Meiosis- The Formation of Sex Cells

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Meiosis- The Formation of Sex Cells
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I. Introduction to Meiosis

• A. Purpose - to make sex cells for reproduction.
• B. Why cant mitosis do this?
• 1. Mitosis would make too many
  chromosomes.

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• 2. Ex. If a fruit fly has 4 chromosomes,
  each of its children would need to have 4
  chromosomes.
• A. Mom and dad each give 4 chromosomes,
  offspring has 8 and would die.

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• 3. The organism would have double the number
  of chromosomes which would cause the organism to
  die.
• 4. Mom gives 2 and dad gives 2, new organism
  has 4 just like parents.

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• C. Diploid Chromosome - the total number of
  chromosomes the individual should have.
• 1. Ex. Human 46
• Ex. Lettuce 18
• 2. Diploid number 2n

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• D. Haploid Chromosome Number - half the number
  of chromosomes an organism should have.
• 1. Ex. Human 23
• 2. Ex. Lettuce 9
• 3. Haploid Chromosome n

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II. Workings of Meiosis

• A. Meiosis is - the formation of sex cells (eggs
  and sperm)
• 1. Gametes - another name for sex cells.
• A. Gametes have haploid chromosome
  number

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• B. Meiosis is the making of eggs and sperm with
  half the number of chromosomes as the parent
  cell.
• 1. Female egg 23
• 2. Male sperm 23

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• C. Two Main Stages of Meiosis
• 1. Meiosis I
• a. Prophase I
• b. Metaphase I
• c. Anaphase I
• d. Telophase I

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• 2. Meiosis II
• a. Prophase II
• b. Metaphase II
• c. Anaphase II
• d. Telophase II
• D. There is only one interphase

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III. Meiosis I

• A. Interphase
• 1. Replication of chromosomes occurs.
• 2. This is the only interphase.

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• B. Prophase I
• 1. Chromatids appear
• 2. Centrosome splits into two centrioles.
• 3. Centrioles move to the opposite poles.
• 4. Spindle fibers appear

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• 5. Nucleolus disappears
• 6. Nuclear membrane disappears
• 7. Tetrads form - two pairs of homologous
  chromosomes joining together.

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• 8. Crossing-over occurs here - genes are
  exchanged between homologous chromsomes.
• a. Creates variation in new offspring

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• C. Metaphase I
• 1. Tetrads line up at the equator.

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• D. Anaphase I
• 1. Tetrads separate.
• 2. Chromatids are pulled to the opposite
  poles.

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• E. Telophase I
• 1. Chromatids still visible.
• 2. Nucleolus and Nuclear Membrane reforms.
• 3. Centrioles replicate
• 4. Two new cells.

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IV. Meiosis II

• A. No interphase begins this phase.
• 1. 2 cells with the diploid chromosome
  number begin this stage.
• 2. Meiosis II will always have two cells
  in each phase.

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• B. Prophase II
• 1. Nuclear membrane and nucleolus
  disappear.
• 2. Centrosome splits into two centrioles.
• 3. Centrioles move to the poles.

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• 4. Spindle fibers form.
• 5. Chromatids are visible and attach to spindle
  fibers.

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• C. Metaphase II
• 1. Chromatids line up on the equator.

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• D. Anaphase II
• 1. Centromeres separate.
• 2. Chromosomes are pulled to opposite
  poles.

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• E. Telophase II
• 1. Centrioles replicate
• 2. Nuclear membrane and nucleolus
  reappear.
• 3. Spindle fibers disappear.

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• 4. 4 new cells are formed
• a. Only have half the number of chromosomes
  as the parent cell.
• b. Males - all 4 remain alive and viable.
• c. Females - one grows rest die off.

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V. Meiosis-Mitosis Comparison

• A. Mitosis
• 1. 4 phases
• 2. Produces 2 diploid cells
• 3. Daughter cells identical to parent

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• B. Meiosis
• 1. 8 phases
• 2. 4 haploid cells produced
• 3. Daughter cells not identical to parent
• 4. Contain tetrads and crossing-over.

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