meiosis
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| gametes | (eggs and sperm) contain half the complement of chromosomes found in other cells
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| fertilization | fusion of gametes
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| zygote | contains two copies of each chromosome
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| n = ? 2n = ? | 23; 46
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| Meiosis 1 | separates the two homologs
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| meiosis II | separates the two sister chromatids
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| prophase I | chromosomes condense (coil); nuclear membrane breaks down; homologous chromosomes undergo synapsis (pair up); cross over occurs (recombination) and chromsomes exchange segments
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| metaphase I | homologous chrosome pairs line up at random at the equatorial midline; microtubules attach to centromeres
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| anaphase I | centromeres do not divide; homologs separate and move to opposite poles
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| telophas I | nuclear membrane reforms; chromosomes uncoil; after cytokinesis two haploid cells are produced
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| meiosis II | similar to mitosis but with two main differences: 1) Haploid set of chromosomes 2) Sister chromatids are not identical because of cross over
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| Prophase II | chromosomes coil; spindle forms; nuclear membrane breaks down; each chromosome is composed of two sister chromatids attached at the centromere
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| metaphase II | chromosomes line up on the midline and attach to spindle fibers
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| anaphase II | centromeres divide; sister chromatids move to opposite poles
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| telophase II | nuclear membrane reforms; chromosomes uncoil; after cytokinesis four unique haploid cells are produced.
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| synapsis | homologous chromosomes pair along their lengths in prophase I; cross over
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| reduction division | homologs separate in anaphase I reducing chromosome # to 1n- this produces haploid gametes
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| 3 key mechanism that increase genetic diversity | crossing over; independent assortment; random fertilization
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| crossing over | DNA exchanges between chromatid pairs
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| nondisjunction | members of a chromosome pair fail to separate during anaphase
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| aneuploidy | -extra chromosome or missing a chromosome
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| trisomy 21 | Down syndrome - extra chromosome at chromosome 21
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