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MeiosisMeiosisFormation of Formation of
GametesGametes (Eggs & (Eggs & Sperm)Sperm)
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Facts About MeiosisFacts About Meiosis
Preceded by interphase which Preceded by interphase which includesincludes chromosome chromosome replicationreplication
TwoTwo meiotic divisions --- meiotic divisions --- Meiosis I and Meiosis IIMeiosis I and Meiosis II
Called Reduction- divisionCalled Reduction- divisionOriginal cell is diploid (2n)Original cell is diploid (2n)Four daughter cellsFour daughter cells produced produced
that are that are monoploid (1n)monoploid (1n)copyright cmassengale
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Facts About MeiosisFacts About MeiosisDaughter cellsDaughter cells contain half the contain half the
number of chromosomesnumber of chromosomes as the as the original celloriginal cell
ProducesProduces gametesgametes (eggs & (eggs & spermsperm))
Occurs in the Occurs in the testestestes in males in males ((SpermatogenesisSpermatogenesis))
Occurs in the Occurs in the ovariesovaries in in females (females (OogenesisOogenesis))
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Start with Start with 46 double stranded 46 double stranded chromosomes (2n)chromosomes (2n)After 1 division - After 1 division - 23 double 23 double stranded chromosomes (n)stranded chromosomes (n)After 2nd division - After 2nd division - 23 single 23 single stranded chromosomes (n)stranded chromosomes (n) Occurs in our Occurs in our germ cellsgerm cells that that produce gametesproduce gametes
More Meiosis FactsMore Meiosis Facts
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Why Do we Need Why Do we Need Meiosis?Meiosis?
It is the fundamental basis It is the fundamental basis of of sexual reproductionsexual reproduction
Two haploid (1n) gametes Two haploid (1n) gametes are brought together are brought together through through fertilizationfertilization to form to form a diploid a diploid (2n) zygote(2n) zygote
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Fertilization – “Putting Fertilization – “Putting it all together”it all together”
1n =31n =3
2n = 62n = 6
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Replication of Replication of ChromosomesChromosomes
ReplicationReplication is is the process of the process of duplicating a duplicating a chromosomechromosome
Occurs Occurs prior to prior to divisiondivision
Replicated Replicated copies are called copies are called sister sister chromatidschromatids
Held together at Held together at centromerecentromere
Occurs in Occurs in InterphasInterphas
ee
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A Replicated A Replicated ChromosomeChromosome
HomologsHomologs (same genes, (same genes, different alleles)different alleles)
SisterSisterChromatidsChromatids(same genes,(same genes,same alleles)same alleles)
Gene XGene X
Homologs separate in meiosis I and Homologs separate in meiosis I and therefore different alleles separate.therefore different alleles separate.
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Meiosis Forms Haploid Meiosis Forms Haploid GametesGametes
Meiosis must Meiosis must reduce the chromosome number by halfreduce the chromosome number by half Fertilization then restoresFertilization then restores the 2n number the 2n number
from mom from dad child
meiosis reducesgenetic content
toomuch!
The right number!
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Meiosis: Two Part Cell Meiosis: Two Part Cell DivisionDivision
HomologsHomologsseparateseparate
Sister Sister chromatidschromatidsseparateseparate
DiploidDiploid
MeiosisMeiosis I I
MeiosisMeiosisIIII
DiploidDiploid
HaploiHaploiddcopyright cmassengale
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Meiosis I: Reduction DivisionMeiosis I: Reduction Division
NucleusNucleus SpindleSpindlefibersfibers NuclearNuclear
envelopeenvelopeEarly Early Prophase IProphase I(Chromoso(Chromosome number me number
doubleddoubled))
Late Late ProphasProphas
e Ie I
MetaphaMetaphase Ise I AnaphasAnaphas
e Ie ITelophase Telophase I I (diploid)(diploid)
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Prophase IProphase I
Early Early prophaseprophaseHomologs Homologs pair.pair.Crossing over Crossing over occursoccurs.
Late prophaseLate prophaseChromosomes Chromosomes condense.condense.Spindle forms.Spindle forms.Nuclear envelope Nuclear envelope fragments.fragments.copyright cmassengale
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Tetrads Form in Tetrads Form in Prophase IProphase I
Homologous Homologous chromosomeschromosomes
(each with sister chromatids) (each with sister chromatids)
Join to form aJoin to form a TETRADTETRAD
Called SynapsisCalled Synapsiscopyright cmassengale
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Crossing-OverCrossing-Over Homologous Homologous
chromosomeschromosomes in a tetrad in a tetrad cross over cross over each othereach other
Pieces of Pieces of chromosomes chromosomes or or genes are genes are exchangedexchanged
Produces Produces Genetic Genetic recombinationrecombination in the offspringin the offspring
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Homologous Homologous Chromosomes During Chromosomes During
Crossing-OverCrossing-Over
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Crossing-over multiplies the already Crossing-over multiplies the already huge number of different gamete huge number of different gamete types produced by independent types produced by independent
assortmentassortment
Crossing-OverCrossing-Over
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Metaphase IMetaphase I
Homologous Homologous pairspairs of of
chromosomes chromosomes align along the align along the equatorequator of the of the
cellcell
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Anaphase IAnaphase I
Homologs separateHomologs separate and move to opposite and move to opposite poles.poles.
Sister chromatids Sister chromatids remain remain attached attached at theirat their centromerescentromeres.
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Telophase ITelophase I
Nuclear envelopes Nuclear envelopes reassemble.reassemble.
Spindle disappears.Spindle disappears.
Cytokinesis Cytokinesis divides cell divides cell into two.into two.
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Meiosis IIMeiosis II
Only one homolog of Only one homolog of each chromosome is each chromosome is present in the cellpresent in the cell.
Meiosis II produces gametes Meiosis II produces gametes with with
one copy of each chromosome one copy of each chromosome and thus one copy of each and thus one copy of each gene.gene.
Sister chromatids carry Sister chromatids carry identical genetic identical genetic
informationinformation. .
Gene XGene X
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Meiosis II: Reducing Meiosis II: Reducing Chromosome NumberChromosome Number
Prophase Prophase IIII
MetaphaMetaphase IIse II
AnaphasAnaphase IIe II
TelophasTelophase IIe II 4 4
Genetically Genetically Different Different haploid haploid
cellscells
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Prophase IIProphase II
Nuclear Nuclear envelope envelope fragments.fragments.
Spindle forms.Spindle forms.
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Metaphase IIMetaphase II
Chromosomes align Chromosomes align along along equatorequator of cell of cell..
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Anaphase IIAnaphase II
Sister Sister chromatidschromatids separate and separate and move to move to opposite polesopposite poles.
EquatorEquator
PolePole
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Telophase IITelophase II
Nuclear envelope Nuclear envelope assembles.assembles.
Chromosomes Chromosomes decondense.decondense.
Spindle Spindle disappears.disappears.
Cytokinesis Cytokinesis divides cell into divides cell into two.two.
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Results of MeiosisResults of MeiosisGametes (egg & Gametes (egg & sperm) formsperm) form
Four haploid cells Four haploid cells with one copy of each with one copy of each chromosome chromosome
One allele of each One allele of each genegene
Different Different combinations of combinations of alleles for different alleles for different genes along the genes along the chromosomechromosome
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GametogenesisGametogenesis
OogenesisOogenesis or or
Spermatogenesis Spermatogenesis
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SpermatogenesisSpermatogenesisOccurs in the Occurs in the
testestestesTwo divisions Two divisions
produce 4 produce 4 spermatidsspermatids
Spermatids Spermatids mature into mature into spermsperm
Men produce Men produce about about 250,000,000 250,000,000 sperm per daysperm per day
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Spermatogenesis in the Spermatogenesis in the TestesTestes
SpermatiSpermatidd
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OogenesisOogenesisOccurs in the Occurs in the ovariesovariesTwo divisions produce Two divisions produce 3 polar 3 polar
bodiesbodies that die and that die and 1 egg1 eggPolar bodies die because of Polar bodies die because of
unequal division of cytoplasmunequal division of cytoplasmImmature egg called Immature egg called oocyteoocyteStarting at puberty, one oocyte Starting at puberty, one oocyte
matures into an matures into an ovum (egg)ovum (egg) every 28 daysevery 28 days
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OogenesisOogenesis
OogoniumOogonium(diploid)(diploid)
MitosisMitosis
PrimaryPrimaryoocyteoocyte(diploid)(diploid)
Meiosis IMeiosis I
SecondarySecondaryoocyteoocyte(haploid)(haploid)
Meiosis IIMeiosis II(if fertilization(if fertilizationoccurs)occurs)
First polar bodyFirst polar bodymay divide may divide (haploid)(haploid) PolarPolar
bodiesbodiesdiedie
Ovum (egg)Ovum (egg)
SecondSecondpolar bodypolar body(haploid)(haploid)
a
A
X
X
a
X
A X
a
X
a
X
MatureMatureeggegg
A
X
A
X
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MitosisMitosis MeiosisMeiosisNumber of Number of
divisionsdivisions 1122
Number of Number of daughter cellsdaughter cells 22 44
Genetically Genetically identical?identical? YesYes NoNo
Chromosome #Chromosome # Same as Same as parentparent Half of parentHalf of parent
WhereWhere Somatic cellsSomatic cells Germ cellsGerm cells
WhenWhen Throughout Throughout lifelife
At sexual At sexual maturitymaturity
RoleRole Growth and Growth and repairrepair
Sexual Sexual reproductionreproduction
Comparison of DivisionsComparison of Divisions
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