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8/14/2019 Ch 7 .1 ,7 .2
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MEIOSISMEIOSISCh 7.1,7 .2Ch 7.1 ,7 .2
8/14/2019 Ch 7 .1 ,7 .2
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MEIOSISMEIOSIS
Organisms reproduce by joining gametes
Gametes are haploid
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MeiosisMeiosis
• Form of cell division whereone cell produces fourgametes, containing half thegenetic material of theparent cell.
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Meiosis VS MitosisMeiosis VS Mitosis
1. Mitosis end result is 2identical daughter cells
Meiosis end result is 4genetically different cells
2.Mitosis occurs in somatic
cellsMeiosis occurs in gametes
3.Mitosis results in 2 diploid
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MeiosisMeiosis
There are only twocells that undergo
Meiosis.Sperm Cells
Male
Reproductive Cells
Eggs Cells
FemaleReproductive Cells
Fertilization
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MeiosisMeiosis
Meiosis divided into twosections with a total of eightphases.
Meiosis 1
Meiosis II These phases are continuous
and flow one right after the
other.
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Meiosis IMeiosis I
Prophase 1Chromosomes
Condense and CoilUp
Nuclear Envelopedissolves
HomologousChromosomes
Line up in Tetrads Tetrad consists of
two homologouschromosomesmade up two
sister
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Meiosis IMeiosis I
Prophase 1Crossing over
occurs
Portions of onechromatidare broken
andexchangedwith the
correspondin ortions
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http:/ /www.accessexcellen ce.org
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Meiosis IMeiosis I
Metaphase 1
Centromerebecomes attachedto spindle fibers
Tetrads line up
along the equator
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Meiosis IMeiosis I
Anaphase 1
HomologousChromosomesmove toopposite ends of
the cell.Chromatids do not
separate
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Meiosis IMeiosis I
Telophase 1Spindle fiber breaks
downChromosomes uncoil
Cytokinesis takes
placeBoth cells contain one
chromosome fromeach pair of
homologous
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MeiosisMeiosis
There is no replication between meiosis I andmeiosis II
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Meiosis IIMeiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
The above steps are exactly the same as mitosis.
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Meiosis IIMeiosis II
Prophase IINew Spindle fibers form
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Meiosis IIMeiosis II
Anaphase II
Centromeres divide
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Meiosis IIMeiosis II
Telophase II
Nuclear Envelope appearsSpindle Fibers break downCytokinesis
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AnimationAnimationhttp://www.csuchico.edu/~ jbell/Biol207/animations/meios i s .h tmlht tp ://www.csu ch ico.e du /~ jbe ll/Biol20 7/a nim a tion s/m eios is .h tml
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MeiosisMeiosis
RESULTS-
4 Haploid cellsGenetically different cells
than the parent cell with half the genetic information.
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MeiosisMeiosis
We need half the informationso that one half comes fromthe male sex cell and theother half comes from thefemale sex cell. Theycombine to have the all thegenetic information needed.
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Formation of Formation of
GametesGametesCalledCalled
g a m e t o g e n eg a m e t o g e n e
s iss is
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Formation of gametesFormation of gametesSpermatogenesi
s (produces 4haploid sperm)Gametes in
males testes
Diploid cellincreasesin size
Undergoes
Meiosis I
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Formation of gametesFormation of gametesProduces 2
cellsEach of these
2 cellsundergoesMeiosis II
Produces 4undifferentiatedhaploid
sperm cells
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Formation of gametesFormation of gametesOogenesis (produces
1 haploid egg and 3polar bodies)Occurs in the ovaries of
females In cytokinesis of
Meiosis I, the
cytoplasm doesn’tdivide equallyProduces 1 cell
with most of thecytoplasm
And 1 polar
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Formation of gametesFormation of gametesBoth undergo
Meiosis IIAgain
unequalcytokinesis
Larger = ovum(egg)Contains a
large
amount of
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MeiosisMeiosis
The random way that these lineup in tetrads is what gives us
the diversity in genes that makeus all different from one another.
This is called Independent
Assortment
About 8 million different possiblecombinations
I d dI d d t
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IndependentIndependent
AssortmentAssortment
The random distribution of homologous chromosomesduring meiosis
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Wh t i th i tWh t i th i t
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What is the importanceWhat is the importance
of genetic variation?of genetic variation?
Increases the chances of survival.
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MitosisMitosis
1. Where it occurs Somatic cells
2. End number of Cells 2
3. Number of divisions 1
4. Number of chromosomes in resulting cell 2n (diploid) same as original cell
5. Genetic Variation? No – end cells are exactly the same as the original
cell (except in mutation)
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MeiosisMeiosis
1. Where it occurs Sex cells (gametes)
2. End number of Cells 4
3. Number of divisions 2
4. Number of chromosomes in resulting cell n (haploid) half the number of the original cell
5. Genetic Variation? yes