Ch13
Meiosis
BIOL221
Chromosomenumber
•
Humancells- Diploid
• 46totalchromosomespercell
• 46- Diploidnumber
•
Humanscells- 23pairsofhomologouschromosomes
• 23- Haploidnumber
• Thenumberofdifferentkindsofchromosomes
OverviewofMeiosis
•
Meiosis
•
Processofasinglediploidcelldividingto
producefourhaploidcells
•
Cellsthatcontainasinglesetof
chromosomes
•
•
•
Forreproduction
Gametes
•
Haploidcellsproducedthroughmeiosis
•
Femalegametesareeggs
•
Malegametesaresperm
Theyarethereproductivecellsofhumanbeings
andmanyotherorganisms.
1
MeiosisComparedtoMitosis
hom ologous
pairs
Hom ologous
m eans the
sam e in size
and function
M itosis
1. Both m itosis and m eiosis are initiated in cells that are
diploid or “ 2n, ” m eaning cells that contain paired sets of
M eiosis
som atic
cell
chrom osom es. The m em bers of each pair are
hom ologous––the sam e in size and function. Tw o pairs
of hom ologous chrom osom es are show n w ithin the cells
in both the m itosis and m eiosis figures. In each
hom ologous pair, one chrom osom e (in red) com es from
the m other of the person w hose cell is undergoing
m eiosis, w hile the other chrom osom
e (in blue) com es
duplication
from the father of this person.
gam ete
precursor
2n
2n
duplication
2. Prior to the initiation of both m itosis and m eiosis, the
chrom osom es duplicate. In both processes, each
2n
chrom osom e is now com posed of tw o sister chrom atids.
2n
3. In m itosis, the chrom osom es line up on the m etaphase
plate, one sister chrom atid on each side of the plate.
2n
In m eiosis, hom ologous chrom osom es— not sister
chrom atids— line up on opposite sides of the
m etaphase plate.
2n
4. In m itosis, the sister chrom atids separate. In m eiosis, the
2n
hom ologous pairs of chrom osom es separate.
2n
division
division
5. In m itosis, cell division takes place, and each of the sister
chrom atids from step 4 is now a full-fledged chrom osom e.
2n
2n
1n
1n
1n
1n
M itosis is finished. In m eiosis, one m em ber of each
hom ologous pair has gone to one cell, the other m em ber
to the other cell. Because each of these cells now has
only a single set of chrom osom es, each is in the haploid
or “ 1n ” state. Next, these single chrom osom es line up on
the m etaphase plate, w ith their sister chrom atids on
6. The
sistersides
chrom
of each chrom osom e then separate.
opposite
ofatids
the plate.
division
division
7. The cells divide again, yielding four haploid cells.
1n
1n
1n
1n
TheStepsinMeiosis
Twoprimarystagesinmeiosis
•
meiosisI
•
Now2haploid daughtercells
•
meiosisII
•
4differentgametes
•
•
Dependingongender
MeiosisI
•
ProphaseI(afterchromosomeduplication)
•
First- pairingofhomologouschromosomes
•
Crossing-overoccurs
•
Homologouschromosomesexchangereciprocalsectionsof
themselves
•
Increasesvariation
•
Resultsinnotwospermoreggsbeingidentical
2
MeiosisI
•
MetaphaseI
•
•
Homologouschromosomepairslineupatthemetaphaseplate
Onememberofeachhomologouspairisononesideoftheplate,theother
memberisontheotherside
•
Randomassortment
MeiosisI
•
AnaphaseI
•
Homologouspairsseparate
•
eachwillbecomepartofa
separatedaughtercell
MeiosisI
•
TelophaseI
•
separatedHomologouspairsreach
opposite poles
3
MeiosisI
CytokinesisI
•
Twodaughtercellsfullyseparated
•
•
Nowhaploid
•
23chromosomespercell
•
Nohomologouspairspresent
•
Eachchromosomestillinduplicatedstate
MeiosisII
•
MeiosisII
•
Sisterchromatidsofthe
duplicatedchromosomesare
separatedintoseparate
daughtercells
•
NosubsequentDNA
replication
•
Proceedsmuchlikemitosis
fromthispoint
•
Only23setsofsister
chromatidspresent
insteadof46
MeiosisII
•
MeiosisII
•
ProphaseII
•
•
Nuclearmembranes
breakdown
•
Iftheyreformedat
allaftermeiosisI
•
Newmitoticspindle
forms
MetaphaseII
•
23sisterchromatids
lineduponmetaphase
plate
•
Attachedtomitotic
spindleatthe
centromere
4
MeiosisII
• MeiosisII
• AnaphaseII
• 23setsofsisterchromatids
separateatcentromere
• Traveltopoles
• TelophaseII
• Separatedchromosomesatthe
poles
• Nuclearenvelopesreform
• Cleavagefurrowbeginstoform
• CytokenesisII
• Cleavagefurrowgrowstopinchoff
cellintotwonewdaughtercells
• NowFOURdaughterhaploid
gametes,readyformaturation
MeiosisGeneratesDiversity
•
Meiosisisunlikemitosis
Inmitosis,TWOdaughtercellsareexact
•
homologous
pairs
Homologous
means the
same in size
and function
geneticcopiesofparentcells
Mitosis
Meiosis
somatic
cell
gamete
precursor
2n
2n
duplication
•
Diploid(46chromosomes)
•
2copiesofeachhomologous
2n
chromosome(23x2)
2n
2n
2n
Inmeiosis,FOURdaughtercells
•
2n
(gametes)arenotidentical
2n
•
Haploid(23chromosomes)
•
1copyofeachchromosome
2n
division
2n
1n
1n
1n
1n
division
1n
1n
1n
1n
Meiosis
•
Meiosisprovidesvariationingametesintwoways
•
Crossingover
•
•
Mixesgeneticmaterialwithinchromosomes
Independentassortment
•
Mixesmaternalandpaternalchromosomes
5
MeiosisGeneratesDiversity
Crossingover
•
• ProphaseI ofmeiosis
• Homologouschromosomespair
witheachother
• Chromosomesexchangereciprocal
segmentswithoneanother
• Tetrads
• Alignedreplicated
homologouspairs
• Chiasma
• Pointonthechromosomes
wherecrossingoveroccurs
MeiosisGeneratesDiversity
•
Independentassortment
•
MetaphaseI ofmeiosis
•
Randomalignmentofmaternalandpaternalchromosomes(homologouspairs)
•
Randomchancealignment
•
•
Oneithersideofthemetaphaseplate
Determineswhichdaughtercelleachchromosome(maternalorpaternal)will
endupin
MeiosisandGenderOutcome
•
Humanfemales
•
23matchedpairsofchromosomes
•
22pairsofautosomes
•
onepairofsex-determining
chromosomes
•
femalesareXX
•
•
homozygous
Humanmales
•
22pairsofautosomes
•
Onepairofsexchromosomes
•
oneXandoneY
•
hemizygous
6
MeiosisandSexOutcome
•
Gametes
•
Eachfemaleeggcontains
•
•
OneXchromosome
Eachmalespermcontains
•
OneX
or
•
OneY
Youshouldnowbeableto:
1.
Distinguishbetweenthefollowingterms:somaticcellandgamete;
autosomeandsexchromosomes;haploidanddiploid
2.
Describetheeventsthatcharacterizeeachphaseofmeiosis
3.
DescribethreeeventsthatoccurduringmeiosisIbutnotmitosis
4.
Nameandexplainthethreeeventsthatcontributetogenetic
variationinsexuallyreproducingorganisms
7