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Meiosis
Formation of Gametes (Eggs
& Sperm)
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Facts About Meiosis
Preceded by interphase which
includes chromosome replication
Two meiotic divisions --- Meiosis
I and Meiosis II
Called Reduction- division
Original cell is diploid (2n)
Four daughter cells produced that
are haploid (1n)
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Facts About Meiosis
Produces gametes (eggs & sperm)
Occurs in the testes in males
starting at puberty (Spermatogenesis)
Occurs in the ovaries in females
(Oogenesis). Meiosis I finishes before the
female is born and Meiosis II happens for one egg each
month before fertilization. Therefore the female is
born with all her eggs but are not ready for ferilization
until Meiosis II is completed.
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Why Do we Need Meiosis?
It is the fundamental basis of
sexual reproduction
Two haploid (1n) gametes are
brought together through
fertilization to form a diploid
(2n) zygote
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Fertilization – “Putting it all
together”
2n = 6
1n =3
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Replication of Chromosomes
Replication is the
process of
duplicating a
chromosome
Occurs prior to
division
Replicated copies
are called sister
chromatids
Held together at
centromere
Occurs in
Interphase
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Meiosis: Two Part Cell Division
Sister
chromatids
separate
Homologs
separate
Meiosis
I
Diploid
Meiosis
II
Diploid
Haploid
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Meiosis I: Reduction Division
Spindle
fibers
Nucleus
Early Prophase I
(Chromosome
number doubled)
Late
Prophase I
Metaphase I
Nuclear
envelope
Anaphase I
Telophase I
(diploid)
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Prophase I
Early prophase
Homologs pair.
Crossing over
occurs.
Late prophase
Chromosomes condense.
Spindle forms.
Nuclear envelope
fragments.
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Tetrads Form in Prophase I
Homologous chromosomes
(each with sister
chromatids)
Join to form a
TETRAD
Called Synapsis
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Crossing-Over
 Homologous
chromosomes in
a tetrad cross
over each other
 Pieces of
chromosomes or
genes are
exchanged
 Produces
Genetic
recombination in
the offspring
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Crossing-Over
Crossing-over multiplies the already huge
number of different gamete types produced
by independent assortment at points called
chiasmata.
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Metaphase I
Homologous pairs
of chromosomes
align along the
equator of the
cell
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Anaphase I
Homologs separate and
move to opposite poles.
Sister chromatids remain
attached at their
centromeres.
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Telophase I
Nuclear envelopes
reassemble.
Spindle disappears.
Cytokinesis divides cell into
two.
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Meiosis II
Gene X
Only one homolog of each
chromosome is present in
the cell.
Sister chromatids carry
identical genetic
information.
Meiosis II produces gametes with
one copy of each chromosome and
thus one copy of each gene.
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Meiosis II: Reducing
Chromosome Number
Prophase II
Metaphase II
Anaphase II
Telophase II
4 Genetically
Different haploid
cells
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Prophase II
Nuclear envelope
fragments.
Spindle forms.
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Metaphase II
Sister chromatids align
along equator of cell.
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Equator
Anaphase II
Pole
Sister chromatids
separate and move
to opposite poles.
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Telophase II
Nuclear envelope
assembles.
Chromosomes
decondense.
Spindle disappears.
Cytokinesis divides
cell into two.
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Results of Meiosis
Gametes (egg & sperm)
form
Four haploid cells with one
copy of each chromosome
One allele of each gene
Different combinations of
alleles for different genes
along the chromosome
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Meiosis Animation
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Is there an advantage to
sexual reproduction?
Yes, Sexual reproduction creates
differences or diversity. Because
sexual reproduction creates so
many different combination, when
the environment changes, there is
a better chance that some
organisms will survive.
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Gametogenesis
Oogenesis
or
Spermatogenesis
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Spermatogenesis
Occurs in the
testes
Two divisions
produce 4
spermatids
Spermatids mature
into sperm
Men produce about
250,000,000
sperm per day
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Spermatogenesis in the Testes
Spermatid
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Spermatogenesis
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Oogenesis
Occurs in the ovaries
Two divisions produce 3 polar bodies
that die and 1 egg
Polar bodies die because of unequal
division of cytoplasm
Immature egg called oocyte
Starting at puberty, one oocyte
matures into an ovum (egg) every 28
days
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Oogenesis in the Ovaries
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Oogenesis
First polar body
may divide
(haploid)
a
X
a
a
X
a
Mitosis
Oogonium
(diploid)
A
X
X
Primary
oocyte
(diploid)
Polar
bodies
die
X
Meiosis I Meiosis II
(if fertilization
occurs)
A
X
A
X
Secondary
oocyte
(haploid)
Ovum (egg)
A
Mature
egg
X
Second
polar body
(haploid)
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Twins
Division after Fertilization
Conjoined (Siamese) Twins
Brittany and Abby Hensel
of Minnesota
http://www.top1.ro/Nicolae/abigail-sibrittany-hensel-unite-pentrutotdeauna.html
Comparing Mitosis
and Meiosis
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Comparison of Divisions
Mitosis
Meiosis
2
Number of
divisions
1
Number of
daughter cells
2
4
Yes
No
Same as parent
Half of parent
Where
Somatic cells
Germ (gamete) cells
When
Throughout life
At sexual maturity
Growth and
repair
Sexual reproduction
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Genetically
identical?
Chromosome #
Role