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Published byMarcus Underwood Modified over 8 years ago
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Making Haploid Gametes
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Both chromosomal problems (monosomy/trisomy & breakage) are result of improper haploid cell formation/division in process known as meiosis Two phases: meiosis I & meiosis II Goal: 2n (diploid) n (haploid)
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Body or somatic cell division is called mitosis, which generates the EXACT same cell Mitosis: cell division which produces cells containing SAME number of chromosomes as parent’s cell ○ Start: One 2n skin cell ○ End: Two 2n skin cells
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Gamete or sex cell division is called meiosis, which generates cell with HALF number of chromosomes as original Start: One 2n cell End: Four n sperm or egg cells n n nnn nnn Video of Mitosis vs. Meiosis 2n2n
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MITOSISMEIOSIS Growth & repairMaking gametes In body/somatic cellsIn sex cells/gametes One 2n cell Two 2n cellsOne 2n cell Four n cells One diploid two diploids One diploid four haploids Exact copy of parentHalf copy of parent 2n n n n n
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Two repeating stages with precursor stage: Steps of meiosis Meiosis I Meiosis II Interphase
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Interphase – DNA replication (2n 4n) 1. Prophase I – homologous chromosomes visible; crossing over occurs to add genetic variety 2. Metaphase I – homologs move to equator 3. Anaphase I – homologs move to opposite poles 4. Telophase I – 2n sets move to poles; cytokinesis 5. Prophase II – new spindle forms around chromosomes 6. Metaphase II – chromosomes move to equator 7. Anaphase II – centromeres divide; n chromatids to poles 8. Telophase II – n sets move to poles; cytokinesis MEIOSIS I = 4n 2n MEIOSIS II – 2n n
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Interphase Doubling of chromosomes (2n 4n) Cell looks same Nucleus is darker because of chromosomal duplication A B C “tetrad” a b c a b c A B C
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A A B B CC aa bb c c
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Prophase I Nuclear envelope disappears Spindle fibers form from centrioles Chromosomes in homologous pairs Crossing-over occurs (genetic diversity!) Homologous pair A A B B CC aa bb c c A B B CC a bb c c a A Crossing over Same gene, different variety Nuclear envelope centrioles spindle fibers
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Metaphase I Homologs move to equator How they line up varies (genetic diversity!) = law of independent assortment Spindle fibers attach to centromeres
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Anaphase I Spindle fibers contract Homologs pulled to poles
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What happens when anaphase goes wrong? If tetrads are not pulled apart evenly, chromosomal abnormalities result Trisomy (extra chromosome Monosomy (missing) Anaphase Mistakes
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Telophase I Chromosomes go to poles Cytokinesis occurs; cells split in two 4n 2n (halfway to goal!)
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Prophase II New spindle fibers form 2n (2 sets of 23 = 46)
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Metaphase II Same as metaphase I Homologs move to equator Spindle fibers attach to centromeres
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Anaphase II Sister chromatids pulled apart at centromere Chromatids move to opposite poles
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What happens when anaphase goes wrong? If chromatids are not pulled apart evenly, chromosomal abnormalities result Trisomy (extra chromosome Deletion/Monosomy (missing) Anaphase Mistakes – A Second Chance
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Telophase II Cytokinesis takes place, splitting two cells into four (sperm or eggs) 2n n
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Meiosis Overview Watch Meiosis Video Watch Meiosis Video
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Overview 2n2n 4n4n 2n2n 2n2n n n n n Precursor cell INTERPHASE MEIOSIS I MEIOSIS II diploid haploid polyploid
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