Transcript Mitosis & Meiosis

Cell Division
Lab 10
Cell Division
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According to cell theory – cells
only come from other living cells.
Information – encoded within DNA –
must be replicated so each new cell
has a copy.
Eukaryote Cell Cycle
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DNA is contained in multiple
chromosomes (a single long DNA
molecule packaged with proteins
into a compact shape).
Cell Division in Eukaryotes
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Mitosis – cell division in somatic
(nonreproductive) cells.
Meiosis – cell division in
reproductive cells.
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Results in eggs or sperm.
The Cell Cycle
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Interphase – G1, S, and G2
together.
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G1 phase – first growth phase
S phase – synthesis phase, replication
of DNA.
G2 phase – second growth phase,
replication of mitochondria,
chromosomes condense.
M phase – mitosis
C phase - cytokinesis
Chromosomes
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Homologous chromosomes or
homologues – two nearly identical
versions of a chromosome.
Diploid cells have two of each type
of chromosome ( 23 pairs or 46 in
humans).
Chromosome Structure
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During S phase,
each of the 2
homologues
replicates,
resulting in
identical copies
called sister
chromatids.
Chromatids
remain
connected at a
linkage site
called the
centromere.
Cell Division
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Interphase –
Cells grow, DNA
replicates and
winds up tightly
(condensation).
Cell Division
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Mitosis – nuclear cell division in
somatic cells.
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Prophase
Metaphase
Anaphase
Telophase
Continuous process where stages
flow one into the next.
Cell Division
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Prophase –
Chromosomes
condense enough to
be seen with a light
microscope.
Cell Division
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Metaphase –
Alignment of the
chromosomes along
center of cell.
Cell Division
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Anaphase –
Separation of
the chromatids.
Sister
chromatids split
apart – one
moving to each
pole.
Cell Division
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Telophase –
re-formation of
the nuclei.
Cell Division
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Cytokinesis –
division of the
cytoplasm.
Two complete,
diploid cells.
Cytokinesis
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In animal cells,
the cell pinches
in two.
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This produces a
cleavage
furrow which
deepens until
the cell splits.
In plants, a cell
plate forms
dividing the cell
in two.
http://www.youtube.com/watch?v=VGV3fv-uZYI
Cellulose is
laid down to
form new cell
wall.
Reproduction
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When organisms reproduce
sexually, a cell from each parent
fuses together.
In order to avoid a doubling of
chromosome number, there must
be a way to reduce the number of
chromosomes in the gametes
(eggs & sperm).
Meiosis
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Meiosis is the special type of cell
division that produces eggs and
sperm.
In meiosis, a diploid cell with two
sets of homologous chromosomes
will divide so that the daughter cells
are haploid and have one set of
chromosomes.
Fertilization
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One haploid
egg and one
haploid sperm
can then fuse in
the process
called
fertilization
forming a
diploid zygote.
Meiosis
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Chromosomes have replicated
during interphase just as in mitosis.
Meiosis actually consists of two
separate divisions.
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Meiosis I – serves to separate the two
versions of the chromosome
(homologues).
Meiosis II – serves to separate the
two replicas of each version (sister
chromatids).
Meiosis
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Because there is only one
replication of DNA but two cell
divisions, each of the four daughter
cells is haploid – has only one set
of chromosomes.
Meiosis I
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Prophase I –
Chromosomes
become visible.
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The 2 versions of
each chromosome
pair up and
exchange
segments. This is
called crossing
over.
Meiosis I
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Metaphase I –
Chromosomes line
up in the middle.
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Which
chromosome faces
which pole is
random. This is
called
independent
assortment.
Meiosis I
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Anaphase I
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Homologues are pulled apart and move
toward opposite poles.
Sister chromatids NOT separated yet.
Each pole has half as many chromosomes
(one set rather than two) as the original
cell.
Telophase I – the chromosomes
gather at the two poles and wait for
the onset of meiosis II.
Meiosis II
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After a brief interphase in which NO
DNA synthesis occurs, meiosis II
begins.
Meiosis II is just like mitosis except
that the sister chromatids are no
longer identical due to crossing
over.
Meiosis II
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Prophase II – nuclear envelopes
break down.
Metaphase II – chromosomes line
up in the middle of the cell.
Meiosis II
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Anaphase II – sister chromatids
move to opposite poles.
Telophase II – The nuclear
envelope reforms around the four
sets of daughter chromosomes.
Meiosis II
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The resulting 4 daughter cells are
haploid.
No 2 cells are alike due to crossing
over.
In animals, these cells develop
directly into gametes (eggs &
sperm).
Unique Features of Meiosis
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Synapsis – occurs
when homologous
chromosomes line up
along their entire
length.
Crossing over occurs when segments
of the chromosomes
are then exchanged.
Unique Features of Meiosis
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Another unique feature is
that the homologous
chromosomes separate in
meiosis I rather than sister
chromatids.
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http://www.youtube.com/watch?v=D1_-mQS_FZ0&NR=1
Sister chromatids separate in
meiosis II.
Meiosis is a reduction
division.