Biological Sciences
Meiosis I
Meiosis I is the first stage of meiosis, a type of cell division that produces gametes (sperm and egg cells). During Meiosis I, a diploid cell divides into two haploid cells through a series of stages including prophase I, metaphase I, anaphase I, and telophase I. This process involves the pairing and exchange of genetic material between homologous chromosomes, leading to genetic diversity in the resulting gametes.
Written by Perlego with AI-assistance
Related key terms
1 of 5
10 Key excerpts on "Meiosis I"
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.
eBook - PDF- D. Peter Snustad, Michael J. Simmons(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
How Many Chromosome Combinations in Sperm? Solve It! 32 Chapter 2 Cellular Reproduction Another reason the cells that emerge from meiosis differ is that during Meiosis I, homologous chromosomes exchange material by crossing over. This process can create countless different combinations of genes. When we superimpose the variabil- ity created by crossing over on the variability created by the random disjunction of homologues, it is easy to see that no two products of meiosis are likely to be the same. KEY POINTS Diploid eukaryotic cells form haploid cells by meiosis, a process involving one round of chromo- some duplication followed by two cell divisions (Meiosis I and Meiosis II). During Meiosis I, homologous chromosomes pair (synapse), exchange material (cross over), and separate (disjoin) from each other. During Meiosis II, chromatids disjoin from each other. When genetics began, the organisms that were used for research were the ones that came to hand from the garden or the barnyard. Some early geneticists branched out to study inheritance in other types of creatures—moths and canaries, for example—and as genetics pro- gressed, research became focused on organisms that were well suited for controlled experimentation in laboratories or field plots. Today a select group of microorganisms, plants, and animals are favored in genetic research. These creatures, often called model organisms, lend themselves well to genetic analysis. For the most part, they are easily cultured in the laboratory, their life cycles are relatively short, and they are genetically variable. In addition, through work over many years, geneti- cists have established large collections of mutant strains for these organisms. We will encounter the model genetic organisms many times in this book. Table 2.1 summarizes information about several of them, and in the sections that follow, we discuss the life cycles of three of these genetically important species.
eBook - PDF- Peter Moens(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
In a microscope preparation of live spermatocytes, metaphase I stage suddenly ends as the separating chromosomes move simultaneously and rapidly (1 1. Introduction to Meiosis 5 /min) to the poles of the cell. This first meiotic anaphase (Fig. lg, h) is the cardinal moment in the meiocyte when the mixture of two genomes is sorted out into two single genomes. Each genome contains a complete set of chromosomes but the two sets are different in genetic detail. If, for example, the metaphase I bivalents of Fig. If divide into the anaphase I of Fig. Ih, then the distribution of thick and thin lines represents the mix of original maternal and paternal contribu- tions in the two separating genomes. No two spermatocytes will have the same mix. The thoroughness of the mix is generated from two sources, the random orientation of bivalents at the metaphase plate in regard to parental origin and the exchange between non-sister chromatids. Since the chromosomes were duplicated before they paired at meiotic pro- phase, the anaphase I chromosomes can enter the second metaphase of Meiosis Immediately, without an intervening duplicating stage (Fig. li). Metaphase II is followed by a second meiotic anaphase which reduces the single genomes of duplicated chromosomes to single genomes of single DNA content (Fig. lj). The remarkable similarity of the meiotic process, even between biological kingdoms, is evident from a comparison of Meiosis In a grasshopper (Fig. 1) with Meiosis In the Easter lily, Lilium longiflorum (Fig. 2). In the lily two sets of 12 chromosomes pair during meiotic prophase producing the pachytene stage of meiosis with an intractable tangle of long bivalents (Fig. 2a). Much of the biochemistry of meiosis discussed in Chapter 10 was done with these chromo- somes. When the 12 bivalents shorten during prophase, they become indi- vidually recognizable (Fig. 2b). The diplotene bivalents obviously resemble, in chromatid and chiasma structure, those of the grasshopper in Fig.
eBook - PDF- Donald Voet, Judith G. Voet(Authors)
- 2023(Publication Date)
- Wiley(Publisher)
b. Germ Cells Are Formed by Meiosis The formation of germ cells, a process known as meiosis (Fig. 1-22), requires two consecutive cell divisions. Before the first meiotic division each chromosome replicates, but the resulting sister chromatids remain attached at their cen- tromere. The homologous pairs of the doubled chromosomes then line up across the equatorial plane of the cell in zipper- like fashion, which permits an exchange of the corresponding sections of homologous chromosomes in a process known as crossing-over. The spindle then moves the members of Figure 1-20 Chromosomes. A photomicrograph of a plant cell (Scadoxus katherinae Bak.) during anaphase of mitosis showing its chromosomes being pulled to opposite poles of the cell by the mitotic spindle. The microtubules forming the mitotic spindle are stained red and the chromosomes are blue. [Courtesy of Andrew S. Bajer, University of Oregon.] Figure 1-21 Mitosis, the usual form of cell division in eukaryotes. Mitosis yields two daughter cells, each with the same chromosomal complement as the parental cell. Section 1-4 Genetics: An Overview 25 each homologous pair to opposite poles of the cell so that, after the first meiotic division, each daughter cell contains N doubled chromosomes. In the second meiotic division, the sister chromatids separate to form chromosomes and move to opposite poles of the dividing cell to yield a total of four haploid cells that are known as gametes. Fertilization consists of the fusion of a male gamete (sperm) with a female gam- ete (ovum) to yield a diploid cell known as a zygote that has received N chromosomes from each of its parents. B. Mendelian Inheritance The basic laws of inheritance were reported in 1866 by Gregor Mendel.
eBook - ePub- Julia E. Richards, R. Scott Hawley(Authors)
- 2010(Publication Date)
- Academic Press(Publisher)
So first, let’s take a pictorial overview of meiosis (Figure 6.15) in terms of where the chromosomes are and how they get moved around and a description of those steps (Box 6.1). Once we have seen how the chromosomes get moved around, then we can move on to discuss more details about some of the critical steps, especially steps 2 and 3, first meiotic prophase and first meiotic metaphase. Figure 6.15 Meiosis starts with cells containing two copies of each chromosome. After two rounds of meiosis, the four resulting sperm each contain only one copy of each chromosome. Production of an egg similarly reduces the copy number from two to one, but as we saw in Figure 6.13, the product of female Meiosis Is a single egg with one copy of each chromosome plus three tiny discarded cells that cannot go on to be fertilized or form a zygote. Thus, in male meiosis one precursor cell produces four sperm but in a female one precursor cell produces one egg. Note that this figure ignores recombination, which we will deal with later in this chapter. Box 6.1 The Stages of Meiosis Meiosis Is the process used to create the germ cells – sperm and eggs. Two homologous chromosomes replicate to become four copies that eventually separate to end up as four single chromosomes in the four separate sperm that are created. (In contrast, only one of the four cells at the end of oogenesis becomes a viable egg.) This list of stages from Figure 6.15 has been greatly simplified. For instance, first meiotic prophase alone is usually divided into five stages with unlikely-sounding names such as zygotene and diakinesis, but we have left out much of that. You don’t have to struggle with distinguishing things such as different levels of condensation of the chromosomes to be able to follow the critical steps we present here and in Figure 6.15. In fact, if you compare this list to the description we gave when we talked about mitosis, you will see that we have even condensed some of the steps
eBook - PDF- Chris Chandler(Author)
- 2016(Publication Date)
- Wiley-Blackwell(Publisher)
Meiosis takes a different pathway when it comes to egg production. From where we have left off with creating the sperm, we can now follow the pathway leading to the egg. Following chromosome duplication and the first set of cell divisions, one of the cells dies. If the remaining cell is fertilized by the sperm, it divides, giving two hap- loid daughter cells. The daughter cell without the sperm also dies. The surviving sperm and egg fuse to produce a complete zygote. That is, the cell has the full comple- ment of chromosomes – the diploid number, 23 pairs. PSYCHOBIOLOGY 42 Of course, at this moment we have a very simple cell and obviously we are composed of many more cells. How does this happen? The answer is further cellular division (see Chapter 3 for detail on embryonic development). To obtain a multicellular organism, the cellular divi- sion that takes place is an ongoing process, called mitosis. Once created, a zygote grows through cellular division. During mitosis the cell doubles its chromosomes before subsequent division (see Figure 2.9). This doubling per- mits the new cells to have a full complement of chro- mosomes. Mitotic division occurs until the organism is complete. During mitosis the 23 pairs of chromosomes in the fertilized cell are duplicated, the chromosomes then separate and divide into separate cells. Our ever-chang- ing bodies make use of this process to repair damaged or dying cells. The production of two identical daughter cells is key to multicellular organisms. Interestingly, neu- rons stop dividing when the development of the nervous system is complete but skin cells divide throughout our lives ( just think of dandruff ). The process of meiosis and mitosis would give us the idea that genetic information is transferred from cell to cell in an identical fashion. However, there is greater vari- ance in genetic transmission than this process suggests. One source of genetic variation is in the process of cross- ing over.
eBook - PDF- Ram J. Singh(Author)
- 2016(Publication Date)
- CRC Press(Publisher)
Tetrad analysis is a powerful scientist’s tool to determine gamete development, cell divi-sion, chromosome dynamics, and recombination (Copenhaver et al., 2000). The difference between mitosis and meiosis has been shown diagrammatically in Figure 4.9 . Mitosis occurs in all growing vegetative tissues; 2 n cells produce only 2 n cells or 1 n cells produce only 1 n cells during mitosis while Meiosis Is confined only specialized in reproductive tissue; 2 n tissues produce 1 n cells and after fertilization with 1 n spore produces 2 n cells ( Figure 4.10a ). Chromosomes usually do not pair in mitotic prophase; 1-chromatid chromosomes replicate and produce 2-chromatid chromosomes and that separates at anaphase while in meiosis synapsis and exchange of 2-chromatid homologue in early prophase leads to the reduction of chromosome number ( Figure 4.10b ). During mitosis, kinetochores lie on the metaphase plate but in meiosis kinetochores lie on either side of the metaphase plate. Kinetochores divide in mitotic anaphase while kinetochores do not divide during meiotic anaphase I. Mitosis concludes with the production of two identical diploid (2 n ) daughter cells, in absence of mutation, while the end of meiosis results in production of four haploid ( n ) daughter nuclei: two identical and two altered (resultant of synapses, crossing over, recombination, and indepen-dent assortment of nonhomologous chromosomes). When egg and sperm unite, a zygote is produced and has the same amount of genetic materials as their parents maintaining the continuity of chromo-some number but variation in genotypic constitution and phenotypic appearance is evolved by meiosis. Although most people use mitosis in reference to division II of meiosis, division II differs from mitosis in three ways. (1) Mitosis always is preceded by an interphase in which DNA synthesis occurs; meiotic division II is not.
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- Carol Bernstein, Harris Bernstein, Carol Bernstein, Harris Bernstein(Authors)
- 2013(Publication Date)
- IntechOpen(Publisher)
However, meiosis by itself has not been silenced, it takes place but is just by-passed by a somatic cell or altered to avoid the reduction of ploidy. Occasional sex, ie. normal meiosis and production of a reduced gametophyte, happens regularly at low frequenices. In animals, automixis is a widespread form of asexuality, but automixis maintains meiosis with meiotic products fused again [6]. Most strikingly, many forms of automixis result in homozygous offspring, with all its detrimental effects. To some extent automixis is more similar to self-fertilization of flowering plants (autogamy) which is usually regarded as a sexual process. In such cases, meiosis cannot be maintained because of recombination – it requires another function which is most likely DNA repair. That is, the key component of sex, the repair at Meiosis I, is still present. Animals with cyclical parthenogenesis like Daphnias alternate between sexual and asexual generations, the former being produced under environmental stress situations [57]. All these forms of asexual or parasexual reproduction demonstrate that Meiosis I has a crucial function other than creating new gene combinations. 5. The purging of deleterious mutations in diploid-haploid cycles The DNA repair hypothesis explains the origin of programmed DSBs, the need for a second chromosome for recombinational repair, and the evolution of Holliday junctions. But, it does not explain the second important part of meiosis, ie. segregation of chromosomes at anaphase I, the lack of a second synthesis phase, and the subsequent mitotic division at Meiosis II which results in four haploid meiotic products. For conducting recombinational repair, permanent Meiosis and the Paradox of Sex in Nature http://dx.doi.org/10.5772/56542 29 diploidy would suffice [11]. This was one of the major points of criticism of the original repair hypothesis. The processes after prophase I cannot be related to DNA repair but must have another functional background.
eBook - PDF- Karen Sermon, Stéphane Viville(Authors)
- 2014(Publication Date)
- Cambridge University Press(Publisher)
For the clinician, embryolo- gist, and geneticist it is of prime importance to develop best strategies that are least invasive, cost- and time- efficient, and of greatest reliability to test and deter- mine risks for abnormal meiosis and improve bene- fits of treatments for patients to conceive a healthy child. 50 Chapter 3: Meiosis: how to get a good start in life References 1. Alberts B, Johnson A, Lewis J et al. Meiosis. In Molecular Biology of the Cell, 4th edn. New York, NY: Garland Science, 2002: http://www.ncbi.nim.gov/ books/NBK26840/ 2. Bowles J, Koopman P. Sex determination in mammalian germ cells: extrinsic versus intrinsic factors. Reproduction 2010; 139: 943–58. 3. Bolcun-Filas E, Schimenti JC. Genetics of meiosis and recombination in mice. In International Review of Cell and Molecular Biology, Vol. 298, Ed. K W Yeon; Burlington, MA: Academic Press, 2012. 4. Davies OR, Maman JD, Pellegrini L. Structural analysis of the human SYCE2–TEX12 complex provides molecular insights into synaptonemal complex assembly. Open Biol 2012; 2: 120099. 5. Hassold T, Sherman S, Hunt P. Counting cross-overs: characterizing meiotic recombination in mammals. Hum Mol Genet 2000; 9: 2409–19. 6. Tease C, Hult´ en MA. Inter-sex variation in synaptonemal complex lengths largely determine the different recombination rates in male and female germ cells. Cytogenet Genome Res 2004; 107: 208–15. 7. San Filippo J, Sung P, Klein H. Mechanism of eukaryotic homologous recombination. Annu Rev Biochem 2008; 77: 229–57. 8. Lichten M, De Massy B. The impressionistic landscape of meiotic recombination. Cell 2011; 147: 267–70. 9. Eichenlaub-Ritter U, Vogt E, Yin H, Gosden R. Spindles, mitochondria and redox potential in ageing oocytes. Reprod Biomed Online 2004; 8: 45–58. 10. Nagaoka SI, Hassold TJ, Hunt PA. Human aneuploidy: mechanisms and new insights into an age-old problem.
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
