Meiosis

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Meiosis

• Cell Divison

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You have 46 Chromosomes

• Each of your body cells has 46 chromosomes, or 23
  pairs.
• Each pair is made of HOMOLOGOUS CHROMOSOMES

Homo same These chromosomes contain the same
genes in the same order
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You have 46 Chromosomes

• Half of your chromosomes (23) came from your mom
• Half of your chromosomes (23) came from your dad

Each parent gave you one chromosome from a
homologous pair.
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Homologous Pairs
From Mom From Dad
gene
From Mom From Dad
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MITOSIS Review

• When your BODY cells (autosomal) divide, they
  make exact copies
• Results two cells with the same type and number
  of chromosomes as the parent cell.
• Many animals reproduce ASEXUALLY through
  mitosis
• ex flatworms, sponges, jellyfish

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This is a problem for most other animals.

• If humans were created through mitosis
• 1st generation
• 46 from mom 46 from dad 92 chromosomes for
  baby
• then
• 2nd generation
• 92 from mom 92 from dad 184 chromosomes for
  baby
• And so on, and so on, and so on

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MEIOSIS

• A second type of cell division that occurs in the
  SEX CELLS (gametes sperm and egg)
• Division creates four cells with half the number
  of chromosomes as the parent cell.

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Haploid vs Diploid

• Body cells (autosomal cells) are DIPLOID
• DI 2 Each cell has 2 copies of each chromosome
• These are considered 2n
• n is the chromosome number

• Sex cells are HAPLOID
• Each sex cell has HALF the normal number of
  chromosomes
• These are considered n

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Sperm and Egg

• Sperm are the sex cells in males
• Eggs are the sex cells in females
• Both are produced through MEIOSIS

Any reproduction that involves SPERM and EGGS is
considered SEXUAL REPRODUCTION.
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Steps of Meiosis

• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

First Division Second Division Meiosis
contains TWO divisions whereas Mitosis only has
one! Similar to MitosisMeiosis does
experience interphase. Chromosomes are
replicated during S phase.
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Steps of Meiosis

• Prophase I
• Chromatin coil and condense, chromosomes are
  visible.
• Spindle form
• Homologous chromosomes line up gene by gene to
  form a tetrad.
• Tetrad two homologous chromosomes made of two
  sister chromatids paired tightly
• Crossing over occurs.

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Crossing Over

• Occurs between two non-sister chromatids.
• Can happen many times or not at all
• Average 2 to 3 crosses per each pair of
  homologues.
• Results in genetic variation
• Explains why siblings look alike but not
  identical.

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Interphase All 23 pairs of chromosomes
duplicate. Prophase begins.
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Steps of Meiosis

• Metaphase I
• Spindle fibers attach to centromere of each
  chromosome.
• Tetrads pulled to the center
• Line up beside each other
• In Mitosis they line up end to endnot paired.

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Meiosis Chromosome line up beside their
homologous pairs. Mitosis Chromosomes line up
end to end
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Steps of Meiosis

• Anaphase I
• Homologous chromosomes separate
• Head to opposite sides of cell
• Centromeres do not split
• Unlike mitosis
• Ensures that each new cell will receive one
  chromosome from each homologous pair

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Meiosis Sister Chromatids stay
together Mitosis Sister Chromatids
separate
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Steps of Mitosis

• Telophase I
• Prophase Reversed
• Cytokenisis Occurs
• Still has one chromosome from each homologous
  pair
• Start the division process over again!

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Meiosis Still have one chromosome from each
homologous pair need to separate them in order
for each sex cell to have ½ of the genetic
material.
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Steps of Meiosis

• Prophase II
• Chromosomes Condense
• Spindle Form and attach to centromere
• NOTICE.did not undergo inerphase therefore the
  chromosomes did not replicate again!
• Metaphase II
• Chromosomes line up at the equator
• End to end this time.like mitosis
• Anaphase II
• Sister chromatids split apart and move to
  opposite poles
• Telophase II
• Nuclei reform
• Spindle Breaks down
• Cytokenisis

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Meiosis I Meiosis II
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Results of Meiosis

• Four haploid cells
• Contains 1 chromosome of each kind

n
2n
n
n
n
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Meiosis Animation

• http//www.youtube.com/watch?vD1_-mQS_FZ0