Meiosis The Production of Reproductive Cells

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Meiosis The Production ofReproductive Cells
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Remember

• Mitosis produced 2 genetically identical cells
  with 2 copies of every chromosome (one from mom
  and one from dad)
• This is how body (somatic) cells are produced.
• REPRODUCTIVE cells do NOT go through MITOSIS
  because these cells MUST have HALF the number of
  chromosomes.

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Mitosis
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Meiosis Overview

• Purpose so that the number of chromosomes gets
  reduced by ½ in reproductive cells
• Body cells have 1 complete set of genes from each
  parent SO there are 2 sets of genes in every cell
• Diploid 2 complete sets of genes (chromosomes)
  Diploid in humans 46
• Gametes sex (reproductive) cells sperm/egg.
  Gametes only have 1 set of chromosomes in each
  cell.
• Haploid 1 set of genes (chromosomes) Haploid
  in humans 23

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Meiosis - Overview
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Meiosis I Interphase

• DNA replication takes place

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Meiosis I Prophase I

• Homologous same gene (1 from each parent)
• Chromatin condenses into chromosomes
• Homologous chromosomes pair up
• Crossing over homologs swap genes to increase
  genetic diversity

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Meiosis I Metaphase I

• Homologous chromosomes are moved to the middle

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Meiosis I Anaphase I

• Homologs are pulled apart
• Crossing over is completed when the chromosomes
  are separated.

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Error in Meiosis

• If the chromosomes fail to separate after
  crossing over, Nondisjunction will occur.
• This can happen during Anaphase I.
• This causes one daughter cell to have both copies
  of the homologous chromosome and the other to not
  have a copy of it.
• Downs Syndrome (Trisomy 21) is caused by
  Nondisjunction.

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Meiosis I Telophase I

• Nuclear envelopes may reform
• Cytokinesis (2 cells)
• Cells are NOT identical
• Cells are HAPLOID

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2 Divisions

• Unlike Mitosis, there are 2 Divisions in Meiosis.
• Meiosis I Big Idea Produce 2 Haploid Cells by
  SEPARATING HOMOLOGOUS CHROMOSOMES
• Meiosis II Big Idea Produce 4 Genetically
  Different Haploid Cells by SEPARATING SISTER
  CHROMATIDS
• There is NO Interphase or DNA replication between
  Meiosis I and Meiosis II

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Meiosis II Prophase II

• Cells are NOT identical
• Haploid cells prepare to divide

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Meiosis II Metaphase II

• Chromosomes line up in the middle

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Meiosis II Anaphase II

• Sister chromatids are pulled apart

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Meiosis II Telophase II

• Nuclear envelopes reform
• Cytokinesis
• 4 genetically DIFFERENT haploid cells

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Mitosis Meiosis

• Gametes (sex cells)
• 4 cells
• Haploid (1 set of genes)
• Genetically Different
• Crossing Over
• Some have moms copy others have dads
• Nondisjunction may occur
• 2 divisions

• Somatic (body) cells
• 2 cells
• Diploid (2 sets of genes 1 from each parent)
• Genetically Identical (to each other to
  original cell)

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• Watch the following Mitosis Meiosis Comparison
  Video http//www.youtube.com/watch?vBa9LXKH2ztU

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Gametes

• Gametogenesis is the production of Gametes (aka
  Meiosis)
• The production of sperm is called
  spermatogenesis.
• The production of eggs is called oogenesis.
• Gametes are haploid.
• 23 chromosomes in human sperm
• 23 chromosomes in human eggs

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Sperm

• Spermatogenesis produces 4 viable sperm cells
  through meiosis.
• Sperm contain either an X or Y chromosome. This
  determines the gender of the offspring.
• This occurs every 2-3 days in human males.

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Eggs

• In females, only one large egg is produced
  through meiosis.
• Cytokinesis is uneven. One cell receives the
  majority of the cytoplasm.
• The other 3 cells are called polar bodies and are
  not involved in reproduction.
• Eggs always contain one X chromosome.
• Prophase I occurs before birth in human females.
  Meiosis I is completed during puberty. No new
  eggs during lifetime.

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Oogenesis
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