Meiosis

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Meiosis

• Cells are divided into two groups
• Reproductive cells gametes
• AND

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Meiosis

• Non-reproductive cells somatic cells
• Somatic cells include nerve cells, skin cells,
  muscle cells, skeletal cells, etc.
• A somatic cell contains a nucleus with TWO sets
  of chromosomes. One set of chromosomes comes
  from each parent (mother and father)

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Meiosis

• Each chromosome from one parent can be matched to
  a chromosome from the other parent.
• The matching chromosomes contain the same genes.
  They are called homologous chromosomes.

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Meiosis

• A somatic cell contains two complete sets of
  homologous chromosomes.
• This makes a somatic cell diploid.
• (Di two)
• The symbol for a diploid cell is 2N

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Meiosis

• In humans the total number of chromosomes in each
  somatic cell 46 (2N 46)
• The total number of genes found in humans is not
  known, but it is estimated at 20,000-25000.
• Each chromosome contains hundreds or thousands of
  genes

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Meiosis

• If an offspring received ALL of the genetic
  information from its parents it would end up with
• And the next generation would have
• And so on

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Meiosis

• Instead an organism produces gametes through
  meiosis.
• Each gamete is haploid contains one set of
  chromosomes.
• The symbol for a haploid cell is N.
• In humans N 23

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Meiosis

• Cell division in which a diploid cell produces 4
  haploid cells (sperm or eggs). Occurs only in
  reproductive organs.
• In flowers stamen and ovary
• In animals testes and ovary

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Meiosis

• Before meiosis begins the cell is in Interphase
  
• At the end of interphase the chromatin doubles
  and forms sister chromatids.
• Each pair of sister chromatids matches up with
  its homologous chromosome.

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Meiosis 1

• Prophase 1-
• The homologous chromosomes come together and form
  tetrads. (tetra 4)
• Nucleus dissolves.
• Centrioles move to opposite ends of cell.
• Spindle fibers form.

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Meiosis 1

• Prophase 1-
• During this first stage of meiosis, crossing over
  occurs.
• Crossing over genetic material gets swapped
  between the homologous chromosomes.

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Meiosis 1

• Metaphase 1
• Tetrads line up in middle of cell.
• Spindle fibers attach to each chromosome

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Meiosis 1

• Anaphase 1
• Tetrads pulled apart to opposite ends of cell
• Homologous chromosomes are separated.
• Sister chromatids remain attached

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Meiosis 1

• Telophase and Cytokinesis
• Nuclear membranes form
• Sister chromatids remain attached
• Cell is pinched into two new cells

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Meiosis 1

• End of Meiosis 1 Two new cells formed
• The cells are NOT identical to each other
• The cells are NOT identical to the parent cell
• Each cell is made of half the number of original
  chromosomes, but each chromosome is made of a
  pair of sister chromatids.

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Meiosis 1-Each new cell is unique!
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Meiosis 2

• Prophase 2 Occurs in the two new cells at the
  same time.
• Centrioles move to opposite ends of cell
• Spindle fibers form

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Meiosis 2

• Metaphase
• Sister chromatids line up in the middle of each
  cell

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Meiosis 2

• Anaphase 2
• Sister chromatids SEPARATE!
• Pulled to opposite ends of the cell.

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Meiosis 2

• Telophase 2 and Cytokinesis
• Two new cells form from each cell resulting in a
  total of 4 NEW haploid cells!
• Each cell is different from the others due to
  crossing over.

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Meiosis

• The description of meiosis above results in the
  production of 4 new cells in males (sperm/pollen).

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Meiosis

• Each egg (ova/ovules) of a female is much larger
  than a sperm.
• During the production of a female gamete, most of
  the cytoplasm ends up in one cell.
• The other three cells are called polar
  bodiesthey are non-functional.

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• Activity on Meiosis!