Download 1.3-Meiosis and Gametogenesis

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s
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Lesson
#1.3
Meiosis
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Mitosis/Meiosis Overview
• Difference between Mitosis and Meiosis
– Mitosis
•Reproduction
– Asexual, required 1 parent cell
– Division increases population
•Growth
– Organism develops from a single cell
– 100 trillion cells in a human body  from single cell
– RBC reproduce  1 million/second
•Repair
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Mitosis/Meiosis Overview
–Meiosis
•Reproduction
–Sexual, requires TWO parents
–Produces gametes: egg and sperm cells
–Leads to genetic variability and diversity
within species
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Meiosis
Formation of Gametes
(Eggs & Sperm)
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Facts About Meiosis
Preceded by interphase which
includes chromosome replication
Goes through Two meiotic divisions
Meiosis I and Meiosis II
Original cell is diploid (2n)
Four daughter cells produced, are
haploid (1n)
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Facts About Meiosis
Daughter cells contain half the
number of chromosomes as the
original cell
Produces gametes (eggs & sperm)
Occurs in the testes in males
Occurs in the ovaries in females
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More Meiosis Facts
 Start
with 46 double stranded
chromosomes
After 1 division - 23 double
stranded chromosomes
After 2nd division - 23 single
stranded chromosomes
 Occurs in our germ cells that
produce gametes
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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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A Replicated Chromosome
Gene X
Homologous
chromosomes
(same genes,
different alleles)
Sister
Chromatids
(same genes,
same alleles)
Meiosis Forms Haploid Gametes
 Meiosis must reduce the chromosome number
by half
 Fertilization then restores the 2n number
from mom
from dad
child
too
much!
meiosis reduces
genetic content
The right
number!
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Meiosis: Two Part Cell
Division
Sister
chromatids
separate
Homologs
separate
Meiosis
I
Meiosis
II
Diploid
Haploid
Haploid
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Meiosis I: Reduction Division
Spindle
fibers
Nucleus
Early
Prophase I
(Chromosome
number
doubled)
Late
Prophase
I
Nuclear
envelope
Metaphase
Anaphase Telophase I
I
I
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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
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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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Homologous Chromosomes
During Crossing-Over
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Crossing-Over
Crossing-over multiplies the already huge
number of different gamete types
produced by independent assortment
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Metaphase I
Homologous pairs of
chromosomes align
along the equator of
the cell
 Microtubules from one pole
are attached to the
centromere of one
chromosome of each tetrad,
while those from the other
pole are attached to the
other.
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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
-Homologous chromosomes move
until there is a haploid set at
each pole.
-Each chromosome consists of
linked sister chromatids.
-Cytokinesis by the same
mechanisms as mitosis
usually occurs simultaneously
Nucleus reforms, but there is
no further replication
of chromosomes.
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Meiosis II
•NO interphase between
Meiosis I and Meiosis II
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Meiosis II: Reducing
Chromosome Number
Prophase
II
Metaphase
Telophase
II
Anaphase
4 unique
II
II
haploid cells
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Prophase II
During prophase II a spindle
apparatus forms, attaches
to centromeres of each
sister chromatids, and
moves them around.
Spindle fibers from one pole
attach to the centromere of
one sister chromatid and
those of the other pole to
the other sister chromatid.
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Metaphase II
Chromosomes align
along equator of cell.
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Anaphase II
Equator
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 is
inherited from each parent
-Different combinations of
alleles for different genes
along the chromosome
Lesson#1.3
Gametogenesis
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Gametogenesis
Oogenesis
or
Spermatogenesis
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Spermatogenesis
Occurs in the
testes…begins at
puberty
Two divisions
produce 4
spermatids
Spermatids mature
into sperm
Men produce about
250,000,000
sperm per day
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Spermatogenesis
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
First polar body
may divide
(haploid)
a
Mitosis
Oogonium
(diploid)
X
A
X
Primary
oocyte
(diploid)
X
Meiosis II a
X
a
a
Polar
bodies
die
X
Meiosis I
(if fertilization
occurs)
A
X
A
X
Secondary
oocyte
(haploid)
Ovum (egg) Mature
egg
A
X
Second
polar body
(haploid)
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Putting it all together
 All your body cells (somatic cells) go through mitosis for
repair and growth
 46 chromosomes, 2n (diploid)
 At puberty, your sex cells (gametes) go through meiosis
to prepare for fertilization
 23 chromosomes, n (haploid)
FERTILIZATION
1 haploid egg + 1 haploid sperm = 1 diploid zygote
mitosis
Mitosis for
repair
Adult
mitosis
baby
embryo
mitosis
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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 cells
When
Throughout life
At sexual maturity
Growth and
repair
Sexual reproduction
Genetically
identical?
Chromosome #
Role
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