lesson overview 11.4 meiosis hill.com/sites/0072495855/student_view0/chapter28/animation__...
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Lesson OverviewLesson Overview11.4 Meiosis11.4 Meiosis
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html
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Chromosome Number
Homologous chromosomes – for each chromosome in normal body cells, there is one that comes from the mother and one that comes from the father
Ex.
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Diploid vs. Haploid Cells
DIPLOID CELLS (somatic, or normal body cells)
• A cell that contains both sets of homologous chromosomes is diploid, meaning “two sets.”
• Represented by the symbol 2n
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Diploid vs. Haploid Cells
HAPLOID CELLS (gametes, or sex cells)
– Some cells contain only a single set of chromosomes, and therefore a single set of genes; such cells are haploid, meaning “one set.”
– Gametes of sexually reproducing organisms are HAPLOID
– can be written as n
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Phases of Meiosis
– Meiosis: process in which the number of chromosomes per cell is cut in half
– Meiosis usually involves two distinct divisions, meiosis I and meiosis II.
– By the end of meiosis II, the ONE diploid cell becomes FOUR haploid cells
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Meiosis I
– Before meiosis I, the cell goes through INTERPHASE
– Each replicated chromosome consists of two identical chromatids joined at the center
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PROPHASE I– Chromosomes pair up forming a tetrad, which
contains four chromatids
– They undergo a process called crossing-over
– Crossing-over is important because it is a source of genetic variation
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METAPHASE I / ANAPHASE I
• Paired homologous chromosomes line up across the center of the cell (metaphase)
• Homologous chromosome pair pulled toward opposite ends of the cell (anaphase)
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TELOPHASE I / CYTOKINESIS
• Nuclear membrane forms around each cluster of chromosomes (telophase)
• Two new cells formed (cytokinesis)
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MEIOSIS I – IMPORTANT EVENTS
1. In prophase I of meiosis, crossing over
occurs
2. During metaphase I of meiosis independent
assortment occurs
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Meiosis II
– The two cells produced by meiosis I now enter a second meiotic division (MEIOSIS II)
– NEITHER CELL GOES THROUGH INTERPHASE (no replicating of DNA)
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MEIOSIS II
• Basically same as phases as mitosis
• By the end of meiosis II, the diploid cell becomes four haploid cells (GAMETES)
• Male gamete = SPERM
• Female gamete = EGG
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Gametes to Zygotes
Fertilization—the fusion of male and female gametes—generates new combinations of alleles in a zygote
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Comparing Meiosis and Mitosis
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__comparison_of_meiosis_and_mitosis__quiz_1_.html
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