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Meiosis Notes Diploid Cells Haploid Cells • 2 chromosomes of each kind in a • 1 chromosome of each kind in a cell. cell. • 1 from mom, 1 from dad. • 1n • All sex cells are haploid. • 2n • All body cells are diploid. • Made by meiosis. • Made by mitosis. • Gametes: Another name for sex cells: egg & sperm. • Zygote: A fertilized cell. • 2 gametes that are now together. Meiosis • The making of gametes (eggs & sperm). • Cells contain 1/2 the normal amount of chromosomes. (haploid) Why is Meiosis necessary? To make sex cells for sexual reproduction What is produced by Meiosis? • 4 haploid cells (1n) • Each cell is genetically different! Meiosis Quick Review • Makes 4 cells genetically different from parent cell & from each other • Makes 1n cells (1n or 2n?) • Makes gametes • Used for reproduction What is different about meiosis? • Many different variations in the gametes because of crossing over. Crossing over is when tetrads bind tightly, parts break off and are swapped between the chromatids • Divides twice, but only copies DNA once. Tetrad: when 2 homologous chromosomes bind together during prophase I of meiosis. Meiosis I (2 new cells are formed) Prophase I: *Tetrads condense *__Crossing Over occurs between chromosomes * Centrosomes move to opposite ends of cell to form spindle fibers Metaphase I: Tetrads line up in the middle of the cell. Anaphase I: Tetrads _Split and each set of sister chromatids moves to Opposite __ ends of the cell Telophase I: Cytokinesis (division of the cytoplasm) results in _2 new cells._ Meiosis II (4 new cells are formed) Prophase II: Metaphase II: Centrosomes move to opposite ends of the cell to form spindle fibers Replicated Chromosomes line up in the middle____ Anaphase II: Replicated Chromosomes seperte and move to opposite ends of the cell Telophase II: *__nuclear membrane__ forms around each set of chromosomes *Cytokinesis *___4 new daughter cells Mistakes in Meiosis Nondisjunction: when homologous chromosomes fail to separate from each other during meiosis Result: One gamete may have too many chromosomes, while the other new gamete is missing that chromosome Organisms with extra chromosomes can survive; organisms with too few usually do not survive Polyploidy: when organisms have more than the normal number of chromosome sets o Can be lethal for animals but not plants (plants just usually get bigger) Examples of Nondisjunction Genotype (X=Female sex chromosome, Y= male sex chromosome) X Turner’s Syndrome Trisomy X XXX Klinefelter’s syndrome XXY Jacob’s syndrome XYY Down syndrome TRISOMY 21 Patau’s syndrome TRISOMY 13 Edward’s syndrome TRISOMY 18 Mitosis DNA replicates in interphase vs. ONE division of chromosomes Separate sister chromatids Meiosis DNA replicates in interphase Makes 2 identical DIPLOID cells TWO divisions of chromosomes Separate homologous chromosomes (meiosis I) Separate Replicated Chromosomes (meiosis II) Makes 4 HAPLOID cells Chromosome # is identical to parent cell Chromosome # is HALF of the parent cell Used for replacing old cells, growth, and repair Used to make gametes (for reproduction)