cell division - meiosis. meiosis process of cell division that reduces the number of chromosomes in...
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Cell Division - Meiosis
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Meiosis
• Process of cell division that reduces the number of chromosomes in new cells to half the number in the original cell.
• The haploid cells produced are called gametes.
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Meiosis
• Cells preparing to divide by meiosis undergo the G1, S and G2 phases of interphase.
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Meiosis
• Cells will then begin meiosis with a duplicate set of chromosomes.
• Meiosis is divided into two stages called Meiosis I and Meiosis II.
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Meiosis I
• Prophase I• DNA coils tightly into
chromosomes, spindles appear, and the nuclear membrane disassembles.
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Prophase I
• Each chromosome lines up next to its homologue. This is called synapsis.
• Each pair of homologous chromosomes is called a tetrad.
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Prophase I
• During this stage, crossing-over occurs.
• This is when genetic material breaks off and attaches to the adjacent chromatids.
• The result is genetic recombination.
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Metaphase I
• Tetrads line up randomly along the midline of the dividing cell.
• Spindles attach to the centromeres of the homologous chromosomes.
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Anaphase I
• Each homologous chromosome moves to an opposite pole.
• The chromosomes are separated randomly. This is called independent assortment.
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Telophase I
• Chromosomes reach the opposite ends of the cell and cytokinesis begins
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Meiosis II
• Meiosis II is not started by the copying of DNA.
• There may be a short resting stage but usually there is no true interphase.
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Prophase II
• Spindle fibers form and begin to move the chromosomes toward the midline of the cell
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Metaphase II
• Chromosomes move to the midline of the cell.
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Anaphase II
• The chromatids separate and move toward opposite poles of the cell.
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Telophase II
• Cytokinesis occurs and produces 4 cells
• A nuclear membrane forms around the chromosomes in each of the 4 new cells.
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Formation of Gametes
• In males, meiosis occurs in the testes. This is called spermatogenesis.
• Four sperm cells result from meiosis.
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Formation of Gametes
• In females, meiosis occurs in the ovaries. This is called Oogenesis.
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Formation of Gametes
• During cytokinesis for females, the cytoplasm is unequally divided.
• This results in only 1 egg cell being formed and 3 polar bodies.
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Asexual Reproduction
• Offspring from only one parent
• Offspring are genetically identical
• Occurs from binary fission or mitosis
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Sexual Reproduction
• Offspring from two parents
• Offspring are genetically different
• Meiosis