Meiosis: (Fig. 7-1) Chp. 7 (pp. 140-157)

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Meiosis (Fig. 7-1)Chp. 7 (pp. 140-157)

• Production of sex cells
• 1/2 the number of chromosomes

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Link!-video
Gametes-reproductive cells
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Stem Cells

• A cell that can differentiate to become a
  specific type of cell
• Stem Cells become cells that can do mitosis or
  meiosis

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Definitions for Meiosis

• Homologous Chromosomes-
• Chromosomes that are similar in size, shape and
  content
• Crossing Over-
• Portions of one chromatid are
• shared with a chromatid on
• another (Homologous
• Chromosome)
• - Occurs during meiosis

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Definitions for Meiosis

• Haploid- containing one set of chromosomes
• 23 for humans (sex cells)
• Diploid- containing two sets of chromosomes
• Zygote- fertilized egg
• 46 chromosomes in humans
• 78 chromosomes in dogs
• Some plants have thousands

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Meiosis contributes to Diversity

• Asexual Reproduction- a single parent passes
  copies of all of its genes to each of its
  offspring. (Clones- not diverse)
• Ex Binary Fission
• MORE GENETIC VARIATION MORE DIVERSITY
• Sexual Reproduction- two parents, each donating a
  haploid reproductive cell
• IMPORTANT for increasing diversity

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More diversity added by

• Mutations- a change in the nucleotide sequence of
  a gene.
• substitution of bases (A replaced by C)
• adding bases (A added to gene)
• deleting bases (A removed from gene)

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Prophase 1

• The chromosomes become visible
• Nuclear envelope breaks down

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Metaphase 1

• Pairs of homologous chromosomes move to the
  equator of the cell

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Anaphase 1

• Homologous chromosomes move to opposite poles of
  cell

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Telophase 1

• Chromosomes gather at the poles of the cell
• Cytoplasm divides

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Prophase 2

• A new spindle forms around the chromosomes

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Metaphase 2

• Chromosomes line up at the equator

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Anaphase 2

• Centromeres divide.
• Chromatids are now called chromosomes
• Chromosomes move to opposite poles of the cell.

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Telophase 2

• Cells begin split
• Cell membrane forms between cells
• Four gamettes are formed

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Comparison of Mitosis and Meiosis
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46 Chromosomes (Diploid)-23 pairs
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Trisomy 21 or Downs Syndrome
Extra Chromosome