Mitosis & Meiosis

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H BOT- 111 Principles of Genetics and Cytogenetics 3 (2+1)

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CELL DIVISION Division of chromosomes & cytoplasm of a cell into two cell. Cell undergoes division- Parent Cell Cell derived from the division of cell – Daughter Cells

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Functions of cell division Growth & development of somatic tissue of organism. Regeneration of damaged tissue Keeping the size of the cells within a limited range. Reproduction

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Amitosis: Simple type of division where the cell content including nucleus divide into two equal halves by inwardly growing constriction in the middle of cell. Mitosis: The two chromosomes of each chromosome separate & move to opposite pole. As a result the two daughter nuclei produced by mitiotic division of a nucleus are identical to parent nucleus. Meiosis: Process of cell division of the reproductive cells of both plant & animals in which the diploid chromosome is reduced to haploid. Cell division: 3 Kinds

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Event taking place during cell division grouped in 2 categories Karyokinesis: Division of chromosome of cell into two groups. Give rise to two daughter nuclei. Cytokinesis: Division of cytoplasm of cell into two halves to produce two daughter cells.

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MITOSIS

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Pl. mitosis confined to meristematic tissue of root & shoot tip, young leaves, flower bud & cambium Squash technique for Root Tip Squash root- common technique- study chromosome in somatic cells. Vigorously growing root tips 2-3 mm in length generally fixed between 11:00 to 14:00 Hrs. 13:00 Hrs is the best time for fixing at which large number of nuclei are found undergoing the mitotic division during that period. Best root tips are obtained from 3 days old seedling.

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MEIOSIS 8

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Place onion in bottle containing water. Root grows well within 3 days Cut root tips & place them in α -bromonapthalene Root tips washed for about 1 hr with 2-3 changes of water 9 MEIOSIS Root tips fixed in Propinol alcohol, 24 hr + few drops ferric acetate

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Root tips washed with 70 & 50% alcohol & kept in each of alcohol for 24 hr Root tips washed 2-3 times with water Root tips hydrolysed in 1N HCl at 60 o C, 10 min 10 MEIOSIS Wash with water, 3 changes , 1 hr

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Squash the tips in drop of propino or acetocarmine Place the coverslip Tap coverslip with blunt end of needle 11 MEIOSIS Observe under microscope

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MEIOSIS 12 STAGES OF MITOSIS

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INTERPHASE Stage between two mitotic division. Nucleus & cytoplasm remain active ; result into increase in nuclear & cytoplasmic substances. Interphase- largest phase of mitiotic cycle. Three sub-phase 13 MEIOSIS

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G 1 Initiallly diploid cell undergoes period of growth & increase in cell mass Synthesis & organization of the substrate & enzyme necessary for DNA synthesis. G1 is marked by transcription of rRNA, tRNA, mRNA & synthesis of different types of protein. 14 MEIOSIS

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S Replication & duplication of chromosomal DNA molecule. After completion of DNA replication cell enters a second growth phase. 15 MEIOSIS

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G 2 Period between the end of S phase & the beginning of prophase of next division. Post DNA synthesis phase, during which all metabolic activities concerning growth of cytoplasm & its constituent organelles & macromolecules are performed. 16 MEIOSIS

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Events in Interphase Nuclear envelope remains intact. Chromosome occur in the form of diffused, long, coiled & distinctly visible chromatin fibers. DNA amount become double. Size of nucleus increase greatly due to accumulation of rRNA & ribosomal proteins. 17 MEIOSIS

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MEIOSIS 20

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PROPHASE Appearance of thin thread like condensing chromosome. Cell become spheroid, more refractile & viscous In the beginning chromosome appears as loose ball of thin wool. As prophase proceed chromosome become increasingly shorter & thicker due to increased condensation. Mid prophase two chromatids of each chromosome become visible Nucleolus & nuclear membrane remain present. 21 MEIOSIS Chromosome condensation- decrease in length with increased thickness-mainly due to coiling of chromosome.

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Two sister chromatids of each chromosome are coiled in relation with each other – relational coiling Plectonimic coiling: Two sister chromatid can not separate from each other without the chromosome being rotated. Observed in prophase of mitosis. Relational coiling between sister chromatid goes on decreasing as chromosome become smalller, it disappear by late prophase. 22 MEIOSIS Paranimic coiling: Two sister chromatid are not twisted round each other, they are simply slipped into those of other, they can easily separated without the chromosome being rotated. Observed in prophase of meiosis.

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Metaphase At the end of prophase 4 important event takes place. Disappearance of nucleolus Breakdown of nuclear envelope & distribution of its components into ER. Appearance of spindle apparatus. Arrangement if chromosome on single plate called “equatorial plate” Movement of chromosome orientation-metakinesis 25 MEIOSIS Centromere of all chromosome lie on the equatorial plate, while arms extends outside this place.

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MEIOSIS 26

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Main features of metaphase are: Absence of nucleolus. Condensation of chromosome. Arrangement of chromosome on the equatorial plate. Coil less in number & larger in diameter. 27 MEIOSIS Disappearance of nuclear membrane. Presence of spindle apparatus. Absence of relational coiling.

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ANAPHASE Two sister chromatids of each chromosome separate & migrate to the opposite poles of the cell. Spindle fibers originate at two points located near the periphery of cell & opposite to each other. These points known as ‘poles’. Centromere is the first portion of each chromosome to reach the pole. Chromosome somewhat condensed as compared to those at metaphase so that they appear relatively smaller in size. 29 MEIOSIS Anaphase begin when centromeres of chromosome appear to divide longitudinally so that sister chromatid separate from each other & ends with reaching of chromosome to opposite pole.

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MEIOSIS 30

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TELOPHASE Begins when sister chromatids of all the chromosome reach to opposite poles. Nuclear membrane reorganized around each group of chromosome Nucleus reappears. End of telophase, middle lamella appears at the equatorial plate of the cell. 33 MEIOSIS Chromosome uncoil – become very long & thin; appeared to be coiled into a loose ball of fine thread.

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CYTOKINESIS Division of cytoplasm. End of mitosis- two identical daughter cells formed. Vesicles – contain cell wall material fuse with one another to form cell membrane & cell wall. 35 MEIOSIS Formation of cell wall begin as cell plate also known as phragmoplast formed by the aggregation of vesicles produced by golgi bodies.

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plant cytokinesis

Mitosis produces identical offspring (2N---2N):

Mitosis produces identical offspring (2N---2N)

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SIGNIFICANCE Two daughter cell identical to each other & identical to mother cell are formed. Asexual reproduction in lower plant possible only by mitosis. Forms basis of continuation of organism. Vegetative reproduction in higher plant by grafting, tissue culture method are also consequence of mitosis. 38 MEIOSIS Mitotic cell division ensures that daughter cells posses a genetical identity, both quantitatively & qualitatively. Mitosis common method of multiplication of cells that helps in growth & development of multicellular organism. Helps in regeneration of lost or damaged tissue & wound healing. Chromosome number maintained constant.

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MEIOSIS 39 MEIOSIS

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Study meiosis – anthers squashed Anthers of flower bud at certain stage of maturity are killed & fixed in alcoholic & aqueous fixatives. 40 MEIOSIS Aceto alcohol fixative – (3 part alcohol: 1 part acetic acid or propionic acid).

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Buds placed in fixative Buds washed Buds transferred in 70% alcohol, 2 hr 41 MEIOSIS Buds washed with water 2-3 times with water for 1 hr Place anther on the slide having drop of acetocarmine

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Heat the slide over the spirit flame for about 2-3 seconds but do not allow carmine to boil. Use blotting paper to absorb drop of carmine & add drop of fresh carmine 42 MEIOSIS Heat it again, squash the material on slide with needle Remove debris , place the cover slip over the material

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Heat the slide gently to remove air bubble Observe under microscope, if chromosome does not take stain add fresh drop of carmine, remove excess of stain with blotting paper Seal the coverslip by paraffin having low melting point 43 MEIOSIS After 24 hr remove the seal of coverslip & make the slide permanent

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10-45% Propionic acid poured in storeware dish Slide should be placed in carefully with coverslip facing downward. Allow the coverslip to separate from the slide (3min) Equal part of Propionic acid & n-butyl alcohol other store ware dish. Invert the slide so that squash material remain upward (5min.) 44 MEIOSIS 1 Propionic Acid : 3 Part of n-butanol (3 min) n-butanol (5 min)

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Term meiosis was coined by J.B. Farmer , 1905 Meiosis is process of cell division of the reproductive cells of both plants & animals in which the diploid number of chromosome is reduced to haploid . Meiosis is K/as reduction division since the number if chromosome is reduced to half Takes place in reproductive cell during gamete formation. Consist of two complete division, as a result this diploid cell produces four haploid cells. Two division of meiosis Meiosis I or heterotypic division Meiosis II or homotypic division 45 MEIOSIS

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First division – reductional - no. of chromosome is reduced to half Second division is mitotic or equational During sexual reproduction 2 gametes male & female each having single set of chromosome (n) fuse to form zygote. Zygote contain twice as many chromosome as a gamete ( n+n =2n) Diploid comes to posses two identical sets of chromosome is called homologous chromosome . Meiosis may take place in the life cycle of plant during one of the following events. At the time of spore formation Gamete formation Zygote formation 46 MEIOSIS

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I P M A T

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MEIOSIS 48 STAGES OF MEIOSIS

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Pre-meiotic Interphase Stage just prior to the entry of cell into meiosis. Event taking place during pre-meiotic interphase are crucial for entry of cell into meiosis During ‘S’ phase of pre-meiotic interphase , chromosome replication takes place. But approximately 0.3% of total DNA present in the nucleus does not replicates during ‘S’ phase. Replicates during Zygotene . Entry of cell into meiosis depend upon some substance synthesized during G 2 sub-stage of pre-meiotic interphase During meiosis ‘S’ phase occurs only once during pre-meiotic interphase . Chromosme replicates once & nuclei divide twice. 49 MEIOSIS

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First Meiotic Division Pairing between homologous chromosome. Crossing over between them during Pachytene stage of Prophase I. Separation of homologous chromosome & their migration to opposite e poles of cell division during anaphase I. Two daughter nuclei are produced by this division receive only half of the chromosome present in somatic complement. For this reason the first division is often referred to as Reduction Division . 50 MEIOSIS

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PROPHASE I Longest in duration Main events in prophase I are Intimate pairing between homologous chromosome Condensation of chromosome Crossing over between homologous chromosome Movement of homologous chromosome away from each other so that chiasma become observable. Terminalization of chiasma Movement of chromosome during chromosome pairing & chiasma terminalization. Prophase I is divided into five sub-stages: Leptotene , Zygotene , Pachytene, Diplotene , Diakinesis 51 MEIOSIS

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LEPTOTENE Marked increase in nuclear volume. Chromosome condensation- visible as fine thread. Each chromosome consist of two chromatids. RNA synthesis- because of which nucleolus volume increase. 52 MEIOSIS Proteins required for condensation of chromsosome are synthesized.

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LEPTOTENE

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ZYGOTENE Stage begin with initiation of pairing between homologous chromosome. Main event Pairing between homologous chromosome. Completion of replication of 0.3% DNA. 54 MEIOSIS Synthesis of specific nuclear protein. Development of synoptnemal complex. Progressive condensation of chromosome. Pairing between homologous chromosome is extremely precise, only the homologous segments of two chromosome pair with each other. Pairing between homologous chromosome is referred as SYNAPSIS .

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ZYGOTENE

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PACHYTENE Begin when synapsis comes to end, and it ends when the homologous chromosome begin to move away from each other. Main event Condensation of chromosome, Pair become shorter & thicker. As consequence of synapsis between homologous chromosome, the no. o f observable chromosome is only half the somatic chro. No. i.e it is only n. 56 MEIOSIS Haploid pair of chromosome is referred to as bivalent, distinctly visible during this stage. Since each of the two chromosome of bivalent consist to two sister chroamtid, each bivalent has four chromatids/strand; bivalents said to four strand/tetrad. Nucleolus distinct & quite large. Crossing over between two homologous chromosome takes place.

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PACHYTENE

Crossing over:

Meiosis KM 58 Crossing over Chiasmata – sites of crossing over, occur in synapsis . Exchange of genetic material between non-sister chromatids. Crossing over produces recombinant chromosomes .

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DIPLOTENE Homologous chrom. Of each bivalent begin to move away from each other, this marks the beginning of this stage. Two homologous of each bivalent appears to be attached with each other at one or more points; these attachments are known as CHAISMATA . Diplotene proceeds, chiasma slowly moves towards the end of homologous chromosome – chiasma terminalization . 59 MEIOSIS Chiasma terminalization occurs mainly due to movement of homologous chromosome from each other. Further condensation of chromosome so that they become progressively shorter & thicker.

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DIPLOTENE

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DIAKINESIS End of chiasma terminalization marks the beginning of this stage. Main events Chromosome become shorter & thicker. 61 MEIOSIS Bivalents move away from each other & spreads towards periphery of cells. Nucleolus & nuclear envelope disappears. Spindle apparatus organized. Bivalents migrate towards equatorial plate of cells; this marks the end of diakinesis.

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DIAKINESIS

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Metaphase I Bivalents arranged at metaphase plate. Metaphase stage terminates as soon as the homologous chromosome begin to separate from each other & migrate to opposite poles of the cell. 64 MEIOSIS Centromere of two homologues of each bivalents lie on either side of equatorial plate.

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Anaphase I Separation of two homologous chromosome of each bivalent marks the beginning of anaphase stage. Two sister chromatid of each chromosome repel to each other. At the end of Anaphase I, the chromosome present in somatic cells are effectively & precisely separated into two identical groups 65 MEIOSIS One chromosome from each bivalent begin to migrate to one pole, while other migrate to opposite pole.

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TELOPHASE I Chromosome uncoil partially. Nucleolus reappear. 66 MEIOSIS Nuclear envelope become organised around two groups of chromosome.

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CYTOKINESIS Cytoplasm of each cell divide into two halves. 67 MEIOSIS Two halves of each cell do not separate, but stay together, this two celled structure is known as dyad.

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MEIOSIS II During second division of meiosis, two haploid nuclei of each dyad divide synchronously Second meiotic division is also divided into four stages As a result the number of chromosome in each of the two haploid nuclei remain the same (i.e haploid) at the end of this division – equational division. 68 MEIOSIS Two sister chromatid of chromosome separate & migrate to the opposite poles.

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PROPHASE II There is no relational coiling between sister chromatid Two sister chromatid of each chromosome is clearly visible. Chromosome relatively much more condensed, hence they appear shorter & thicker 69 MEIOSIS At the end of nucleolus & nuclear envelope disappears, spindle apparatus organized.

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METAPHASE II Chromosome arranged at equatorial plate Spindle apparatus present. 70 MEIOSIS Nucleolus & nuclear envelope absent Centromere of all the chromosome arranged in single plane at equatorial plate. Two sister chromatids of each chromosome are distinctly separated from each other, but they remain attached at the centromere. Chromosome become more condensed, thicker & shorter Relatively shorter duration.

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Anaphase II Centromere of chromosome observably longitudinally & the two sister chromatids of each chromosome begin to separate & move away to the opposite poles. 71 MEIOSIS Stage comes to an end when sister chromatids of the chromosome reach to the opposite pole.

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TELOPHASE II Chromatid uncoil partially so that they resume appearance of loose ball of thread. Nucleolus reappear. 72 MEIOSIS Nuclear envelope reorganised from the elements of ER

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CYTOKINESIS By the end of telophase, cytoplasm of each of the two cells of dyad divides into two parts. Four daughter cells produced from parent cells are present together and are known as tetrad. 73 MEIOSIS One parent cell produce four haploid daughter cells after completion of two meiotic division.

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SIGNIFICANCE Meiosis helps to maintain the chromosome number constant in each plant & animal species. Genetic variation forms raw material for variation. Recombination of genes results in genetic variation. 74 MEIOSIS Occurrence of crossing over results in recombination of genes.

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Mitosis Meiosis Consists of one nuclear division Consists of two nuclear division One cell cycle results in production of two daughter cells One cell cycle results in the formation of 4 daughter cells The chromosome number of daughter cells is the same as that of mother cell (2n) Daughter cells contain half the chromosome number of mother cell (n) Daughter cells are identical with mother cell in structure and chromosome composition Daughter cells are different from mother cell in chromosome number and composition. Mitosis occurs in somatic tissues It occurs in reproductive tissues. Total DNA of nucleus replicates during S phase 99.7% DNA replicates during S phase and remaining 0.3% during zygotene stage. Homologous chromosomes pair during pachytene .

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Mitosis Meiosis There is no pairing between homologous chromosomes. Homologues chromosomes pair during pachytene . Segregation and recombination do not occur . Crossing over takes place during pachytene . Chromosomes are in the form of dyad at metaphase Chromosomes are in the form of tetrad at metaphase. At metaphase, centromere of each bivalent divides longitudinally. The centromere does not divide at metaphase. One member of sister chromatids move to opposite pole during anaphase One member of homologous chromosomes moves to opposite pole during anaphase I Maintains purity due to lack of segregation and recombination Generates variability due to segregation and recombination.

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MEIOSIS 77

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MEIOSIS 78

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MEIOSIS 79

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MEIOSIS 80

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Meiosis KM 83

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Meiosis KM 84

Animation:

Meiosis KM 85 Animation

Mitosis vs. meiosis:

Meiosis KM 86 Mitosis vs. meiosis

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Meiosis KM 87

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