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BIOL100

CH4Meiosis

Chromosomenumber•  Humancells-Diploid

•  46totalchromosomespercell

•  46-Diploidnumber

•  Humanscells-23pairsofhomologouschromosomes

•  23-Haploidnumber

•  Thenumberofdifferentkindsofchromosomes

OverviewofMeiosis•  Meiosis

•  Processofasinglediploidcelldividingto

producefourhaploidcells

•  Cellsthatcontainasinglesetof

chromosomes

•  ForreproducCon

•  Gametes

•  Haploidcellsproducedthroughmeiosis

•  Femalegametesareeggs

•  Malegametesaresperm

•  TheyarethereproducCvecellsofhumanbeingsandmanyotherorganisms.

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MeiosisComparedtoMitosis

Both mitosis and meiosis are initiated in cells that are diploid or “2n,” meaning cells that contain paired sets of chromosomes. The members of each pair are homologous––the same in size and function. Two pairs of homologous chromosomes are shown within the cells in 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, the chromosomes duplicate. In both processes, each chromosome is now composed of two sister chromatids.

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

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

The cells divide again, yielding four haploid cells.

somatic cell

duplication duplication

gamete precursor

Homologous means the

same in size and function

2n 2n

2n 2n

2n 2n

2n 2n

1n 2n 1n

1n 1n

1n 1n 1n 1n

2n

homologous pairs

Mitosis Meiosis

In mitosis, cell division takes place, and each of the sister chromatids from step 4 is now a full-fledged chromosome. Mitosis is finished. In meiosis, one member of each homologous pair has gone to one cell, the other member to the other cell. Because each of these cells now has only a single set of chromosomes, each is in the haploid or “1n” state. Next, these single chromosomes line up on the metaphase plate, with their sister chromatids on opposite 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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4.

7.

5.

6.

TheStepsinMeiosis

•  Twoprimarystagesinmeiosis

•  meiosisI

•  Now2haploiddaughtercells

•  meiosisII

•  4differentgametes

•  Dependingongender

MeiosisI•  ProphaseI(aKerchromosomeduplicaCon)

•  First-pairingofhomologouschromosomes

•  Crossing-overoccurs

•  HomologouschromosomesexchangereciprocalsecConsofthemselves

•  IncreasesvariaCon

•  ResultsinnotwospermoreggsbeingidenCcal

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MeiosisI•  MetaphaseI

•  Homologouschromosomepairslineupatthemetaphaseplate

•  Onememberofeachhomologouspairisononesideoftheplate,theother

memberisontheotherside

•  Randomassortment

MeiosisI

•  AnaphaseI

•  Homologouspairsseparate

•  eachwillbecomepartofaseparatedaughtercell

MeiosisI

•  TelophaseI

•  separatedHomologouspairsreachoppositepoles

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MeiosisI•  CytokinesisI

•  Twodaughtercellsfullyseparated

•  Nowhaploid

•  23chromosomespercell

•  Nohomologouspairspresent

•  Eachchromosomes+llinduplicatedstate

MeiosisII•  MeiosisII

•  SisterchromaCdsofthe

duplicatedchromosomesare

separatedintoseparate

daughtercells

•  NosubsequentDNA

replicaCon

•  Proceedsmuchlikemitosis

fromthispoint

•  Only23setsofsister

chromaCdspresent

insteadof46

MeiosisII•  MeiosisII

•  ProphaseII

•  Nuclearmembranesbreakdown

•  IftheyreformedatallaKermeiosisI

•  NewmitoCcspindleforms

•  MetaphaseII

•  23sisterchromaCdslineduponmetaphaseplate

•  AUachedtomitoCcspindleatthecentromere

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MeiosisII•  MeiosisII

•  AnaphaseII

•  23setsofsisterchromaCdsseparateatcentromere

•  Traveltopoles•  TelophaseII

•  Separatedchromosomesatthepoles

•  Nuclearenvelopesreform

•  Cleavagefurrowbeginstoform

•  CytokenesisII

•  Cleavagefurrowgrowstopinchoffcellintotwonewdaughtercells

•  NowFOURdaughterhaploidgametes,readyformaturaCon

MeiosisGeneratesDiversity

•  Meiosisisunlikemitosis

•  Inmitosis,TWOdaughtercellsareexact

geneCccopiesofparentcells

•  Diploid(46chromosomes)

•  2copiesofeachhomologous

chromosome(23x2)

•  Inmeiosis,FOURdaughtercells

(gametes)arenotidenCcal

•  Haploid(23chromosomes)

•  1copyofeachchromosome

somatic cell

duplication

gamete precursor

Homologous means the

same in size and function

2n 2n

2n 2n

2n 2n

2n 2n

1n 2n 1n

1n 1n

1n 1n 1n 1n

2n

homologous pairs

Mitosis Meiosis

division

division

Meiosis

•  MeiosisprovidesvariaConingametesintwoways

•  Crossingover

•  MixesgeneCcmaterialwithinchromosomes

•  Independentassortment

•  Mixesmaternalandpaternalchromosomes

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MeiosisGeneratesDiversity•  Crossingover

•  ProphaseIofmeiosis

•  Homologouschromosomespairwitheachother

•  Chromosomesexchangereciprocalsegmentswithoneanother

•  Tetrads

•  Alignedreplicatedhomologouspairs

•  Chiasma

•  Pointonthechromosomeswherecrossingoveroccurs

MeiosisGeneratesDiversity•  Independentassortment

•  MetaphaseIofmeiosis

•  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.  DisCnguishbetweenthefollowingterms:somaCccellandgamete;autosomeandsexchromosomes;haploidanddiploid

2.  Describetheeventsthatcharacterizeeachphaseofmeiosis

3.  DescribethreeeventsthatoccurduringmeiosisIbutnotmitosis

4.  NameandexplainthethreeeventsthatcontributetogeneCc

variaConinsexuallyreproducingorganisms