Meiosis: Formation of gametes

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Meiosis Formation of gametes

• Utilizing http//www.biology.com
• BioCoach Activity
• Meiosis

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Introduction to Meiosis

• Meiosis occurs only in sexually reproducing
  organisms.
• In animals, where the somatic (body) cells are
  diploid, the products of meiosis are the gametes.
  
• Some plants, algae and fungi have more complex
  cycles involving both mitosis and meiosis.

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Introduction to Meiosis

• A two-step cell-division process in sexually
  reproducing eukaryotes.
• Results in cells (typically gametes) with
  one-half the chromosome number of the original
  parental cell.
• One diploid cell (2N) results in 4 haploid cells
  (N).

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Introduction to Meiosis

• Meiosis consists of two successive nuclear
  divisions
• meiosis I
• meiosis II
• Each division consists of
• Prophase
• Metaphase
• Anaphase
• Telophase

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Premeiotic Interphase in Animal Cells

• All chromosomes are duplicated
• The two centriole are produced by the duplication
  of a single centriole

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Meiosis I Prophase I

• Chromosomes become visible as tetrads
• A tetrad is a pair of homologous chromosomes (ex.
  both copies of chr 3)
• Crossing-over occurs
• Homologous chromosomes exchange portions of their
  chromatids
• Produces new combinations of alleles

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Meiosis I Prophase I

• Nucleolus disappears
• Meiotic spindle forms
• Nuclear envelope disappears
• Prophase I consumes
• 90 of the time for meiosis

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Crossing Over During Meiosis

• crossing-over results in a physical exchange of
  equivalent segments of maternal and paternal
  homologous chromosomes

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Crossing Over During Meiosis
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Prophase I
Meiotic prophase I
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Meiosis I Metaphase I

• The centrioles are at opposite poles of the cell
• The pairs of chromosomes (tetrads) are arranged
  along the center of cell
• Spindle fibers are attached to one chromosome of
  each pair

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Metaphase I
Meiotic metaphase I
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Meiosis I Anaphase I

• Chromosomes of each tetrad separate and move to
  opposite poles
• Note Sister chromatids remain attached and move
  together to the pole

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Anaphase I
Meiotic anaphase I
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Meiosis I Telophase I

• Homologous chromosome pairs reach the poles of
  the cell
• Nuclear envelopes form around them
• Cytokinesis follows to produce two cells

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Telophase I
Meiotic telophase I
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Product of Meiosis I

• Two cells with a haploid number of chromosomes is
  produced. Each cell has two copies of each
  chromatid.
• The two cells have sets of chromosomes that are
  different from each other and from the parent
  cell that entered meiosis.

Animation
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Meiosis II

• There is no interphase between the two stages
  (chromosomes are not duplicated).

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Meiosis II Prophase II

• Centrioles duplicate
• Nuclear envelope breaks down
• Spindle apparatus forms

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Prophase II
Meiotic prophase II
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Comparing Prophase Iand Prophase II
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Meiosis II Metaphase II

• Spindle apparatus attaches to each chromosome
• Single chromosomes align along center of each cell

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Metaphase II
Meiotic metaphase II
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Comparing Metaphase Iand Metaphase II
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Meiosis II Anaphase II

• Centromeres separate
• Sister chromatids now individual chromosomes
  move toward the opposite poles of the cell.

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Anaphase II
Meiotic anaphase II
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Comparing Anaphase Iand Anaphase II
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Meiosis II Telophase II

• Nuclear envelope forms around each set of
  chromosomes
• Cytokinesis occurs
• Four daughter cells are produced, each with a
  haploid set of chromosomes.

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Telophase II
Meiotic telophase II
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Comparing Telophase Iand Telophase II
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Product of Meiosis II

• Four cells with a haploid number of chromosomes
  is produced. Each cell has a single copy of each
  chromatid.
• The four cells have sets of chromosomes that are
  genetically different from each other and from
  the parent cell that entered meiosis.

Animation
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Mitosis and Meiosis Reviewed
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(No Transcript)
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Compare meiosis to mitosis
How are they alike?

• In interphase prior to both, replication of DNA
  occurs
• Both are methods of cell division
• Both include Prophase, Metaphase, Anaphase and
  Telophase
• Cytokinesis occurs in both

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Compare meiosis to mitosis
How are they different?
Meiosis Two cell divisions Sexual Reproduction
--makes gametes Results in 4 haploid cells
Mitosis One cell division Asexual
reproduction --makes body cells Also for growth
and regeneration Results in 2 diploid cells
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Compare meiosis to mitosis
How are they different?
Meiosis Two parts - eight phases DNA in daughter
cells differs from parent cell Crossing over
occurs
Mitosis One part four phases DNA in daughter
cells is same as parent cell Crossing over
doesnt occur
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Compare anaphase in mitosis to anaphase I in
meiosis
Mitotic Anaphase
Meiotic Anaphase I
How are they different?