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Unit 3 Part II – The Cell
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A form of cell division happening in sexually reproducing organisms. Consists of 2 consecutive cell divisions (meiosis I and meiosis II). Produces 4 sex cells. Distributes maternal and paternal chromosomes evenly and randomly.
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Meiosis makes sex cells. For women it creates egg cells in the ovaries. For men it produces sperm cells in the testes.
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Meiosis is necessary to halve the number of chromosomes going into the sex cells. Why halve the chromosomes in gametes? At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents.
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Homologous Chromosome corresponding (matching) chromosomes from a female and male parent. Diploid Cell (2n) – a cell that contains both sets of homologous chromosomes. Haploid Cell (n) – a cell that contains one set of homologous chromosomes. Gene – segment of DNA that codes for a trait. Replicated Chromosome – a chromosome that has doubled it’s DNA. Non-Replicated Chromosome – a chromosome that has split and contains only one copy of DNA.
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1. Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). 2. Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives through crossing over and independent assortment.
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Same steps as Mitosis, but it divides twice to produce 4 daughter cells instead of 2. Daughter cells are not identical but similar. Meiosis I PMAT 2 similar cells Meiosis II PMAT 4 similar cells
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Interphase Cells duplicate their DNA. Prophase I sister chromatids pair up forming a homologous chromosome. Crossing Over When two chromosomes exchange portions of their chromatids. Occurs during Prophase I.
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Metaphase I Spindle fibers attach to the centromeres on each chromosome.
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Anaphase I Spindle fibers pull the homologous chromosomes toward opposite ends of the cell.
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Telophase I Meiosis I results in 2 haploid (n) daughter cells. Each with half the number of chromosomes as the original cell. Still replicated chromosomes.
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Prophase II 2 haploid (n) daughter cells. Each with half the number of chromosomes as the original cell.
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Metaphase II The chromosomes line up in a similar way to the metaphase stage of mitosis.
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Anaphase II The sister chromatids separate and move toward opposite ends of the cell.
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Telophase II Meiosis II results in four haploid (n) daughter cells. Daughter cells are non-replicated.
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OOGENESIS EGG PRODUCTION SPERMATOGENESIS SPERM PRODUCTION
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The haploid sperm (carrying 1 set of chromosomes) combines with the haploid egg (carrying 1 set of chromosomes).
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