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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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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