Meiosis

Report
Meiosis
Reduction Division
Mike Clark, M.D.
Meiosis
• Meiosis is nicknamed reduction division
• It is a process where a cell divides (division) but
reduces the genetic material to ½ (reduction)
• This type of cell division occurs in the gametes
(sex cells)
• The original parent gamete cells
(spermatogonium and oogonium) are diploid
(2n) like a somatic cell but the final daughter
gamete cells (sperm and egg term ovum) are
haploid (1n)
Differences between Mitosis and Meiosis
• Mitosis occurs in somatic cells – meiosis
occurs in gametes
• Mitosis has one nuclear division – meiosis has
two nuclear divisions
• Mitosis produces two new daughter cells –
meiosis produces four new daughter cells
• The resultant daughter cells in mitosis have 46
pieces of genetic material – the resultant
daughter cells in meiosis has 23 pieces of
genetic material
Mother cell
(before chromosome replication)
Chromosome
replication
Chromosome
replication
2n = 4
MITOSIS
MEIOSIS
Replicated
chromosome
Prophase
Metaphase
Chromosomes
align at the
metaphase plate
Sister chromatids
separate during
anaphase
Metaphase I
Tetrads align at the
metaphase plate
Homologous chromosomes
separate but sister
chromatids remain together
during anaphase I
Daughter
cells of
mitosis
2n
Tetrad formed by
synapsis of replicated
homologous
chromosomes
Prophase I
Daughter cells
of meiosis I
2n
No further chromosomal
replication;
sister chromatids
Meiosis II
separate during
anaphase II
n
n
n
Daughter cells of meiosis II
(usually gametes)
n
Figure 27.5 (1 of 2)
Fig. 13-7-1
Interphase
Homologous pair of chromosomes
in diploid parent cell
Interphase in meiosis occurs
prior to the start of meiosis I.
46 pieces of genetic material
It consists of the same three
in parent cell
Phases as in mitosis – G1,S and
Chromosomes
replicate
G2.
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
In the S- phase of interphase DNA is duplicated. As noted before the
new DNA stays attached to the old (chromatid/chromosome) – thus
though we say there are 46 chromosomes – there is actually enough
genetic material for 92 chromosomes since one chromosome contains
two chromatids. When the chromatids separate they are considered
full chromosomes thus there is enough genetic material for 4 haploid
(gamete) cells. 92 divided by 4 equals 23 – thus 23 chromosomes in a
cell is termed haploid (1n). This is the amount of genetic material that
the sperm and egg contain.
Fig. 13-7-2
Interphase
Homologous pair of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Diploid cell with
replicated
chromosomes
Sister
chromatids
Meiosis I
1
Haploid cells with
replicated chromosomes
Homologous
chromosomes
separate
At the end of meiosis I
have two daughter cells
with 23 doublets of
genetic material
(23 chromosomes) but
each chromosome has
two chromatids – thus
enough for 46 singlet
chromosomes
Fig. 13-7-3
Interphase
Homologous pair of chromosomes
in diploid parent cell
At the end of
meiosis II – have
4 daughter cells
each with ½ the
amount of
genetic material
(haploid).
At the completion of
meiosis I (after
Chromosomes
cytokinesis I) - the two
replicate
Homologous pair of replicated chromosomes
cells enter into a phase
termed Interkinesis.
Interkinesis is similar to
Sister
Diploid
cell
with
chromatids
replicated
Interphase – but it lacks
chromosomes
Meiosis I
the S-phase – thus DNA
is not replicated – it is
Homologous
already enough DNA for
chromosomes
separate
4 haploid cells.
Haploid cells with
1
replicated chromosomes
Meiosis II
2 Sister chromatids
separate
Haploid cells with unreplicated chromosomes
Fig. 13-7-3
Interphase
Homologous pair of chromosomes
in diploid parent cell
46 pieces of genetic
material in parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
S- phase in
interphase
duplicates DNA (but
stays attached
chromatid/
chromosome– thus
enough genetic
material
for 4 haploid
(gamete)
cells
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
1 Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
2 Sister chromatids
separate
Haploid cells with unreplicated chromosomes
At the end of meiosis I
have two daughter
cells with 23 doublets
of genetic material
(23 chromosomes) but
each chromosome has
two chromatids – thus
enough for 46 singlet
chromosomes
At the end of meiosis II –
have 4 daughter cells each
with ½ the amount of
genetic material (haploid).
• Division in meiosis I occurs in four phases:
–
–
–
–
Prophase I
Metaphase I
Anaphase I
Telophase I and cytokinesis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
•
Three events are unique to meiosis, and all three
occur in meiosis l:
–
1. Synapsis and crossing over in prophase I:
Homologous chromosomes physically connect and
exchange genetic information
2. At the metaphase plate, there are paired
homologous chromosomes (tetrads), instead of
individual replicated chromosomes
3. At anaphase I, it is homologous chromosomes,
instead of sister chromatids, that separate
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-8a
Prophase I
Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Sister chromatids
remain attached
Metaphase
plate
Homologous
chromosomes
separate
Homologous
chromosomes
Fragments
of nuclear
envelope
Telophase I and
Cytokinesis
Anaphase I
Microtubule
attached to
kinetochore
Cleavage
furrow
Prophase I
• Prophase I typically occupies more than 90% of the
time required for meiosis
• Chromosomes begin to condense
• In synapsis, homologous chromosomes loosely pair
up, aligned gene by gene
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
1. Synapsis and crossing over in prophase I:
Homologous chromosomes physically connect and
exchange genetic information
• In crossing over, nonsister chromatids exchange
DNA segments
• Each pair of chromosomes forms a tetrad, a group
of four chromatids
• Each tetrad usually has one or more chiasmata, Xshaped regions where crossing over occurred
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Crossing Over
• Crossing over produces recombinant
chromosomes, which combine genes inherited from
each parent
• Crossing over begins very early in prophase I, as
homologous chromosomes pair up gene by gene
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• In crossing over, homologous portions of two
nonsister chromatids trade places
• Crossing over contributes to genetic variation by
combining DNA from two parents into a single
chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-12-1
Prophase I
of meiosis
Pair of
homologs
Nonsister
chromatids
held together
during synapsis
Fig. 13-12-2
Prophase I
of meiosis
Pair of
homologs
Chiasma
Centromere
TEM
Nonsister
chromatids
held together
during synapsis
Fig. 13-12-3
Prophase I
of meiosis
Pair of
homologs
Chiasma
Centromere
TEM
Anaphase I
Nonsister
chromatids
held together
during synapsis
Fig. 13-12-4
Prophase I
of meiosis
Pair of
homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Nonsister
chromatids
held together
during synapsis
Fig. 13-12-5
Prophase I
of meiosis
Pair of
homologs
Nonsister
chromatids
held together
during synapsis
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes
Without crossing over the newly formed cells would inherit either a
full chromosome containing only mom’s or dad’s genes on that
chromosome.
Possibility 2
Possibility 1
Metaphase II
By crossing over the situation above would not happen in that
each chromosome would have a piece of dad’s genetic
material and a piece of mom’s genetic material.
Without crossing over the 4 daughter cells below
would have no genetic recombination.
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
2. At the metaphase plate, there are paired
homologous chromosomes (tetrads), instead of
individual replicated chromosomes Metaphase I
• In metaphase I, tetrads line up at the metaphase
plate, with one chromosome facing each pole
• Microtubules from one pole are attached to the
kinetochore of one chromosome of each tetrad
• Microtubules from the other pole are attached to
the kinetochore of the other chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-8b
Prophase I
Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore
3. At anaphase I, it is homologous chromosomes, instead
of sister chromatids, that separate
Anaphase I
• In anaphase I, pairs of homologous chromosomes
separate
• One chromosome moves toward each pole, guided
by the spindle apparatus
• Sister chromatids remain attached at the
centromere and move as one unit toward the pole
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Telophase I and Cytokinesis
• In the beginning of telophase I, each half of the cell
has a haploid set of chromosomes; each
chromosome still consists of two sister chromatids
• Cytokinesis usually occurs simultaneously, forming
two haploid daughter cells
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• In animal cells, a cleavage furrow forms; in plant
cells, a cell plate forms
• No chromosome replication occurs between the
end of meiosis I and the beginning of meiosis II
because the chromosomes are already replicated
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-8c
Telophase I and
Cytokinesis
Anaphase I
Sister chromatids
remain attached
Homologous
chromosomes
separate
Cleavage
furrow
• Division in meiosis II also occurs in four phases:
–
–
–
–
Prophase II
Metaphase II
Anaphase II
Telophase II and cytokinesis
• Meiosis II is very similar to mitosis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-8d
Prophase II
Metaphase II
Anaphase II
Sister chromatids
separate
Telophase II and
Cytokinesis
Haploid daughter cells
forming
Prophase II
• In prophase II, a spindle apparatus forms
• In late prophase II, chromosomes (each still
composed of two chromatids) move toward the
metaphase plate
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Metaphase II
• In metaphase II, the sister chromatids are arranged
at the metaphase plate
• Because of crossing over in meiosis I, the two sister
chromatids of each chromosome are no longer
genetically identical
• The kinetochores of sister chromatids attach to
microtubules extending from opposite poles
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-8e
Prophase II
Metaphase II
Anaphase II
• In anaphase II, the sister chromatids separate
• The sister chromatids of each chromosome now
move as two newly individual chromosomes
toward opposite poles
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Telophase II and Cytokinesis
• In telophase II, the chromosomes arrive at opposite
poles
• Nuclei form, and the chromosomes begin
decondensing
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Cytokinesis separates the cytoplasm
• At the end of meiosis, there are four daughter cells,
each with a haploid set of unreplicated
chromosomes
• Each daughter cell is genetically distinct from the
others and from the parent cell
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-8f
Anaphase II
Sister chromatids
separate
Telephase II and
Cytokinesis
Haploid daughter cells
forming
Oogenesis
• Production of female gametes
• Begins in the fetal period
– Oogonia (2n ovarian stem cells) multiply by
mitosis and store nutrients
– Primary oocytes develop in primordial follicles
– Primary oocytes begin meiosis but stall in
prophase I and stay there for years – until the
woman ovulates
– This suspended prophase 1 can late in life lead to
Down’s Syndrome in the woman’s offspring
Fig. 15-16
Fig. 15-16b
Fig. 15-17
Error – crossing over occurred improperly – the exchange was with
non-homologous chromosomes.
Normal chromosome 9
Normal chromosome 22
Reciprocal
translocation
Translocated chromosome 9
Translocated chromosome 22
(Philadelphia chromosome)
Oogenesis
• Each month after puberty, a few primary
oocytes are activated
• One is selected each month to resume
meiosis I (the one to be ovulated)
• Result is two haploid cells
– Secondary oocyte
– First polar body
Oogenesis
• The secondary oocyte arrests in metaphase II
and is ovulated
• If penetrated by sperm the second oocyte
completes meiosis II, yielding
– Ovum (the functional gamete)
– Second polar body
Follicle development
in ovary
Meiotic events
Before birth
Oogonium (stem cell)
Follicle cells
Oocyte
Mitosis
Primary oocyte
Primordial follicle
Primary oocyte
(arrested in prophase I;
present at birth)
Primordial follicle
Growth
Infancy and
childhood
(ovary inactive)
Each month from
puberty to
menopause
Primary follicle
Primary oocyte (still
arrested in prophase I)
Secondary follicle
Spindle
Meiosis I (completed
by one primary oocyte
each month in response
to LH surge)
First polar body
Meiosis II of polar
body (may or may
not occur)
Polar bodies
(all polar bodies
degenerate)
Vesicular (Graafian)
follicle
Secondary oocyte
(arrested in
metaphase II)
Ovulation
Sperm
Second
Ovum
polar body
Meiosis II
completed
(only if
sperm
penetration
occurs)
Degenating
Ovulated secondary
oocyte
In absence of
fertilization, ruptured
follicle becomes a
corpus luteum and
ultimately degenerates.
corpus luteum
Figure 27.17
Final Result of Oogenesis
(formation of the egg)
• Four cells are produced – all 4 with a haploid set of
genetic material - but three of the cells are nonfunctional – termed polar bodies
• Only one viable cell is produced - the egg cell (termed
the ovum) – this is the cell to be ovulated for the
month
• The one viable cell (ovum) receives most of the cell
cytoplasm
• Inasmuch as the placenta will not develop till much
later if the egg is fertilized – the developing embryo
must live off the food in the ovum’s cytoplasm till the
after birth (placenta) is formed
Mitosis of Spermatogonia
• Begins at puberty
• Spermatogonia
– Stem cells in contact with the epithelial basal
lamina
– Each mitotic division  a type A daughter cell
and a type B daughter cell
Spermatogonium
(stem cell)
Mitosis
Growth
Enters meiosis I
and moves to
adluminal
compartment
Meiosis I
completed
Meiosis II
Basal lamina
Type A daughter cell
remains at basal lamina
as a stem cell
Type B daughter cell
Primary
spermatocyte
Secondary
spermatocytes
Early
spermatids
Late spermatids
Spermatozoa
(b) Events of spermatogenesis,
showing the relative position
of various spermatogenic cells
Figure 27.7b
Approximately 24 days
Golgi
apparatus
Acrosomal
vesicle
Mitochondria
Acrosome
Nucleus
1
(a)
2
Spermatid
nucleus
Centrioles
3
Midpiece Head
Microtubules
Flagellum
Excess
cytoplasm
4
Tail
5
6
7
(b)
Figure 27.8a, b
Final Result of Spermatogenesis
• All the four cells (sperm) are viable – thus
differing from the female situation

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