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MeiosisMeiosisMeiosisMeiosis& Sexual Reproduction

Cell Division –Asexual reproduction

• Mitosis

– produce cells with same information• identical daughter cells

– exact copies•clones

– same amount of DNA •same number of chromosomes

• same genetic information

Asexual reproduction

• Single-celled eukaryotes and simple multicellular eukaryotes reproduce asexually

Binary fission in AmoebaBinary fission in AmoebaBinary fission in AmoebaBinary fission in Amoeba

Budding in YeastBudding in YeastBudding in YeastBudding in Yeast Budding in Budding in Budding in Budding in HydraHydraHydraHydra

How about the rest of us?

• What if a complex multicellular organism (like us) wants to reproduce?

– joining of egg + sperm

• Do we make egg & sperm by mitosis?

46464646 46464646++++ 92929292

eggeggeggegg spermspermspermsperm zygotezygotezygotezygote

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How do we make sperm & eggs?

• reduce 46 chromosomes → 23 chromosomes– half the number of chromosomes

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2323232346464646

eggeggeggegg

spermspermspermsperm

46464646

meiosismeiosismeiosismeiosis 46464646

fertilizationfertilizationfertilizationfertilization

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Review: MitosisReview: MitosisReview: MitosisReview: Mitosis• Division Division Division Division of theof theof theof the NUCLEUSNUCLEUSNUCLEUSNUCLEUS

• Purpose: growth & repairPurpose: growth & repairPurpose: growth & repairPurpose: growth & repair

• Four StagesFour StagesFour StagesFour Stages

• Can you identify them?Can you identify them?Can you identify them?Can you identify them?

Four Mitotic StagesFour Mitotic StagesFour Mitotic StagesFour Mitotic Stages

•ProProProProphasephasephasephase

•MetaMetaMetaMetaphasephasephasephase

•AnaAnaAnaAnaphasephasephasephase

•TeloTeloTeloTelophasephasephasephase

MitosisMitosisMitosisMitosis

• Produces two new cellsidentical to the original cell

• Cells are diploid (2n)

Chromosomes during Chromosomes during Chromosomes during Chromosomes during Metaphase of mitosisMetaphase of mitosisMetaphase of mitosisMetaphase of mitosis

ProphaseProphaseProphaseProphase MetaphaseMetaphaseMetaphaseMetaphase AnaphaseAnaphaseAnaphaseAnaphase TelophaseTelophaseTelophaseTelophaseCytokinesisCytokinesisCytokinesisCytokinesis

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Facts About MeiosisFacts About MeiosisFacts About MeiosisFacts About MeiosisFacts About MeiosisFacts About MeiosisFacts About MeiosisFacts About Meiosis•• Preceded by interphase which includes Preceded by interphase which includes chromosome replicationchromosome replication

•• TwoTwo meiotic divisions meiotic divisions ------

Meiosis I and Meiosis IIMeiosis I and Meiosis II

•• Called Called reductionreduction--divisiondivision

•• Original cell is diploid (2n)Original cell is diploid (2n)

•• Four daughter cells produced that are Four daughter cells produced that are haploid (1n)haploid (1n)

Facts About MeiosisFacts About MeiosisFacts About MeiosisFacts About Meiosis

•Produces gametes (eggs & sperm)Produces gametes (eggs & sperm)Produces gametes (eggs & sperm)Produces gametes (eggs & sperm)

•Occurs in theOccurs in theOccurs in theOccurs in the testes testes testes testes in malesin malesin malesin males

= = = = SpermatogenesisSpermatogenesisSpermatogenesisSpermatogenesis

• Occurs in theOccurs in theOccurs in theOccurs in the ovaries ovaries ovaries ovaries in femalesin femalesin femalesin females

= = = = OogenesisOogenesisOogenesisOogenesis

---- 2 copies2 copies2 copies2 copies

---- diploiddiploiddiploiddiploid

---- 2n2n2n2n

---- 1 copy1 copy1 copy1 copy

---- haploidhaploidhaploidhaploid

---- 1n1n1n1n

Meiosis = reduction divisionMeiosis = reduction divisionMeiosis = reduction divisionMeiosis = reduction division Homologous chromosomes• Paired chromosomes – one from each parent

– both chromosomes of a pair carry genes

• control same inherited characters

• homologous = same information

diploiddiploiddiploiddiploid2n2n2n2n

homologoushomologoushomologoushomologouschromosomeschromosomeschromosomeschromosomes

Replicated Replicated Replicated Replicated homologous chromosomeshomologous chromosomeshomologous chromosomeshomologous chromosomes

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2n = 2n = 2n = 2n = 6666doubledoubledoubledoublestrandedstrandedstrandedstranded

2n = 2n = 2n = 2n = 6666singlesinglesinglesingle

strandedstrandedstrandedstranded

Preparing for meiosis

• Preceded by Interphase

– Duplication of DNA

– S phase of interphaseof MEIOSIS(just like in mitosis)

M1 prophaseM1 prophaseM1 prophaseM1 prophase

Meiosis: Two Part Cell DivisionMeiosis: Two Part Cell Division

HomologsHomologsHomologsHomologsseparateseparateseparateseparate

Sister Sister Sister Sister chromatidschromatidschromatidschromatidsseparateseparateseparateseparate

Meiosis I

DiploidDiploidDiploidDiploid

HaploidHaploidHaploidHaploid

Meiosis II

HaploidHaploidHaploidHaploid

Homologous ChromosomesHomologous ChromosomesHomologous ChromosomesHomologous ChromosomesHomologous ChromosomesHomologous ChromosomesHomologous ChromosomesHomologous Chromosomesseparate in meiosis Iseparate in meiosis Iseparate in meiosis Iseparate in meiosis Iseparate in meiosis Iseparate in meiosis Iseparate in meiosis Iseparate in meiosis I

SisterSisterSisterSisterSisterSisterSisterSisterChromatidsChromatidsChromatidsChromatidsChromatidsChromatidsChromatidsChromatidssame genes,same genes,same genes,same genes,same genes,same genes,same genes,same genes,same allelessame allelessame allelessame allelessame allelessame allelessame allelessame alleles

HomologsHomologsHomologsHomologsHomologsHomologsHomologsHomologssame genes,same genes,same genes,same genes,same genes,same genes,same genes,same genes,different allelesdifferent allelesdifferent allelesdifferent allelesdifferent allelesdifferent allelesdifferent allelesdifferent alleles

Meiosis I: Reduction DivisionMeiosis I: Reduction DivisionMeiosis I: Reduction DivisionMeiosis I: Reduction DivisionMeiosis I: Reduction DivisionMeiosis I: Reduction DivisionMeiosis I: Reduction DivisionMeiosis I: Reduction Division

NucleusNucleusNucleusNucleusNucleusNucleusNucleusNucleus SpindleSpindleSpindleSpindleSpindleSpindleSpindleSpindlefibersfibersfibersfibersfibersfibersfibersfibers NuclearNuclearNuclearNuclearNuclearNuclearNuclearNuclear

envelopeenvelopeenvelopeenvelopeenvelopeenvelopeenvelopeenvelopeEarly Prophase IEarly Prophase IEarly Prophase IEarly Prophase IEarly Prophase IEarly Prophase IEarly Prophase IEarly Prophase I(Chromosome (Chromosome (Chromosome (Chromosome (Chromosome (Chromosome (Chromosome (Chromosome

number doubled)number doubled)number doubled)number doubled)number doubled)number doubled)number doubled)number doubled)

Late Late Late Late Late Late Late Late Prophase IProphase IProphase IProphase IProphase IProphase IProphase IProphase I

Metaphase IMetaphase IMetaphase IMetaphase IMetaphase IMetaphase IMetaphase IMetaphase IAnaphase IAnaphase IAnaphase IAnaphase IAnaphase IAnaphase IAnaphase IAnaphase I

Telophase ITelophase ITelophase ITelophase ITelophase ITelophase ITelophase ITelophase I

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Prophase IProphase IProphase IProphase I

Early prophaseEarly prophaseEarly prophaseEarly prophaseLate prophaseLate prophaseLate prophaseLate prophase

•HomologsHomologsHomologsHomologs pairpairpairpair

•Crossing over occursCrossing over occursCrossing over occursCrossing over occurs

•Chromosomes condenseChromosomes condenseChromosomes condenseChromosomes condense

•Spindle formsSpindle formsSpindle formsSpindle forms

•Nuclear envelope fragmentsNuclear envelope fragmentsNuclear envelope fragmentsNuclear envelope fragments

Tetrads Form in Prophase ITetrads Form in Prophase ITetrads Form in Prophase ITetrads Form in Prophase IHomologous chromosomesHomologous chromosomesHomologous chromosomesHomologous chromosomes

(each with sister chromatids) (each with sister chromatids) (each with sister chromatids) (each with sister chromatids) Join to form aJoin to form aJoin to form aJoin to form a

TETRADTETRADTETRADTETRAD

Called SynapsisCalled SynapsisCalled SynapsisCalled Synapsis

Crossing over- homologous pairs swappieces of chromosome

- Creates new combinations of traits

tetradtetradtetradtetrad

synapsissynapsissynapsissynapsisfrom mom

from dad

Sister chromatids homologs

from mom

from dad

Sister chromatids homologs

Metaphase IMetaphase IMetaphase IMetaphase IMetaphase IMetaphase IMetaphase IMetaphase I

Homologous pairsHomologous pairsHomologous pairsHomologous pairsof chromosomes of chromosomes of chromosomes of chromosomes align along the align along the align along the align along the

equator of the cellequator of the cellequator of the cellequator of the cell

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Independent Assortment

- chromosomes line up randomly

- independent of one another

- How many possibilities???

- 2n (n=number of chromosomes)

Independent Assortment

- in humans produces 223 (8,388,608)different combinations

Before meiosis

After meiosis

Maternal

Paternal

Anaphase IAnaphase IAnaphase IAnaphase IAnaphase IAnaphase IAnaphase IAnaphase I

Homologs separateHomologs separateHomologs separateHomologs separateHomologs separateHomologs separateHomologs separateHomologs separate and and and and and and and and move to opposite poles.move to opposite poles.move to opposite poles.move to opposite poles.move to opposite poles.move to opposite poles.move to opposite poles.move to opposite poles.

Sister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatids remain remain remain remain remain remain remain remain attachedattachedattachedattachedattachedattachedattachedattached at theirat theirat theirat theirat theirat theirat theirat theircentromerescentromerescentromerescentromerescentromerescentromerescentromerescentromeres.

Telophase ITelophase ITelophase ITelophase ITelophase ITelophase ITelophase ITelophase I

Nuclear envelopesNuclear envelopesNuclear envelopesNuclear envelopesNuclear envelopesNuclear envelopesNuclear envelopesNuclear envelopes reassemblereassemblereassemblereassemblereassemblereassemblereassemblereassemble

Spindle disappearsSpindle disappearsSpindle disappearsSpindle disappearsSpindle disappearsSpindle disappearsSpindle disappearsSpindle disappears

CytokinesisCytokinesisCytokinesisCytokinesisCytokinesisCytokinesisCytokinesisCytokinesis divides cell into two.divides cell into two.divides cell into two.divides cell into two.divides cell into two.divides cell into two.divides cell into two.divides cell into two.

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2n = 2n = 2n = 2n = 4444doubledoubledoubledoublestrandedstrandedstrandedstranded

2n = 2n = 2n = 2n = 4444singlesinglesinglesingle

strandedstrandedstrandedstranded

Review: Meiosis I

prophase1prophase1prophase1prophase1

2n = 2n = 2n = 2n = 4444doubledoubledoubledoublestrandedstrandedstrandedstranded

• 1st division of meiosis separateshomologous pairs

1n1n1n1n = = = = 2222doubledoubledoubledoublestrandedstrandedstrandedstranded

tetrad

synapsis

telophase1telophase1telophase1telophase1

metaphase1metaphase1metaphase1metaphase1

Meiosis 2

1n1n1n1n = = = = 2222doubledoubledoubledoublestrandedstrandedstrandedstranded

1n1n1n1n = = = = 2222singlesinglesinglesingle

strandedstrandedstrandedstranded

metaphase2metaphase2metaphase2metaphase2

telophase2telophase2telophase2telophase2

prophase2prophase2prophase2prophase2

1n1n1n1n = = = = 2222doubledoubledoubledoublestrandedstrandedstrandedstranded

• 2nd division of meiosis separatessister chromatids

Meiosis IIMeiosis IIMeiosis IIMeiosis IIMeiosis IIMeiosis IIMeiosis IIMeiosis II

Only one homolog of each Only one homolog of each Only one homolog of each Only one homolog of each Only one homolog of each Only one homolog of each Only one homolog of each Only one homolog of each chromosome is present in the cellchromosome is present in the cellchromosome is present in the cellchromosome is present in the cellchromosome is present in the cellchromosome is present in the cellchromosome is present in the cellchromosome is present in the cell.

Meiosis II produces gametes with Meiosis II produces gametes with Meiosis II produces gametes with Meiosis II produces gametes with Meiosis II produces gametes with Meiosis II produces gametes with Meiosis II produces gametes with Meiosis II produces gametes with

one copy of each chromosome and thus one copy of each chromosome and thus one copy of each chromosome and thus one copy of each chromosome and thus one copy of each chromosome and thus one copy of each chromosome and thus one copy of each chromosome and thus one copy of each chromosome and thus one copy of each gene.one copy of each gene.one copy of each gene.one copy of each gene.one copy of each gene.one copy of each gene.one copy of each gene.one copy of each gene.

Sister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsare still attachedare still attachedare still attachedare still attachedare still attachedare still attachedare still attachedare still attached

Meiosis II: Reducing Chromosome Meiosis II: Reducing Chromosome Meiosis II: Reducing Chromosome Meiosis II: Reducing Chromosome Meiosis II: Reducing Chromosome Meiosis II: Reducing Chromosome Meiosis II: Reducing Chromosome Meiosis II: Reducing Chromosome NumberNumberNumberNumberNumberNumberNumberNumber

Prophase IIProphase IIProphase IIProphase IIProphase IIProphase IIProphase IIProphase II Metaphase IIMetaphase IIMetaphase IIMetaphase IIMetaphase IIMetaphase IIMetaphase IIMetaphase II

Anaphase IIAnaphase IIAnaphase IIAnaphase IIAnaphase IIAnaphase IIAnaphase IIAnaphase IITelophase IITelophase IITelophase IITelophase IITelophase IITelophase IITelophase IITelophase II

4 haploid cells4 haploid cells4 haploid cells4 haploid cells4 haploid cells4 haploid cells4 haploid cells4 haploid cells

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Prophase IIProphase IIProphase IIProphase IIProphase IIProphase IIProphase IIProphase II

Nuclear envelope Nuclear envelope Nuclear envelope Nuclear envelope Nuclear envelope Nuclear envelope Nuclear envelope Nuclear envelope fragments.fragments.fragments.fragments.fragments.fragments.fragments.fragments.

Spindle forms.Spindle forms.Spindle forms.Spindle forms.Spindle forms.Spindle forms.Spindle forms.Spindle forms.

Metaphase IIMetaphase IIMetaphase IIMetaphase IIMetaphase IIMetaphase IIMetaphase IIMetaphase II

Chromosomes align Chromosomes align Chromosomes align Chromosomes align Chromosomes align Chromosomes align Chromosomes align Chromosomes align along equator of cellalong equator of cellalong equator of cellalong equator of cellalong equator of cellalong equator of cellalong equator of cellalong equator of cell........

Anaphase IIAnaphase IIAnaphase IIAnaphase IIAnaphase IIAnaphase IIAnaphase IIAnaphase II

Sister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsSister chromatidsseparate and move separate and move separate and move separate and move separate and move separate and move separate and move separate and move to opposite polesto opposite polesto opposite polesto opposite polesto opposite polesto opposite polesto opposite polesto opposite poles.

EquatorEquatorEquatorEquatorEquatorEquatorEquatorEquator

PolePolePolePolePolePolePolePole

Telophase IITelophase IITelophase IITelophase IITelophase IITelophase IITelophase IITelophase II

Nuclear envelope Nuclear envelope Nuclear envelope Nuclear envelope Nuclear envelope Nuclear envelope Nuclear envelope Nuclear envelope assemblesassemblesassemblesassemblesassemblesassemblesassemblesassembles

Chromosomes Chromosomes Chromosomes Chromosomes Chromosomes Chromosomes Chromosomes Chromosomes decondensedecondensedecondensedecondensedecondensedecondensedecondensedecondense

Spindle disappearsSpindle disappearsSpindle disappearsSpindle disappearsSpindle disappearsSpindle disappearsSpindle disappearsSpindle disappears

Cytokinesis divides cell Cytokinesis divides cell Cytokinesis divides cell Cytokinesis divides cell Cytokinesis divides cell Cytokinesis divides cell Cytokinesis divides cell Cytokinesis divides cell into twointo twointo twointo twointo twointo twointo twointo two

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Results of MeiosisResults of MeiosisResults of MeiosisResults of MeiosisResults of MeiosisResults of MeiosisResults of MeiosisResults of Meiosis

Gametes (egg & sperm) formGametes (egg & sperm) formGametes (egg & sperm) formGametes (egg & sperm) formGametes (egg & sperm) formGametes (egg & sperm) formGametes (egg & sperm) formGametes (egg & sperm) form

Four haploid cellsFour haploid cellsFour haploid cellsFour haploid cellsFour haploid cellsFour haploid cellsFour haploid cellsFour haploid cells with one with one with one with one with one with one with one with one copy of each chromosome copy of each chromosome copy of each chromosome copy of each chromosome copy of each chromosome copy of each chromosome copy of each chromosome copy of each chromosome

One allele of each geneOne allele of each geneOne allele of each geneOne allele of each geneOne allele of each geneOne allele of each geneOne allele of each geneOne allele of each gene

Different combinations of Different combinations of Different combinations of Different combinations of Different combinations of Different combinations of Different combinations of Different combinations of alleles for different genes alleles for different genes alleles for different genes alleles for different genes alleles for different genes alleles for different genes alleles for different genes alleles for different genes along the chromosomealong the chromosomealong the chromosomealong the chromosomealong the chromosomealong the chromosomealong the chromosomealong the chromosome

Meiosis 1

Meiosis 2

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The value of meiosis

• Meiosis introduces genetic variation– gametes of offspring do not have same genes as gametes from parents

– This is the raw material for EVOLUTIONEVOLUTIONEVOLUTIONEVOLUTION

from Momfrom Momfrom Momfrom Mom from Dadfrom Dadfrom Dadfrom Dadnew gametesnew gametesnew gametesnew gametes

made by offspringmade by offspringmade by offspringmade by offspring

Sources of genetic variability

• Genetic variability in sexual reproduction

– independent assortment•homologous chromosomes in Meiosis 1

– crossing over•between homologous chromosomes in prophase 1

– random fertilization• random ovum fertilized by a random sperm

Any 2 parents will produce a zygote with over 70 trillion(223 x 223) diploid combinations

Sexual reproduction creates variability

Sexual reproduction allows us to maintain both genetic similarity & differences.

Martin & Charlie Sheen, Emilio EstevezMartin & Charlie Sheen, Emilio EstevezMartin & Charlie Sheen, Emilio EstevezMartin & Charlie Sheen, Emilio Estevez

Michael & KirkMichael & KirkMichael & KirkMichael & KirkDouglasDouglasDouglasDouglas

Baldwin brothersBaldwin brothersBaldwin brothersBaldwin brothers