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BIOL221

Ch13Meiosis

Chromosomenumber• Humancells- Diploid

• 46totalchromosomespercell

• 46- Diploidnumber

• Humanscells- 23pairsofhomologouschromosomes

• 23- Haploidnumber

• Thenumberofdifferentkindsofchromosomes

OverviewofMeiosis• Meiosis

• Processofasinglediploidcelldividingto

producefourhaploidcells

• Cellsthatcontainasinglesetof

chromosomes

• Forreproduction

• Gametes

• Haploidcellsproducedthroughmeiosis

• Femalegametesareeggs

• Malegametesaresperm

• Theyarethereproductivecellsofhumanbeingsandmanyotherorganisms.

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MeiosisComparedtoMitosis

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

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2.

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TheStepsinMeiosis

• Twoprimarystagesinmeiosis

• meiosisI

• Now2haploid daughtercells

• meiosisII

• 4differentgametes

• Dependingongender

MeiosisI• ProphaseI(afterchromosomeduplication)

• First- pairingofhomologouschromosomes

• Crossing-overoccurs

• Homologouschromosomesexchangereciprocalsectionsofthemselves

• Increasesvariation

• Resultsinnotwospermoreggsbeingidentical

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MeiosisI• MetaphaseI

• Homologouschromosomepairslineupatthemetaphaseplate

• Onememberofeachhomologouspairisononesideoftheplate,theother

memberisontheotherside

• Randomassortment

MeiosisI

• AnaphaseI

• Homologouspairsseparate

• eachwillbecomepartofa

separatedaughtercell

MeiosisI

• TelophaseI

• separatedHomologouspairsreach

opposite poles

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MeiosisI• CytokinesisI

• Twodaughtercellsfullyseparated

• Nowhaploid

• 23chromosomespercell

• Nohomologouspairspresent

• Eachchromosomestillinduplicatedstate

MeiosisII• MeiosisII

• Sisterchromatidsofthe

duplicatedchromosomesare

separatedintoseparate

daughtercells

• NosubsequentDNA

replication

• Proceedsmuchlikemitosis

fromthispoint

• Only23setsofsister

chromatidspresent

insteadof46

MeiosisII• MeiosisII

• ProphaseII

• Nuclearmembranesbreakdown

• IftheyreformedatallaftermeiosisI

• Newmitoticspindleforms

• MetaphaseII

• 23sisterchromatidslineduponmetaphaseplate

• Attachedtomitoticspindleatthecentromere

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MeiosisII• MeiosisII

• AnaphaseII

• 23setsofsisterchromatidsseparateatcentromere

• Traveltopoles

• TelophaseII

• Separatedchromosomesatthepoles

• Nuclearenvelopesreform

• Cleavagefurrowbeginstoform• CytokenesisII

• Cleavagefurrowgrowstopinchoffcellintotwonewdaughtercells

• NowFOURdaughterhaploidgametes,readyformaturation

MeiosisGeneratesDiversity

• Meiosisisunlikemitosis

• Inmitosis,TWOdaughtercellsareexact

geneticcopiesofparentcells

• Diploid(46chromosomes)

• 2copiesofeachhomologous

chromosome(23x2)

• Inmeiosis,FOURdaughtercells

(gametes)arenotidentical

• Haploid(23chromosomes)

• 1copyofeachchromosome

somaticcell

duplication

gameteprecursor

Homologousmeans the

same in sizeand function

2n2n

2n2n

2n2n

2n2n

1n2n 1n

1n1n

1n1n 1n1n

2n

homologouspairs

Mitosis Meiosis

division

division

Meiosis

• Meiosisprovidesvariationingametesintwoways

• Crossingover

• Mixesgeneticmaterialwithinchromosomes

• Independentassortment

• Mixesmaternalandpaternalchromosomes

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MeiosisGeneratesDiversity• Crossingover

• ProphaseI ofmeiosis

• Homologouschromosomespairwitheachother

• Chromosomesexchangereciprocalsegmentswithoneanother

• Tetrads

• Alignedreplicatedhomologouspairs

• Chiasma

• Pointonthechromosomeswherecrossingoveroccurs

MeiosisGeneratesDiversity• Independentassortment

• MetaphaseI ofmeiosis

• Randomalignmentofmaternalandpaternalchromosomes(homologouspairs)

• Oneithersideofthemetaphaseplate

• Randomchancealignment

• Determineswhichdaughtercelleachchromosome(maternalorpaternal)willendupin

MeiosisandGenderOutcome• Humanfemales

• 23matchedpairsofchromosomes

• 22pairsofautosomes

• onepairofsex-determiningchromosomes

• femalesareXX

• homozygous

• Humanmales

• 22pairsofautosomes

• Onepairofsexchromosomes

• oneXandoneY

• hemizygous

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MeiosisandSexOutcome• Gametes

• Eachfemaleeggcontains

• OneXchromosome

• Eachmalespermcontains

• OneX

or

• OneY

Youshouldnowbeableto:

1. Distinguishbetweenthefollowingterms:somaticcellandgamete;autosomeandsexchromosomes;haploidanddiploid

2. Describetheeventsthatcharacterizeeachphaseofmeiosis

3. DescribethreeeventsthatoccurduringmeiosisIbutnotmitosis

4. Nameandexplainthethreeeventsthatcontributetogeneticvariationinsexuallyreproducingorganisms