Post on 06-Jul-2020
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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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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