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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 replicationchromosome replication

TwoTwo meiotic divisions --- meiotic divisions --- Meiosis Meiosis I and Meiosis III 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)

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Facts About MeiosisFacts About Meiosis

Daughter 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 females in 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 to form a diploid form 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 the is the process of process of duplicating a duplicating a chromosomechromosome

Occurs Occurs prior to prior to divisiondivision

Replicated Replicated copies are called copies are called sister chromatidssister chromatids

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

HaploiHaploidd

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Meiosis I: Reduction DivisionMeiosis I: Reduction Division

NucleusNucleus SpindleSpindlefibersfibers NuclearNuclear

envelopeenvelopeInterphaseInterphase(Chromoso(Chromosome number me number

doubleddoubled))

ProphasProphase I e I

((tetrads tetrads form)form)

MetaphaMetaphase Ise I AnaphasAnaphas

e Ie ITelophase Telophase I I (diploid)(diploid)

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Prophase IProphase I

ProphaseProphaseHomologs Homologs pair.pair.Crossing over Crossing over occursoccurs.

ProphaseProphaseChromosomes Chromosomes condense.condense.Spindle forms.Spindle forms.Nuclear envelope Nuclear envelope fragments.fragments.

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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 Synapsis

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Crossing-OverCrossing-Over Homologous Homologous

chromosomeschromosomes in a tetrad in a tetrad cross over each cross over each otherother

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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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 Identical 4 Identical

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 IINuclear 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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SpermatogenesisSpermatogenesis Occurs in the Occurs in the testestestes

Two divisions Two divisions produce 4 produce 4 spermatidsspermatids

Spermatids Spermatids mature into spermmature into sperm

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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SpermatogenesisSpermatogenesis

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OogenesisOogenesis

Occurs 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 cytoplasm Immature 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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Oogenesis in the Oogenesis in the OvariesOvaries

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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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Comparing Comparing Mitosis and Mitosis and

MeiosisMeiosis

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MitosisMitosis MeiosisMeiosis

Number of Number of divisionsdivisions 11

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Number of Number of daughter cellsdaughter cells 22 44

Genetically Genetically identical?identical? YesYes NoNo

Chromosome #Chromosome # Same as parentSame as parent Half of parentHalf of parent

WhereWhere Somatic cellsSomatic cells Germ cellsGerm cells

WhenWhen Throughout lifeThroughout life At sexual maturityAt sexual maturity

RoleRole Growth and repairGrowth and repair Sexual reproductionSexual reproduction

Comparison of Comparison of DivisionsDivisions