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Crossing Over in Meiosis I

Crossing Over in Meiosis I. Essential Question. What kinds of changes to chromosome occur during meiosis?. Concepts for Review. What happens to the number of chromosomes during meiosis? Describe 4 key differences between mitosis and meiosis Describe homologous chromosome

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Crossing Over in Meiosis I

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  1. Crossing Over in Meiosis I

  2. Essential Question What kinds of changes to chromosome occur during meiosis?

  3. Concepts for Review • What happens to the number of chromosomes during meiosis? • Describe 4 key differences between mitosis and meiosis • Describe homologous chromosome • T/F Sister chromatids are homologous

  4. Concepts for Review #1

  5. Concepts for Review #2 • Pairing of homologous chromosomes to form a tetrad • # of cell divisions • # of daughter cells • # of chromosomes in each daughter cell

  6. Concepts for Review #3 • Same length • Same genes • Not necessarily identical, i.e. can have different forms of a gene

  7. Concepts for Review #4

  8. Key Term • Crossing-overis a process during which chromosome segments are exchanged • Occurs during prophase I • Only occurs between homologous chromosomes

  9. Crossing-OverInvolves homologous chromosomes

  10. Crossing-OverResults in genetic variation

  11. You already observed this in your initial look at meiosis…

  12. Meiotic Mistakes • Nondisjunction is when chromosomes fail to separate during meiosis I or when chromatids fail to separate during meiosis II • Results in abnormal number of chromosomes that is usually, but not always, fatal • Example: Down’s syndrome (extra chromosome #21)

  13. Down’s Syndrome is a trisomy condition The resulting 2N gametes, if fertilized, create 3N zygotes (triploid).

  14. Consequences of Meiotic Mistakes • Polyploidy • when whole sets of chromosomes fail to separate at meiosis I or II • the resulting 2n gametes, if fertilized by normal sperm, create 3n zygotes (triploid) • organisms with an odd number of chromosome sets cannot produce viable gametes (Example: seedless fruits)

  15. TOTD • What do the letters A, B, a, b represent on the chromosomes pictured today? • Why do crossed chromosomes still work? • Compare the number of different types of chromatids before crossing-over with the number after crossing-over. • How does crossing-over relate to genetic recombination?

  16. Meiosis Online Animations  • Click here for animation of meiosis • Click here for animation of unique features of meiosis • Click here for Meiosis Square Dance

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