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Meiosis and Sexual Reproduction
Lecture 11
Objectives
 At the end of this series of lectures you should be able to:  Define terms.  Explain why sexual reproduction requires meiosis.
 List the phases of meiosis I and meiosis II and describe the events characteristic of each phase. Recognize the phases of meiosis from diagrams and micrographs.
 Describe the similarities and differences between mitosis and meiosis.  Explain how and why karyotyping is performed.
 Describe the causes and symptoms of Down syndrome.
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Meiosis
 Reduces the chromosome number from diploid to haploid.  Double division  Similar to mitosis in function and naming conventions
National Institutes of Health, Public Domain, http://commons.wikimedia.org/wiki/File:Meiosis_Overview.svg
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Meiosis
 Meisosis I
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Interphase – DNA replication
Prophase I
Metaphase I
Anaphase I
Telophase I
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Prophase II
Metaphase II
Anaphase II
Telophase II
 Meiosis II
Meiosis
 Prophase I  Chromatin condenses and chromosomes become visible
 Homologous chromosomes pair – synapsis
 Tetrads
 Crossing‐over
 Metaphase I
 Tetrad line‐up in middle of cell
 Anaphase I
 Homologous chromosomes separate and move to opposite poles.
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Marek Kultys, CC BY‐SA 3.0, http://en.wikipedia.org/wiki/Meiosis#mediaviewer/File:Meiosis_diagram.jpg Meiosis
 Telophase I
 Chromosomes reach opposite poles  New nucleus is haploid
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Marek Kultys, CC BY‐SA 3.0, http://en.wikipedia.org/wiki/Meiosis#mediaviewer/File:Meiosis_diagram.jpg Meiosis
 Prophase II
 If DNA uncoiled, it condenses.  If nuclear envelop formed it disappears.
 Metaphase II
 Chromosomes form a line in the middle of the cell.
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Marek Kultys, CC BY‐SA 3.0, http://en.wikipedia.org/wiki/Meiosis#mediaviewer/File:Meiosis_diagram.jpg Meiosis
 Anaphase II
 Sister chromatids separate and move toward opposite poles.  Telophase II
 Chromosomes reach opposite poles
 Nuclear envelope reforms
 DNA uncoils
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Marek Kultys, CC BY‐SA 3.0, http://en.wikipedia.org/wiki/Meiosis#mediaviewer/File:Meiosis_diagram.jpg Mitosis vs. Meiosis
 Mitosis results in 2 cells while meiosis results in 4 cells.  Mitosis is a single division while meiosis is a double division.
 Cells produced through mitosis are diploid while cells produced through meiosis are haploid.  Mitosis occurs throughout the body while meiosis occurs only in the gonads (ovaries and testes). 7
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Biology by OpenStax College is licensed under a Creative Commons Attribution 3.0 Biology by OpenStax College is licensed under a Creative Commons Attribution 3.0 8
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Karyotype
 A photograph of chromosomes during mitosis
 Homologous pairs
 Largest to smallest autosomes
 Sex chromosomes at end
 Examine chromosomes to identify abnormalities and study inheritance. Biology by OpenStax College is licensed under a Creative Commons Attribution 3.0 9
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Down Syndrome
 Trisomy 21  Three copies of chromosome 21
 Symptoms
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varying degrees of mental retardation
characteristic facial features
short stature
heart defects
shortened life span
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