproductive processes. Relatively early, they developed sexual reproduction, which is associated with meiosis, a process of two division cycles with accompanying recom- bination of genetic material. After meiosis four haploid daughter cells arise from one diploid cell. Each daughter cell exhibits little change in gene arrangement comparised to the previous generation. There is a difference between the sexual reproduction of plants and animals. In animals, meiosis occurs within a sexual gland producing gametes, so meiosis is directly connected with sexual reproduc- tion in every generation. In plants, meiosis also occurs in diploid cells, but results in spores, which develop into a haploid gen- eration. As a result of evolution, plants undergo alternation of generations in their life cycle with meiosis occurring between the diploid and haploid phases (Fig. 9.1A). The haploid generation is called the gametophyte because it produces gametes, cells capable of fusing during fertilization. The latter leads to the diploid generation which produces spores after meiosis. These postmeiotic spores develop into the haploid generation. This scheme of haplo- and diplophase alternation is differently real- ized depending on the systematic level. In the life cycle, the haplophase or diplophase can be the state in which the plant lives longer and develops its vegetative organs. The proportion between haplo- and diplo- phase is dependent upon systematic position and the environment.
eBook - PDF- David Jagiello(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
T h e principal purpose of meiosis is to effect chromosome reduction, by w h i c h " t h e diploid or so-called 'somatic' n u m b e r must b e r e d u c e d by half to the haploid or gametic. . . . Re- duction results from a regrouping of the chromosomes of the diploid group a n d their segregation into two single or haploid groups corre- sponding in a general way to those that originally came together in the e g g " (1). At the time of Wilson's writing, "the main facts (seemed) well established," but " m a n y intricate and difficult questions of de- tail . . . are still matters of controversy." Many, if not most, of these questions remain unsettled today. T h r e e central events characterize meiosis: pairing, crossing over, a n d distribution (separation) of parental chromosomes. Pairing (two- by-two association) may b e regarded as a general property of chromo- somes, most often restricted to homologs, b u t not infrequently ex- pressed by nonhomologous chromosomes and chromosomal regions. T h o u g h pairing is usually thought of as a meiotic p h e n o m e n o n , it is also expressed to various extents in somatic chromosomes (the poly- tene "bivalent" of dipteran salivary glands and other tissues is the most familiar, if extreme, example). In meiosis, pairing in some form is an absolute, though not sufficient, r e q u i r e m e n t for equal segregation of maternal and paternal chromosomes, and their distribution to daughter gametocytes. 187 BIOREGULATORS OF REPRODUCTION Copyright © 1981 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-379980-5 188 MONTROSE J. MOSES S Y N A P S I S A N D T H E S Y N A P T O N E M A L C O M P L E X * Synapsis is a special form of pairing in which chromosomes are closely apposed, so intimately that the bivalent thus formed often ap- pears as a single structure.
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