MEIOSIS

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MEIOSIS
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MEIOSIS

• Genes
• Lined up on chromosomes
• Chromosome gt1,000 genes

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MEIOSIS

• Diploid and Haploid Cells
• Chromosomes occur in pairs, 1 from male and one
  from female
• Diploid 2 of each kind of chromosome (2n)
• Zygote
• Haploid 1 of each kind of chromosome (n)
• Sperm or Egg (Gamete)

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MEIOSIS
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MEIOSIS

• Homologous Chromosomes
• Chromosomes matched in homologous pairs
• Humans have 46 chromosomes (23 from egg, 23 from
  sperm)
• 23 SETS of matching pairs of chromosomes

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MEIOSIS

• Homologous Chromosomes
• 2 chromosomes make matched pair
• Both carry same genes
• Genes exist at same location
• Similar length
• Centromere in same location
• Not always identical
• Can carry different alleles for same gene
• Hair color location same, color different

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MEIOSIS
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MEIOSIS

• Lets review mitosis first!
• What are the 4 phases? What happens in each?
• Cell division produces new cells exactly like
  original cell

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MEIOSIS

• Why cant Mitosis be only means of cell
  division?? (DONT WRITE!)
• If pea plant, for example, produces 14
  chromosomes
• Gametes would each contain 14 chromosomes
• Each offspring would have 28
• Offspring of next generation would have 56
• DOES NOT WORK!!!

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MEIOSIS

• Meiosis
• Sexual reproduction
• Production of gametes
• Reduction of chromosomes
• 2 Phases

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MEIOSIS

• Sexual Reproduction
• Sperm (male gamete) and Egg (female gamete) fuse
• Each gives 23 chromosomes to zygote 46 total
  chromosomes in offspring

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MEIOSIS

• Production of gametes
• BOTTOM LINE Meiosis occurs in specialized body
  cells of each parent to produce gametes for
  sexual reproduction.

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MEIOSIS

• Reduction of Chromosomes
• Allows for offspring to have same number of
  chromosomes as parents (not double)
• Gametes (egg or sperm) with ½ number of
  chromosomes as parents body cells

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MEIOSIS

• 2 Phases
• MEIOSIS I Genetic Variation
• MEIOSIS II Mitosis-Like

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MEIOSIS

• MEIOSIS I INTERPHASE
• Cell replicates chromosomes
• Result 2 identical sister chromatids held
  together by centromeres

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MEIOSIS

• MEIOSIS I PROPHASE I
• Similar to Mitosis
• DNA coils to form Chromosomes
• Homologous chromosomes line up gene by gene along
  length
• Tetrad 2 homologous chromosomes w/ 2 sister
  chromatids each

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MEIOSIS

• MEIOSIS I PROPHASE I
• TETRAD 4 chromatids
• Chromatids pair tightly
• Crossing over non-sister chromatids can break
  and exchange genetic information
• Estimation 2 or 3 crossovers
• NEW GENETIC COMBINATION!

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MEIOSIS

• MEIOSIS I Metaphase I
• Centromere attaches to spindle
• Spindles pull tetrads into middle
• Unique to meiosis
• Tetrad homologous pair of chromosomes

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MEIOSIS

• MEIOSIS Anaphase I
• Begins as homologous chromosomes separate and
  move to opposite ends of cells
• Each has two chromatids
• Centromeres do not split
• Why is this important?
• Ensures that each new cell receives only 1
  chromosome from each homologous pair

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MEIOSIS

• MEIOSIS I Telophase I
• Spindle is broken down
• Chromosomes uncoil
• Cytoplasm divides ? 2 new cells
• One of each chromosome per new cell
• ANOTHER DIVISION IS NEEDED! WHY?

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MEIOSIS

• MEIOSIS II
• Like Mitotic division
• Phases Prophase II, Metaphase II, Anaphase II,
  Telophase II
• The only difference from mitotic division is that
  chromosomes do not replicate before they divide
  at centromeres.

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MEIOSIS

• Meiosis provides for genetic variation
• Crossing over provides rearrangement of genes
• Can happen anywhere at random on chromosome
• Endless possibilities of different chromosomes
• Genetic Recombination reassortement of
  chromosomes and genetic information they carry by
  crossing over

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MEIOSIS

• Nondisjunction
• Caused when chromosomes do not separate properly
  in meiosis I
• Chromosomes in tetrad move to one side of cell
  rather than separate
• Two gametes are result
• 1 has extra chromosome
• Other has missing chromosome

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MEIOSIS

• Nondisjunction
• When zygote formed with an extra chromosome,
  condition called trisomy
• Ex In humans, having an extra Chromosome 21,
  zygote has 47 chromosomes instead of 46. This
  zygote develops into a baby with Down syndrome.
• When zygote formed with a missing chromosome,
  condition called monosomy
• Ex In humans, a non-lethal monosomy zygote
  develops Turner syndrome. This results in a
  female that has only one x chromosome instead of
  two.

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MEIOSIS

• Polyploidy
• Organisms with more than the usual number of
  chromosome sets
• Rare in animalscauses death
• Frequent in plants
• Plants appear larger and healthier
• Ex Apples triploid (3n)
• Wheat hexaploid (6n)

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MEIOSIS