Biology Chap. 6.2 – Process of Meiosis
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Meiosis
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-A form of nuclear division that creates four haploid cells from one diploid cell -Reduces chromosome number -Creates genetic diversity
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Two Rounds of Cell Division
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-Meiosis I -Meiosis II
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How many phases does each round of cell division have?
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4 phases
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Homologous Chromosomes
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- Two separate chromosomes: one from your rather and one from your mother -Similar to each other but are not copies of each other -Divided inMeiosis I
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Sister Chromatids
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-Duplicated chromosomes(copies) -Attached by centromeres -Divided in Meiosis II
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Meiosis I
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- Divides homologous chromosomes, producing two haploid cells with duplicated chromosomes. -DNA is copied before meiosis occurs
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Steps of Meiosis I
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- Prophase I - Metaphase I - Anaphase I - Telophase I
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Prophase I
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- Nuclear membrane breaks down - Centrosomes and centrioles move to opposite sides of the cell - Spindle fibers start to assemble - Duplicated chromosomes condense - Homologous chromosomes pair up
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Metaphase I
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- Homologous chromosomes line up on the middle of the cell by spindle fibers. - Result is 23 pairs of chromosomes (some from mother and father) line up along each side of the cell equator
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Anaphase I
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- Paired homologous chromosomes separate and move to opposite sides of the cell - Sister chromatids remain connected.
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Telophase I
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- Nuclear membrane forms again - Spindle fibers disassemble - Cell undergoes cytokinesis
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Result of Meiosis I
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- Two cells that each have a unique combination of 23 duplicated chromosomes coming from both parent.
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Meiosis II
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- Divides sister chromatids -Results in undoubled chromosomes -DNA isn't copied again before this stage
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Prophase II
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- Nuclear membrane breaks down - Centrosomes and centrioles move to opposite sides of the cell - Spindle fibers assemble
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Metaphase II
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- Spindle fibers align the 23 chromosomes along the cell equator
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Anaphase II
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- Sister chromatids are pulled apart and move to opposite sides of the cell
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Telophase II
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- Nuclear membranes form around each set of chromosomes at opposite ends of the cell - Spindle fibers break apart - Cell undergoes cytokinesis
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End Result of Meiosis II
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- Four haploid cells with a combination of chromosomes from both the mother and father.
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Gametogenesis
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- The production of gametes - Includes meiosis and other changes that produce a mature cell
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Sperm
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- Male gamete cell - Much smaller than egg - Main contribution to embryo is DNA
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Egg
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- Female gamete cell
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Sperm Formation
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- Starts with a round cell and ends by making a trees lined cell that can move rapidly
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Polar Bodies
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- Cells with little more than DNA that are eventually broken down
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How many of the 4 cells produced in each round actually become an egg?
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1 cell
