Meiosis - The Special Cell Division

Meiosis - The Special Cell Division

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  MEIOSIS (THE REDUCTION DIVISION) By PBMallikharjuna PhD GFGC Yelahanka Bangalore 7th and 8th June 2021 July 13, 2021 GFGC Yelahanka 1
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  Introduction : Meiosisis a very important and compulsory biological process of all eukaryotic organisms including plants. • It is a component of the true sexual reproduction in order to produce the haploid gametes (gametogenesis) or the spores (sporogenesis). • Therefore, meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organisms, whereas fertilization restores the diploid phase. July 13, 2021 GFGC Yelahanka 2
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   Meiosis isthe specialized cell division that mainly occurs in the reproductive or sex cells such as the pollen mother cells (PMC) and megaspore mother cells(MMC) in higher plants. Thereby, the haploid microspore or megaspore tetrads are produced.  Meiocytes are the cells which undergo meiosis.  The term Meiosis was coined by Farmer(1905).  The process of meiosis was first described by Oscar Hertwig, who observed it in sea urchin eggs. Later Edouard Van Beneden expanded it.  It can be defined as the special type of cell division where a single diploid reproductive cell(2N) is divided into four haploid gamete cells(N). July 13, 2021 GFGC Yelahanka 3
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  • The keyfeatures of meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II, but only a single cycle of DNA replication. Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase. • Meiosis involves pairing of homologous chromosomes and recombination between them. • Four haploid cells are formed at the end of meiosis II. • Meiotic events can be grouped under the following phases:-  Meiosis-I ( ): Prophase I, Metaphase I, Anaphase I, and Telophase I.  Meiosis-II( ): Prophase II , Metaphase II, Anaphase II, and Telophase II July 13, 2021 GFGC Yelahanka 4
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  KARYOKINESIS CYTOKINESIS July 13,2021 GFGC Yelahanka 5
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  MITOSIS - I ProphaseI • It is of longest phase of karyokinesis of meiosis which is about 200 times more complex than that of the prophase of Mitosis. • Hence it is further divided into five sub stages i.e.,  Leptotene,  Zygotene,  Pachytene,  Diplotene, and  Diakinesis. July 13, 2021 GFGC Yelahanka 7
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  Leptotene/ Leptonema : (a)Chromosomes are long thread like with chromomeres (beaded) on it. (b) the volume of nucleus increases. (c) Half of the chromosomes are from male(paternal) and the remaining half are from female (maternal)parent. (d) Chromosome with similar structure are known as homologous chromosomes. (e) Leptonemal chromosomes have a definite polarization and forms loops whose ends are attached to the nuclear envelope at points near the centrioles, contained within an aster. Such peculiar arrangement is termed as bouquet stage (in animals) and syndet knot (in plants). (f) E.M. (electron microscope) reveals that chromosomes are composed of paired chromatids, a dense proteinaceous filament or axial core lies within the groove between the sister- chromatids of each chromosome. July 13, 2021 GFGC Yelahanka 8
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  Leptotene Homologous Chromosomes July 13, 2021GFGC Yelahanka 9
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  Synapsis July 13, 2021GFGC Yelahanka 10
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  Zygotene / Zygonema (a)Pairing or “synapsis” of homologous chromosomes takes place in this stage. (b) Synapsis may be of following types.  Procentric : Starting at the centromere.  Proterminal : Starting at the end.  Localised random : Starting at various points. (c) Paired chromosomes are called bivalents, which by further molecular packing and spiralization becomes shorter and thicker. (d) Pairing of homologous chromosomes in a zipper-fashion. Number of bivalents (paired homologous chromosomes) is half to total number of chromosomes in a diploid cell. Each bivalent is formed of one paternal and one maternal chromosome (i.e. one chromosome derived from each parent). Ex in Allium cepa eight bivalents are present (in PMC/MMC) (e) Under EM, a filamentous ladder like nucleoproteinaceous complex, called Synaptenemal Complex between the homologous chromosomes, which is discovered by “Moses” (1953). July 13, 2021 GFGC Yelahanka 11
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  Pachytene/Pachynema (a) In thetetrad, two similar chromatids of the same chromosome are called sister- chromatids and those of two homologous chromosomes are termed non-sister chromatids. (b Crossing over i.e. exchange of segments between non-sister chromatids of homologous chromosome occurs at this stage. It takes place by breakage and reunion of chromatid segments. Breakage called nicking, is assisted by an enzyme endonuclease and reunion termed annealing is added by an enzyme ligase. Breakage and reunion hypothesis proposed by Darlington (1937). (c) Chromatids of pachytene chromosome are attached with centromere. (d) A tetrad consists of two sets of homologous chromosomes each with two sister chromatids. (e) A number of electron dense bodies about 100 nm in diameter are seen at irregular intervals within the centre of the synaptonemal complex, known as recombination nodules. July 13, 2021 GFGC Yelahanka 12
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  Diplotene/Diplonema (a) At thisstage, the paired chromosomes begin to separate (desynapsis). (b) A cross is formed at the place of crossing over between non-sister chromatids (X) called Chiasma (c) Homologous chromosomes move apart they remain attached to one another at specific points called chiasmata. (d) At least one chiasma is formed in each bivalent. (e) Chromosomes are attached only at the place of chiasmata. (f)Chromatin bridges are formed in place of synaptonemal complex on chiasmata. (g) This stage remains as such for long time. July 13, 2021 GFGC Yelahanka 13
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  Diakinesis (a) Chiasmata movestowards the ends of chromosomes. This is called terminalization. (b) Chromatids remain attached at the place of chiasma only. (c) Nuclear membrane and nucleolus degenerates. (d) Chromosome recondense and tetrad moves to the metaphase plate. (e) Formation of spindle apparatus by the involvement of microtubules. (f) Bivalents are irregularly and freely scattered in the nucleocytoplasmic matrix. July 13, 2021 GFGC Yelahanka 14
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  Metaphase I :involves; (i) Chromosome come on the equator. (ii) Due to repulsive force the chromosome segment get exchanged at the chiasmata. (iii) Bivalents arrange themselves in two parallel equatorial or metaphase plates. Each equatorial plate has one genome. (iv) Centromeres of homologous chromosomes lie equidistant from equator and are directed towards the poles while arms generally lie horizontally on the equator. (v) Each homologous chromosome has two kinetochores and both the kinetochores of a chromosome are joined to the chromosomes July 13, 2021 GFGC Yelahanka 15
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  Anaphase-I (i) It involvesseparation of homologous chromosomes which start moving opposite poles so each tetrad is divided into two daughter dyads. So anaphase-I involves the reduction of chromosome number, this is called disjunction. (ii) The shape of separating chromosomes may be i or J or V-shape depending upon the position of centromere. (iii) Segregation of Mendelian factors or independent assortment of chromosomes take place. In which the paternal and maternal chromosomes of each homologous pair segregate during anaphase-I that introduces genetic variability. July 13, 2021 GFGC Yelahanka 16
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  The sequential events inthe separation of a pair of homologous chromosomes during Meta –and Anaphases -I Disjunction July 13, 2021 GFGC Yelahanka 17
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  Telophase-I (i) Two daughternuclei are formed but the chromosome number is half of the mother cell. (ii) Nuclear membrane reappears. (iii) After telophase I cytokinesis may or may not occur. (iv) At the end of Meiosis I either two daughter cells will be formed or a cell may have two daughter nuclei. (v) Meiosis I is also termed as reduction division. (vi) After meiosis I, the cells in animals are reformed as secondary spermatocytes or secondary oocytes; with haploid number of chromosomes but diploid amount of DNA. (vi) Chromosomes undergo decondensation by hydration and despiralization and change into long and thread like chromatin fibres. July 13, 2021 GFGC Yelahanka 18
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  Interphase : Generallythere is no interphase between meiosis-I and meiosis-II. A brief interphase called interkinesis, or intermeiotic interphase sometimes may exist. During this interphase (S phase), there is no replication of chromosomes, Cytokinesis-I : It may or may not be present. When present, it occurs by cell- furrow formation in animal cells and cell plate formation in plant cells. Significance of meiosis-I (i) It separates the homologous chromosomes to reduce the chromosome number to the haploid state, a necessity for sexual reproduction. (ii) It introduces variation by forming new gene combinations through crossing over and random assortment of paternal and maternal chromosomes. (iii) Sometimes, it may cause chromosomal mutation by abnormal disjunction. (iv) It induces the cells to produce gametes for sexual reproduction or spores for asexual reproduction. July 13, 2021 GFGC Yelahanka 19
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  Meiosis-I Dyad cell Metaphase -IIAnaphase -II Telophase -II Cytokinesis-II MICROSPORE TETRAD July 13, 2021 GFGC Yelahanka 20
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  Meiosis-II ( Homotyticor equational division) • Meiosis-II is exactly like mitosis, so it is also known as meiotic mitosis. • In this division, two haploid chromosome splits longitudinally and distributed equally to form 4 haploid cells. • It completes in 4 stages - Prophase-II, Metaphase-II, Anaphase-II, Telophase-II. Prophase-II: • The dyad chromosomes becomes shorter and thicker. • Nuclear membrane and nucleolus disappear. • Spindle fiber starts to form. July 13, 2021 GFGC Yelahanka 21
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  Metaphase-II: • The dyadschromosomes comes to equatorial plane. • Spindle fibers organize between poles and attaches to centromere of chromosome. Anaphase-II: • Centromere of each chromosome divides and sister-chromatids separates to form two daughter chromosomes. • Spindle fiber contracts and pull the daughter chromosome apart towards opposite pole. Telophase-II: • Chromosome become organize at respective pole into nuclei. • Chromosome elongates to form thin networks of chromatin. • Nuclear membrane and nucleolus reappears July 13, 2021 GFGC Yelahanka 22
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  Microspore Tetrad July 13,2021 GFGC Yelahanka 23
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  Cytokinesis-II: • The resultof cytokinesis is four haploid daughter cells (gametes or spores). • Cytokinesis takes place by cell plate formation in plant cells • Successive methods: cytokinesis followed by each nuclear division resulting in 4 haploid cells. E.g., Monocot plants (Allium cepa) • Simultaneous methods: cytokinesis occurs only after meiosis- II to form 4 haploid cells. E.g., Dicot plants • In animal cells, cytokinesis occurs by furrow formation or depression. July 13, 2021 GFGC Yelahanka 24
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  Stages of Meiosis-I Stages of MeiosisII Stages of Meiosis in Allium cepa (pollen mother cells) Late Prophase Metaphase I Anaphase I Telophase-I Dyad stage Cell plate Prophase II Metaphase II Anaphase II Telophase II Microspore Tetrad July 13, 2021 GFGC Yelahanka 25
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  July 13, 2021GFGC Yelahanka 26
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  Significance of Meiosis: •Essential for sexual reproduction in higher animals and plants. • It helps in the formation haploid gametes and spores for sexual reproduction. • Number of chromosome remain fixed in a species from generation to generation due to meiosis. • Crossing over brings the genetic variations in offspring, which helps in evolution of organisms. • Failure of disjunction in meiosis leads to mutation whereby polyploidy may arise. • The random distribution of maternal and paternal chromosomes takes place into daughter cells during meiosis, and it is a sort of independent assortment which leads to variation. July 13, 2021 GFGC Yelahanka 27
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  Synaptonemal Complex  Thesynaptonemal complex (SC) is a protein structure that forms between homologous chromosomes during meiosis.  It is a tripartite structure consisting of two parallel lateral regions and a central element. The fibres of lateral filaments are called synaptomeres.  This "tripartite structure" is seen during the pachytene stage of the first meiotic prophase, both in males and in females during gametogenesis.  It mediates synapsis, recombination and chiasmata formation in eukaryotes including plants.  Further, SC functions primarily as a scaffold to allow interacting chromatids to complete their crossover activities.
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   During Leptotenesub stage, the lateral elements begin to form, and they initiate and complete their pairing during the zygotene stage. After pachytene ends, the SC usually becomes disassembled and can no longer be identified.  Montrose and J. Moses (1956) first described synaptonemal complex.
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  How SC isformed?  The lateral elements of the synaptonemal complex start to attach to individual chromosomes during leptotene, but the central element, which actually joins homologous chromosomes together, does not form until zygotene.  During early zygotene, the ends (telomeres) of each chromosome become clustered on one side of the nucleus and attach to the nuclear envelope, with the body of each chromosome looping out into the nucleus. This type of chromosome configuration, called a bouquet, is thought to promote chromosome alignment. Chromomeres
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   The alignmentof similar-sized chromosomes facilitates formation of synaptonemal complexes, which become fully developed during pachytene  Formation of synaptonemal complexes is closely associated with the process of crossing over in higher eukaryotes, and some electron micrographs reveal additional protein complexes, called recombination nodules, that may mediate the crossing over process. The synaptonemal complexes then disassemble during diplotene, allowing the homologous chromosomes to separate (except where they are joined by chiasmata).
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  Protein Electron micrograph Diagrammatic Pairinglike a zipper The sequential events of SC in the meiosis-I
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  Functions of SC It helps in the proper alignment of homologous chromosomes during pairing (Synapsis) in the meiotic division.  It stabilizes the pairing of homologous chromosomes  It facilitates the crossing over and thereby genetic recombination.  Lack of SC as in male Drosophila no crossing over occurs (Complete Linkage).
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  The Cytological Basisof Crossing over Introduction : It is not necessary that chiasmata should be associated with exchange of chromosome segments. Therefore, to demonstrate that crossing over is associated with actual exchange of chromosome segments, special experiments were devised by C. Stern in Drosophila and by H.S. Creighton and B. McClintock in corn. These experiments were reported in 1931 and had utilized chromosomes, whose morphology was altered due to chromosomal aberrations in order to make it identifiable from its homologue.
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  Stern’s Experiment onDrosophila : • The flies which are classified as crossovers on the basis of phenotype i.e., carnation (with normal eye shape) and barred (with normal eye colour) were studied cytologically. • It was found that carnation flies did not have any fragmented X-chromosome, but rather had normal X-chromosome. • On the other hand, barred flies had a fragmented X-chromosome with a segment of Y-chromosome attached to one of the two fragments of X- chromosome. • Such cytological observations suggested that genetic crossing over was accompanied with an actual exchange of chromosome segments.
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  Test cross Gametogenesis The flowchartof Stern’s experiment in Drosophila Created by X-ray exposure
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   The femaleDrosophila carries XX chromosome and the male Drosophila carries one X chromosome and one Y chromosome.  He made the two X chromosomes of the female are different from each other by treating such flies with X-rays.  One X chromosome had a part of a Y chromosome attached to one end.  The other X chromosome has been broken into two unequal segments.  Thus, both aberrant X chromosomes were cytologically detectable.  The broken X chromosome contains a recessive gene (c) for carnation eye colour and a dominant gene (B) for bar eye shape (one fragment having both of the genes).  While its homologue X chromosome contains the dominant gene (C) for red colour and the recessive gene (b) for round eye shape.
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   Female fliesheterozygous for these two morphologically distinguishable X-chromosomes were produced by crossing. These heterozygous females with trans-configuration were crossed to males with carnation eye colour (c) and round eye shape (b).  Fertilization produced following four kinds of female offspring, o Carnation – Bar o Red – Round o Carnation - Round & o Red – Bar.  The crossing over was indicated phenotypically showed microscopic evidence of exchanges between homologous chromosomes.  The physical or cytological basis of crossing over was thus established Parental combinations Recombinants
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  Experiments by HarrietCreighton and Barbara McClintock on Maize : • Harriet Creighton and Barbara McClintock (1931) demonstrated the correlation between genetic crossing over and the exchange of parts of homologous chromosomes. • They analyzed crosses involving two loci on chromosome 9 of maize. ▪ C = Coloured seed ▪ c = colourless seed ▪ Wx – Starchy endosperm, and ▪ wx = waxy endosperm • They made use of a chromosome with a densely stained “knob” at one end and an extra (translocated) piece of chromosome at the other end.
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  • They createda heterozygote with the following characteristics:  repulsion configuration of genetic markers  cytological landmarks on both ends of one chromosome. • They next performed a test cross to this stock with a cc wx wx tester. • The plant heterozygous for coloured aleurone and starchy (non-waxy) endosperm characters and carried these genes in repulsion phase, i.e., Cwx / cWx. • Cwx was carried on the and cWx on the knobless chromosome. • Such a plant was test crossed with plant homozygous recessive for both characters, i.e., colourless and waxy. • Crossing over involves the physical exchange of chromosomal material , then the recombinant phenotypes should each contain one of the cytological landmarks.
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  Test cross Repulsion configuration Diagrammatic Couplingconfiguration
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  Comparative diagrams ofthe experiments of Creighton and McClintock with maize and of Stern with Drosophila, mutually supporting the hypothesis of physical and genetic exchange in two key experimental species. [Reproduced with permission from ref....PNAS ©2005 by National Academy of Sciences https://www.pnas.org/content/pnas/102/19/6641.full.pdf Further reading