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MEIOSIS OBJECTIVES At the end of the lesson, students should be able to:Explain & compare the processes in meiosis I & meiosis II.Explain the position and changes of the chromosomes at each stages.
Objectives• Define chromatid, synapsis, bivalent, tetrad, chiasma, crossing over & centromere• State the significance of meiosis• Compare meiosis & mitosis
CONTENT OF MEIOSISMeiosis- a form of nuclear division in which the chromosome number is halved from the diploid number (2n) to the haploid number (n).involves interphasefollowed by two cycle of cell division, known as meiosis I and meiosis II (first & second meiotic division).gives rise to four haploid cells.
occurs during gametogenesis in animal and during spores formation in plants.a continuous process but is conveniently divided into prophase, metaphase, anaphase and telophase.These stages occur in the first meiotic division and again in the second meiotic division.
Importance term s in meiosis:b) Chromatid - prior to meiosis 1,DNA replication occurs and each chromosomes has two sister chromatids. - After crossing over occurs,sister chromatid of a chromosome are not longer identical - the chromosomes in the four daughter cells of meiosis are actually chromatids from the original parent cell.b) Synapsis - during meiosis 1,homologous chromosomes pair,come together and line up in synapsis
c) Bivalent - during synapsis,the two sets of paired chromosomes lay alongside each other as a bivalentd) Tetrad - is a group of four homologous chromatids synapsed together during prophase 1 of meiosise) Chiasma - a special structure,chiasma,occurs during crossing over. - it is the point at which a chromatid of one chromosome crosses with a chromatid of the homologous chromosome - it can be usually seen in prophase 1
f) Crossing over - chromosomes have equal exchanges of genetic materials while paired up.d) Centromere - specialised region of a chromosome at which siser chromatids are held together
MEIOSIS I• Prophase Isister chromatids begin to condense & homologousIn a process called synapsis, visible as a tetrad
Stages in Prophase I• There’s 5 substages in Prophase I: leptotene, zygotene, pachytene, diplotene & diakinesis• Things that occur during these stages:- Chromosomes become visible as fine thread- Spindle starts to form
• Homologous chromosomes attracted toeach other forming a bivalent or tetrad(consist of 4 chromatids) in a processknown as synapsis• Nucleolus disappeared• chromatids become visible as theymove apart from each other; theyremain in contact at points calledchiasmata.
• chromatids continue to move apart as they shorten and thicken• shortening and thickening continues; chiasmata move to ends; nuclear envelope breaks down- continue to repel each other- bivalents assume particular shapes depending upon the number of chiasmata.
By the end of prophase I:• chromosomes are fully contracted and deeply stained;• centrioles migrated to the poles;• nucleoli and nuclear envelope dispersed;• spindle fibres form.
MEIOSIS I• Metaphase IThe bivalents become arranged on the metaphase plateKinetochore microtubules from each pole of the cell attached to kinetochore of chromosomes
MEIOSIS I• Anaphase ISpindle fibers pull homologous chromosomes, centromeres firstThis separate the chromosomes into two haploid sets
MEIOSIS I• Telophase IThe arrival of homologous chromosomes at opposite poles marks the ends of meiosis I.
MEIOSIS I• Telophase ICleavage (animals) or cell wall formation (plants) THEN occurs as in mitosis.Halving of chromosome number has occurred but the chromosomes are still composed of two chromatids.
MEIOSIS II• Interphase - In some species; chromosomes decondense, nuclear membrane & nucleoli re-form- No further DNA replication occurs.Meiosis II is similar to mitosis.
MEIOSIS II • Prophase II absent if interphase II is absent. nucleoli and nuclear envelopes disperse & the chromatids shorten, thicken. Centrioles, move to opposite poles new spindle fibers appear.
MEIOSIS II• Metaphase II Chromosomes line up separately on the equator of the spindle. Kinetochores pointing towards opposite poles• Anaphase II The centromeres divide and the spindle fibers pull the chromatids to opposites poles
MEIOSIS II• Telophase IIfour haploid daughter cells are formed.The chromosomes uncoiled, lengthen and become very indistinct.spindle fibers disappear
MEIOSIS IINuclear envelope re-formSubsequent cleavage (animals) or cell wall formation (plants) will produce four daughter cells from the original single parent cell.
SIGNIFICANCE OF MEIOSIS Halving the chromosome number ensures that when gametes with the haploid number fuse to form a zygote the normal diploid number is restored. Meiosis leads to increased variation because: When fertilization there is recombination of parental genes.
SIGNIFICANCE OF MEIOSISDuring metaphase I, homologous chromosomes are together at the equator of the spindle, but they separate into daughter cells independently of each other. Chiasmata and crossing-over can separate and rearrange genes located on the same chromosome.
Three events, unique to meiosis, occur during the first division cycle. 1. During prophase I, homologous chromosomes pair up in a process called synapsis. A protein zipper, the synaptonemal complex, holds homologous chromosomes together tightly.
Three events, unique to meiosis,occur during the first division cycle Later in prophase I, the joined homologous chromosomes are visible as a tetrad. At X-shaped regions called chiasmata, sections of nonsister chromatids are exchanged. Chiasmata is the physical manifestation of crossing over, a form of genetic rearrangement.
Three events, unique to meiosis, occurduring the first division cycle 2. At metaphase I homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase plate. In humans, you would see 23 tetrads.
Three events, unique to meiosis,occur during the first division cycle3. At anaphase I, it is homologous chromosomes, non sister chromatids are separated and carried to opposite poles of the cell. Sister chromatids remain attached at the centromere until anaphase II.
COMPARISON BETWEEN MEIOSIS AND MITOSISNo. MITOSIS MEIOSIS1. Conserves Reduces the chromosome chromosome number number by half (n). (2n).
COMPARISON BETWEEN MEIOSIS AND MITOSISNo. MITOSIS MEIOSIS2. no synapsis occur to Synapsis occurs to form form bivalent. bivalent at the homologous chromosomes during prophase I3. No chiasma occurs so Some chiasma occurs to there is no cross – form cross – over. over.
COMPARISON BETWEEN MEIOSIS AND MITOSISNo. MITOSIS MEIOSIS4. genetic composition in Genetic composition in daughter cells are daughter cells are different identical to parental to parental cell. cells.5. Two daughter cells Four daughter cells each each diploid (2n) haploid (n)6. Cytokinesis occurs Cytokinesis occurs once or once. twice.7. The daughter cell can The daughter cell can undergoes mitosis . undergoes mitosis but not meiosis.