Download Meiosis - Norman Public Schools

Cell Divison
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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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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.
gene
From Mom
From Dad
From Mom
From Dad
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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
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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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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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”
Sperm are the sex cells in males
 Eggs are the sex cells in females
Both are produced through MEIOSIS
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Any reproduction that involves SPERM
and EGGS is considered SEXUAL
REPRODUCTION.
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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 Mitosis…Meiosis does
experience interphase.
**Chromosomes are replicated
during “S” phase.
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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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.
Interphase: All 23 pairs of
chromosomes duplicate.
Prophase begins.
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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 end…not paired.
Meiosis: Chromosome line up
beside their homologous pairs.
Mitosis: Chromosomes line up
end to end
Anaphase I
 Homologous chromosomes separate
◦ Head to opposite sides of cell
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Centromeres do not split
◦ Unlike mitosis
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Ensures that each new cell will receive one
chromosome from each homologous pair
Meiosis: Sister Chromatids stay
together
Mitosis: Sister Chromatids
separate
Telophase I
 Prophase Reversed
 Cytokenisis Occurs
 Still has one chromosome from each
homologous pair
 Start the division process over again!
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.
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
Meiosis I
Meiosis II
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Four haploid cells
Contains 1 chromosome of each kind
n
n
2n
n
n
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http://www.youtube.com/watch?v=D1_mQS_FZ0