meiosis - warren county public schools notes.pdf · mitosis primary oocyte (diploid) meiosis i...
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Facts About Meiosis ü 4 haploid daughter cells are
produced (each contain half the number of chromosomes as the original cell)
ü Produces gametes (eggs & sperm) ü Occurs in the testes in males
(Spermatogenesis) ü Occurs in the ovaries in females
(Oogenesis)
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Meiosis: Two Part Cell Division
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Homologous Chromosomes separate
Sister chromatids separate
Diploid
Meiosis I
Meiosis II
Diploid
Haploid
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Why Do we Need Meiosis? ü sexual reproduction ü two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote
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Meiosis I: Reduction Division
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Nucleus Spindle fibers
Nuclear envelope
Early Prophase I (Chromosome number doubled because has gone through S phase of interphase)
Late Prophase I Metaphase I
Anaphase I Telophase I (haploid)
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Prophase I
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Early prophase ü Homologs pair through synapsis forming tetrads. ü Crossing over occurs.
Late prophase ü Chromosomes condense. ü Spindle forms. ü Nuclear envelope breaks down.
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Crossing-Over
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Crossing-over multiplies the already huge number of different gamete types produced by independent assortment
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Metaphase I
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Homologous pairs of chromosomes align across from each other along the equator of the cell. Random alignment is called Independent Assortment.
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Anaphase I
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Homologs separate and move to opposite poles. Sister chromatids remain
attached at their centromeres.
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Telophase I
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Nuclear envelopes reassemble.
Spindle disappears. Cleavage furrow forms and cytokinesis divides cell into two.
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Telophase II
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Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two.
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Results of Meiosis
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Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome
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Spermatogenesis ü Occurs in the testes ü Meiosis produces 4
spermatids ü Spermatids mature into
sperm ü Men produce about
250,000,000 sperm per day
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Oogenesis ü Occurs in the ovaries before birth
ü Two divisions produce 3 polar bodies that die and 1 egg
ü Polar bodies die because of unequal division of cytoplasm
ü Immature egg called oocyte
ü Starting at puberty, one oocyte matures into an ovum (egg) every 28 days
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Oogenesis
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Oogonium (diploid)
Mitosis
Primary oocyte (diploid)
Meiosis I
Secondary oocyte (haploid)
Meiosis II (if fertilization occurs)
First polar body may divide (haploid) Polar
bodies die
Ovum (egg)
Second polar body (haploid)
a
A
X
X
a
X
A X
a
X
a
X
Mature egg
A
X
A
X
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Comparison of Divisions
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Mitosis Meiosis Number of
divisions 1 2
Number of daughter cells 2 4
Genetically identical cells
made? Yes No
Chromosome # Same as parent Half of parent
Where Somatic cells Germ cell to make gametes
When Throughout life At sexual maturity
Role Growth and repair Sexual reproduction
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