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MEIOSIS
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1. The division of the nucleus of a diploid germ cells
D. Meiosis 1. The division of the nucleus of a diploid germ cells * Produces haploid gametes = egg or sperm * Reduces the chromosomes in the gametes to half of what is found in the somatic cells of the organism. (= reduction division)
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2. Why is meiosis necessary?
* For SEXUAL REPRODUCTION! * = egg sperm zygote * The union of egg and sperm is called Fertilization.
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* Without meiosis, the chromosome number would double with each generation.
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3. Meiosis Overview 1st Division 2nd Division 2nd Division
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Meiosis Bozeman - MEIOSIS
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A Germ Cell In Interphase
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4. Germ Cell Life Cycle: Interphase = Meiosis Stage I begins:
*G1 = diploid germ cell is growing in the testes or ovaries *S = DNA Replication occurs (the only time this happens during the entire process) *G2 = Cell prepares to divide Meiosis Stage I begins: * Homologous pairs of chromosomes will be separated into different cells.
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Prophase I
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MEIOSIS STAGE I 1. PROPHASE I: * Chromosomes become visible
* Homologous chromosomes pair = Synapsis * Crossing over occurs= a portion of a chromatid breaks off and is exchanged with corresponding chromatid portions on the other homologous chromosome.
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Crossing -Over
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Metaphase I
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MEIOSIS STAGE I METAPHASE I * Homologous pairs of chromosomes
line up on the equator of the cell. * Spindles attach to the centromeres
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Anaphase I
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MEIOSIS STAGE I 3. ANAPHASE I * Homologous pairs separate and
move to opposite centrioles * Nondisjunction could occur. (The failure of homologous chromosomes to separate during Anaphase I or failure of sister chromatids to separate during Anaphase II of meiosis which can result in a gamete having an abnormal chromosome number)
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Telophase I followed by Cytokinesis I
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MEIOSIS STAGE I TELOPHASE I then Cytokinesis
* Chromosomes gather at the centrioles. * Cytokinesis = The cytoplasm divides.
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Prophase II
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MEIOSIS STAGE II PROPHASE II * New spindles form in each of
the two new cells.
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Metaphase II
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MEIOSIS STAGE II METAPHASE II * Individual chromosomes line
up on the equator in each of the two cells. * Spindles attach to each chromosome in each cell.
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Anaphase II
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MEIOSIS STAGE II ANAPHASE II * Chromatids (copies of the DNA) are
pulled apart and move to opposite centrioles in each of the two cells. * Nondisjunction could occur.
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Telophase II and Cytokinesis II
OR
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MEIOSIS STAGE II TELOPHASE II and cytokinesis
* A nuclear envelope forms around each set of chromosomes in each of the two cells *Chromosomes return to chromatin *Nucleoli reappear *Cytokinesis = The cytoplasm divides *Gametes are formed (1 egg females; 4 sperm in males) *Each gamete is haploid.
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F. Spermatogenesis vs Oogenesis
Diploid Germ Cells Meiosis I Meiosis II * Uneven distribution of the cytoplasm during Meiosis I and Meiosis II results in the formation of 1 haploid egg cell and 3 polar bodies that will be destroyed by the body and not used for reproduction. *Results in the formation of 4 haploid sperm cells.
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Comparison of Mitosis to Meiosis
MEIOSIS ANIMATION Bozeman Biology Meiosis Video Bozeman animation – meiosis
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Begins with a diploid cell
Allows for growth and to maintain structure A cell with 16 chromosomes divides to produce 2 cells each having 16 chromosomes. Is preceded by DNA Replication Synapsis occurs Starts with a somatic cell Forms gametes. Results in 2 diploid cells A cell with 8 chromosomes divides to produce 4 cells each having 4 chromosomes Starts with a germ cell Occurs only in the testes or ovaries Crossing over occurs Results in haploid cells Produces somatic cells
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