Transcript of Meiosis Reduction Division Mike Clark, M.D.. Meiosis Meiosis is nicknamed reduction division It is a...
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- Meiosis Reduction Division Mike Clark, M.D.
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- Meiosis Meiosis is nicknamed reduction division It is a process
where a cell divides (division) but reduces the genetic material to
(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)
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- 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
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- Figure 27.5 (1 of 2) Mother cell (before chromosome
replication) Chromosome replication Chromosome replication 2n = 4
MITOSIS Replicated chromosome Prophase Chromosomes align at the
metaphase plate Sister chromatids separate during anaphase 2n2n2n2n
Metaphase Daughter cells of mitosis Tetrad formed by synapsis of
replicated homologous chromosomes Tetrads align at the metaphase
plate Homologous chromosomes separate but sister chromatids remain
together during anaphase I No further chromosomal replication;
sister chromatids separate during anaphase II Daughter cells of
meiosis II (usually gametes) nnnn Prophase I Metaphase I Daughter
cells of meiosis I Meiosis II MEIOSIS
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- Fig. 13-7-1 Interphase Homologous pair of chromosomes in
diploid parent cell Chromosomes replicate Homologous pair of
replicated chromosomes Sister chromatids Diploid cell with
replicated chromosomes 46 pieces of genetic material in parent cell
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. Interphase in
meiosis occurs prior to the start of meiosis I. It consists of the
same three Phases as in mitosis G1,S and G2.
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- Fig. 13-7-2 Interphase Homologous pair of chromosomes in
diploid parent cell Chromosomes replicate Homologous pair of
replicated chromosomes Sister chromatids Diploid cell with
replicated chromosomes Meiosis I Homologous chromosomes separate 1
Haploid cells with replicated 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
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- Fig. 13-7-3 Interphase Homologous pair of chromosomes in
diploid parent cell Chromosomes replicate Homologous pair of
replicated chromosomes Sister chromatids Diploid cell with
replicated chromosomes Meiosis I Homologous chromosomes separate 1
Haploid cells with replicated chromosomes Meiosis II 2 Sister
chromatids separate Haploid cells with unreplicated chromosomes At
the end of meiosis II have 4 daughter cells each with the amount of
genetic material (haploid). At the completion of meiosis I (after
cytokinesis I) - the two cells enter into a phase termed
Interkinesis. Interkinesis is similar to Interphase but it lacks
the S-phase thus DNA is not replicated it is already enough DNA for
4 haploid cells.
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- Fig. 13-7-3 Interphase Homologous pair of chromosomes in
diploid parent cell Chromosomes replicate Homologous pair of
replicated chromosomes Sister chromatids Diploid cell with
replicated chromosomes Meiosis I Homologous chromosomes separate 1
Haploid cells with replicated chromosomes Meiosis II 2 Sister
chromatids separate Haploid cells with unreplicated chromosomes 46
pieces of genetic material in parent cell S- phase in interphase
duplicates DNA (but stays attached chromatid/ chromosome thus
enough genetic material for 4 haploid (gamete) cells 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 the amount of genetic material
(haploid).
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- 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
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- 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
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- Metaphase I Fig. 13-8a Prophase IAnaphase I Telophase I and
Cytokinesis Centrosome (with centriole pair) Sister chromatids
Chiasmata Spindle Homologous chromosomes Fragments of nuclear
envelope Centromere (with kinetochore) Metaphase plate Microtubule
attached to kinetochore Sister chromatids remain attached
Homologous chromosomes separate Cleavage furrow
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- 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
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- 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, X- shaped regions where
crossing over occurred Copyright 2008 Pearson Education Inc.,
publishing as Pearson Benjamin Cummings
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- 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
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- 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
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- Fig. 13-12-1 Prophase I of meiosis Pair of homologs Nonsister
chromatids held together during synapsis
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- Fig. 13-12-2 Prophase I of meiosis Pair of homologs Nonsister
chromatids held together during synapsis Chiasma Centromere
TEM
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- Fig. 13-12-3 Prophase I of meiosis Pair of homologs Nonsister
chromatids held together during synapsis Chiasma Centromere
Anaphase I TEM
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- Fig. 13-12-4 Prophase I of meiosis Pair of homologs Nonsister
chromatids held together during synapsis Chiasma Centromere
Anaphase I Anaphase II TEM
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- Fig. 13-12-5 Prophase I of meiosis Pair of homologs Nonsister
chromatids held together during synapsis Chiasma Centromere
Anaphase I Anaphase II Daughter cells Recombinant chromosomes
TEM
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- Possibility 1 Possibility 2 Metaphase II Without crossing over
the newly formed cells would inherit either a full chromosome
containing only moms or dads genes on that chromosome. By crossing
over the situation above would not happen in that each chromosome
would have a piece of dads genetic material and a piece of moms
genetic material.
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- Without crossing over the 4 daughter cells below would have no
genetic recombination. Metaphase II Daughter cells Combination
1Combination 2Combination 3Combination 4
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- 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
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- Fig. 13-8b Prophase IMetaphase I Centrosome (with centriole
pair) Sister chromatids Chiasmata Spindle Centromere (with
kinetochore) Metaphase plate Homologous chromosomes Fragments of
nuclear envelope Microtubule attached to kinetochore
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- 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
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- 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
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- 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
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- Fig. 13-8c Anaphase I Telophase I and Cytokinesis Sister
chromatids remain attached Homologous chromosomes separate Cleavage
furrow
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- 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
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- Fig. 13-8d Prophase II Metaphase II Anaphase II Telophase II
and Cytokinesis Sister chromatids separate Haploid daughter cells
forming
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- 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
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- 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
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- Fig. 13-8e Prophase IIMetaphase II
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- 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
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- 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
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- 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
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- Fig. 13-8f Anaphase II Telephase II and Cytokinesis Sister
chromatids separate Haploid daughter cells forming
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- 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 Downs Syndrome in the womans
offspring
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- Fig. 15-16
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- Fig. 15-16b
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- Fig. 15-17 Normal chromosome 9 Normal chromosome 22 Reciprocal
translocation Translocated chromosome 9 Translocated chromosome 22
(Philadelphia chromosome) Error crossing over occurred improperly
the exchange was with non-homologous chromosomes.
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- 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
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- 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
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- Figure 27.17 Meiotic eventsFollicle development in ovary Before
birth Infancy and childhood (ovary inactive) Primary oocyte Primary
oocyte (still arrested in prophase I) Vesicular (Graafian) follicle
Primary follicle Primordial follicle Oocyte Ovulated secondary
oocyte In absence of fertilization, ruptured follicle becomes a
corpus luteum and ultimately degenerates. Degenating corpus luteum
Secondary follicle Primary oocyte (arrested in prophase I; present
at birth) Oogonium (stem cell) Each month from puberty to menopause
Meiosis I (completed by one primary oocyte each month in response
to LH surge) First polar body Mitosis Growth Meiosis II of polar
body (may or may not occur) Polar bodies (all polar bodies
degenerate) OvumSecond polar body Meiosis II completed (only if
sperm penetration occurs) Sperm Ovulation Secondary oocyte
(arrested in metaphase II) Follicle cells Spindle
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- 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 non- functional 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 ovums cytoplasm till the after
birth (placenta) is formed
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- 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
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- Figure 27.7b Basal lamina Spermatogonium (stem cell) Mitosis
Growth Late spermatids Early spermatids Secondary spermatocytes
Primary spermatocyte Spermatozoa Type B daughter cell Enters
meiosis I and moves to adluminal compartment Meiosis I completed
Meiosis II Type A daughter cell remains at basal lamina as a stem
cell (b) Events of spermatogenesis, showing the relative position
of various spermatogenic cells
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- Figure 27.8a, b Centrioles Spermatid nucleus Golgi apparatus
Acrosomal vesicle Mitochondria Approximately 24 days Excess
cytoplasm Nucleus Acrosome Microtubules Flagellum Tail MidpieceHead
(a) (b) 1 2 3 4 5 6 7
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- Final Result of Spermatogenesis All the four cells (sperm) are
viable thus differing from the female situation