Meiosis

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Meiosis
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Meiosis Overview

• Meiosis - cell division in which gametes are made
  (eggs and sperm). The cells made will have half
  the number of chromosomes of the parent cell.
• Homologous chromosomes with a corresponding
  partner. (one from mom one from Dad)

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• Haploid a cell that contains only one of each
  homologous chromosome pair half the
  total.(sperm, egg)
• Diploid a cell that contains both homologous
  chromosomes full set.(body cells)

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Meiosis I
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Nucleus
Interphase I

1. Cell starts out as a DIPLOID cell. Either a
   SPERMATOCYTE or an OOCYTE
2. Chromatin replicates.

Cell Membrane
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Prophase I

• Nucleus starts to disappear.
• Sister Chromatids pair up to form chromosomes,
  which now can be seen.
• Homologous Chromosomes pair up to form tetrads,
• (2 homologs together).
• 4. Centrioles move to the poles
• 5. Crossing-over can occur during the latter part
  of this stage.

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Crossing Over

• Crossing over the swapping of DNA which occurs
  between homologs.
• It gives us genetic variation or differences.
• This is one reason why you are your own unique
  person.

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Metaphase I

1. Spindle Fibers appear.
2. Tetrads line up along the middle of the spindle.
3. Fibers attach to centromeres.

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Anaphase I

• Spindle fibers pull the tetrads apart, with
  sister chromatids remaining together.
• 2. Chromosomes are pulled to the centrioles.

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Telophase I

• Spindle Fibers and centrioles disappear.
• Nucleus reappears around each of the new sets of
  chromosomes.
• Cell starts to form a break at the middle and
  starts to pinch in at the sides.

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Telophase I (continued)

1. Cytokinesis occurs

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Meiosis II
Continues with both cells, but this will only
show one
Movie
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Prophase II

• 1. Chromosomes are already visible
• 2. Nucleus disappears
• 3. Centrioles move to the poles

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Metaphase II

• 1. Spindle Fibers appear
• 2. Chromosomes are lined up along middle of cell

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Anaphase II

1. Spindle fibers pull sister chromatids to each
   pole.

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Telophase II

• 1. Spindle fibers and centrioles disappear
• 2. A nucleus appears around each set of
  chromosomes
• 3. Cell starts to pinch in at the center

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Telophase II (Continued)

• 1. Both cells divide (cytokinesis), leaving 4 new
  cells
• 2. Each with half the number of chromosomes the
  original cell had.
• 3. Each cell is now HAPLOID, called a gamete.
• 4. Gametes can be sperms or eggs.

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Spermatogenesis- making sperm cells by meiosis.
Makes 4 sperm
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Oogenesis - making egg cells by meiosis. Makes 1
egg and 3 polar bodies
Movie
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Comparing Mitosis and Meiosis

• Mitosis
• Happens all over body
• IPMAT
• Makes 2 cells
• Diploid cellsfull set of chromosomes
• Make new body cells
• 1 set of divisions

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Comparing Mitosis and Meiosis

• Meiosis
• Reproductive organs
• IPMAT - PMAT
• Makes 4 cells
• Haploid1/2 set of chromosomes
• Makes sex cells (Gametes)
• 2 sets of divisions
• Crossing over

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Mitosis and Meiosis

• Mitosis allows for genetic continuity within an
  organism.
• Meiosis allows genetic variety during sexual
  reproduction.