1

Chapter13MeiosisandSexualLifeCycles

Chromosomenumber

• Humancells‐Diploid– 46totalchromosomespercell

• 46‐Diploidnumber

• Humanscells‐23pairsofhomologouschromosomes– 23‐Haploidnumber– Thenumberofdifferentkindsofchromosomes

AnOverviewofMeiosis• Humancellsareconsidereddiploidbecauseeachcellhastwocopies– Someorganisms

• Haploid• triploid• tetraploid

OverviewofMeiosis

• Meiosis– Processofasinglediploidcelldividingtoproducefourhaploidcells• Cellsthatcontainasinglesetofchromosomes• ForreproducKon

2

OverviewofMeiosis• Gametes

– Haploidcellsproducedthroughmeiosisare– Femalegametesareeggs

– Malegametesaresperm.

• TheyarethereproducKvecellsofhumanbeingsandmanyotherorganisms.

MeiosisComparedtoMitosis

Figure 10.1

Both mitosis and meiosis are initiated in cells that are diploid or “2n,” meaning cells that contain paired sets ofchromosomes. The members of each pair are homologous––the same in size and function. Two pairsof homologous chromosomes are shown within the cellsin both the mitosis and meiosis figures. In each homologous pair, one chromosome (in red) comes from the mother of the person whose cell is undergoing meiosis, while the other chromosome (in blue) comes from the father of this person.

Prior to the initiation of both mitosis and meiosis, thechromosomes duplicate. In both processes, eachchromosome is now composed of two sister chromatids.

In mitosis, the chromosomes line up on the metaphaseplate, one sister chromatid on each side of the plate.In meiosis, homologous chromosomes—not sisterchromatids—line up on opposite sides of themetaphase plate.

In mitosis, the sister chromatids separate. In meiosis, thehomologous pairs of chromosomes separate.

The cells divide again, yielding four haploid cells.

somaticcell

duplication duplication

gameteprecursor

Homologousmeans the

same in sizeand function

2n2n

2n2n

2n2n

2n2n

1n2n 1n

1n1n

1n1n 1n1n

2n

homologouspairs

Mitosis Meiosis

In mitosis, cell division takes place, and each of the sisterchromatids from step 4 is now a full-fledged chromosome.Mitosis is finished. In meiosis, one member of eachhomologous pair has gone to one cell, the other memberto the other cell. Because each of these cells now hasonly a single set of chromosomes, each is in the haploidor “1n” state. Next, these single chromosomes line up onthe metaphase plate, with their sister chromatids onopposite sides of the plate.The sister chromatids of each chromosome then separate.

division division

division division

1.

2.

3.

4.

7.

5.

6.

OverviewofMeiosis

• Whenthehaploidspermandhaploideggfuse,adiploidferKlizedegg(orzygote)isproduced,seSngintodevelopmentanewgeneraKonoforganism.

Chapter13MeiosisandSexualLifeCycles

10.2TheStepsinMeiosis

3

TheStepsinMeiosis• Meiosis

– oneroundofchromosomeduplicaKon

– followedbytworoundsofcelldivision• NosecondchromosomeduplicaKonaVerfirstdivision

Both mitosis and meiosis are initiated in cells that are diploid or “2n,” meaning cells that contain paired sets ofchromosomes. The members of each pair are homologous––the same in size and function. Two pairsof homologous chromosomes are shown within the cellsin both the mitosis and meiosis figures. In each homologous pair, one chromosome (in red) comes from the mother of the person whose cell is undergoing meiosis, while the other chromosome (in blue) comes from the father of this person.

Prior to the initiation of both mitosis and meiosis, thechromosomes duplicate. In both processes, eachchromosome is now composed of two sister chromatids.

In mitosis, the chromosomes line up on the metaphaseplate, one sister chromatid on each side of the plate.In meiosis, homologous chromosomes—not sisterchromatids—line up on opposite sides of themetaphase plate.

In mitosis, the sister chromatids separate. In meiosis, thehomologous pairs of chromosomes separate.

The cells divide again, yielding four haploid cells.

somaticcell

duplication duplication

gameteprecursor

Homologousmeans the

same in sizeand function

2n2n

2n2n

2n2n

2n2n

1n2n 1n

1n1n

1n1n 1n1n

2n

homologouspairs

Mitosis Meiosis

In mitosis, cell division takes place, and each of the sisterchromatids from step 4 is now a full-fledged chromosome.Mitosis is finished. In meiosis, one member of eachhomologous pair has gone to one cell, the other memberto the other cell. Because each of these cells now hasonly a single set of chromosomes, each is in the haploidor “1n” state. Next, these single chromosomes line up onthe metaphase plate, with their sister chromatids onopposite sides of the plate.The sister chromatids of each chromosome then separate.

division division

division division

1.

2.

3.

4.

7.

5.

6.

TheStepsinMeiosis

• Twoprimarystagesinmeiosis– meiosisI

– meiosisII

MeiosisI• ProphaseI(aVerchromosomeduplicaKon)

– First‐pairingofhomologouschromosomes

– Crossing‐overoccurs• HomologouschromosomesexchangereciprocalsecKonsofthemselves

• IncreasesvariaKon• ResultsinnotwospermoreggsbeingidenKcal

MeiosisI• MetaphaseI

– Homologouschromosomepairslineupatthemetaphaseplate

• Onememberofeachhomologouspairisononesideoftheplate,theothermemberisontheotherside– Randomassortment

4

MeiosisI

• AnaphaseI– Homologouspairsseparate

• eachwillbecomepartofaseparatedaughtercell.

MeiosisI

• TelophaseI– separatedHomologouspairsreachoppositepoles

MeiosisI

• CytokinesisI– Twodaughtercellsfullyseparated

• Nowhaploid• 23chromosomespercell

– Nohomologouspairspresent– EachchromosomesKllinduplicatedstate

MeiosisII

• MeiosisII– SisterchromaKdsoftheduplicatedchromosomesareseparatedintoseparatedaughtercells• NosubsequentDNAreplicaKon• Proceedsmuchlikemitosisfromthispoint

– Only23setsofsisterchromaKdspresentinsteadof46

5

MeiosisII

• MeiosisII– ProphaseII

• Nuclearmembranesbreakdown– IftheyreformedatallaVermeiosisI– NewmitoKcspindleforms

– MetaphaseII• 23sisterchromaKdslineduponmetaphaseplate

• A^achedtomitoKcspindleatthecentromere

MeiosisII

• MeiosisII– AnaphaseII

• 23setsofsisterchromaKdsseparateatcentromere– Traveltopoles

– TelophaseII• Separatedchromosomesatthepoles• Nuclearenvelopesreform• Cleavagefurrowbeginstoform

– CytokenesisII• Cleavagefurrowgrowstopinchoffcellintotwonewdaughtercells

– NowFOURdaughterhaploidgametes,readyformaturaKon

MeiosisII

Figure 10.2

(a) Meiosis I

(b) Crossing over (c) Independent assortment

Homologous chromosomeslink as theycondense, formingtetrads.

DNA has alreadyduplicated

Microtubulesmove homologouschromosomesto metaphase plate. Microtubules

separate homologouschromosomes(sister chromatidsremain together).

Crossing overoccurs.

Independentassortmentoccurs.

duplicated paternal

chromosome

Exchange of parts of non-sister chromatids.

sister chromatids

non-sisterchromatids

Diploid

Prophase IEnd ofinterphase

Metaphase I

Anaphase I

duplicated maternal

chromosometetrad

First important sourceof genetic variation

Two haploiddaughter cellsresult fromcytokinesis.

(Brief)Sister chromatidsline up at newmetaphase plate. Sister chromatids

separate.Four haploidcells result.

Telophase I IMetaphase II

Metaphase I

In the sequence above,homologous chromosomes lined up this way inMetaphase I . . .

... but they could have lined up this way, yielding a different outcome.

Compare these cellsto the cells above

Haploid

Telophase I Prophase IIMetaphase II

Anaphase II Telophase I I

cytokinesis

cytokinesis

Meiosis II

Second important sourceof genetic variation

Metaphase I

Random alignment of maternal/paternal chromosomes at the metaphase plate.

Chapter13MeiosisandSexualLifeCycles

10.3WhatistheSignificanceofMeiosis?

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WhatistheSignificanceofMeiosis?

• Meiosis– GeneratesdiversitybyensuringthatthegametesitgivesrisetowilldiffergeneKcallyfromoneanother.

MeiosisGeneratesDiversity

• Meiosisisunlikemitosis– Inmitosis,TWOdaughtercellsareexactgeneKccopiesofparentcells• Diploid(46chromosomes)• 2copiesofeachhomologouschromosome(23x2)

– Inmeiosis,FOURdaughtercells(gametes)arenotidenKcal• Haploid(23chromosomes)

• 1copyofeachchromosome

MeiosisComparedtoMitosis

Figure 10.1

Both mitosis and meiosis are initiated in cells that are diploid or “2n,” meaning cells that contain paired sets ofchromosomes. The members of each pair are homologous––the same in size and function. Two pairsof homologous chromosomes are shown within the cellsin both the mitosis and meiosis figures. In each homologous pair, one chromosome (in red) comes from the mother of the person whose cell is undergoing meiosis, while the other chromosome (in blue) comes from the father of this person.

Prior to the initiation of both mitosis and meiosis, thechromosomes duplicate. In both processes, eachchromosome is now composed of two sister chromatids.

In mitosis, the chromosomes line up on the metaphaseplate, one sister chromatid on each side of the plate.In meiosis, homologous chromosomes—not sisterchromatids—line up on opposite sides of themetaphase plate.

In mitosis, the sister chromatids separate. In meiosis, thehomologous pairs of chromosomes separate.

The cells divide again, yielding four haploid cells.

somaticcell

duplication duplication

gameteprecursor

Homologousmeans the

same in sizeand function

2n2n

2n2n

2n2n

2n2n

1n2n 1n

1n1n

1n1n 1n1n

2n

homologouspairs

Mitosis Meiosis

In mitosis, cell division takes place, and each of the sisterchromatids from step 4 is now a full-fledged chromosome.Mitosis is finished. In meiosis, one member of eachhomologous pair has gone to one cell, the other memberto the other cell. Because each of these cells now hasonly a single set of chromosomes, each is in the haploidor “1n” state. Next, these single chromosomes line up onthe metaphase plate, with their sister chromatids onopposite sides of the plate.The sister chromatids of each chromosome then separate.

division division

division division

1.

2.

3.

4.

7.

5.

6.

Meiosis

• MeiosisprovidesvariaKoningametesintwoways– Crossingover

– Independentassortment

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MeiosisGeneratesDiversity

• Crossingover– ProphaseIofmeiosis

• Homologouschromosomespairwitheachother• Chromosomesexchangereciprocalsegmentswithoneanother

• Tetrads– Alignedreplicatedhomologouspairs

• Chiasma– Pointonthechromosomeswherecrossingoveroccurs