MeiosisMeiosisFormation of Formation of GametesGametes (Eggs & (Eggs & Sperm)Sperm)
Facts About MeiosisFacts About MeiosisPreceded by interphase which Preceded by interphase which
includesincludes chromosome replicationchromosome replicationTwoTwo meiotic divisions --- meiotic divisions --- Meiosis Meiosis
I and Meiosis III and Meiosis IICalled Reduction- divisionCalled Reduction- divisionOriginal cell is diploid (2n)Original cell is diploid (2n)Four daughter cellsFour daughter cells produced produced
that are that are monoploid (1n)monoploid (1n)
Facts About MeiosisFacts About Meiosis
Daughter cellsDaughter cells contain half the contain half the number of chromosomesnumber of chromosomes as the as the original celloriginal cell
ProducesProduces gametesgametes (eggs & sperm(eggs & sperm))Occurs in the Occurs in the testestestes in males in males
((SpermatogenesisSpermatogenesis))Occurs in the Occurs in the ovariesovaries in females in females
((OogenesisOogenesis))
Start with Start with 46 double stranded 46 double stranded chromosomes (2n)chromosomes (2n)After 1 division - After 1 division - 23 double 23 double stranded chromosomes (n)stranded chromosomes (n)After 2nd division - After 2nd division - 23 single 23 single stranded chromosomes (n)stranded chromosomes (n) Occurs in our Occurs in our germ cellsgerm cells that that produce gametesproduce gametes
More Meiosis FactsMore Meiosis Facts
Why Do we Need Why Do we Need Meiosis?Meiosis?It is the fundamental basis of It is the fundamental basis of
sexual reproductionsexual reproductionTwo haploid (1n) gametes Two haploid (1n) gametes
are brought together are brought together through through fertilizationfertilization to form to form a diploid a diploid (2n) zygote(2n) zygote
Fertilization – “Putting Fertilization – “Putting it all together”it all together”
1n =31n =3
2n = 62n = 6
Replication of Replication of ChromosomesChromosomesReplicationReplication is the is the
process of process of duplicating a duplicating a chromosomechromosome
Occurs Occurs prior to prior to divisiondivision
Replicated copies Replicated copies are called are called sister sister chromatidschromatids
Held together at Held together at centromerecentromere
Occurs in Occurs in InterphasInterphas
ee
A Replicated A Replicated ChromosomeChromosome
HomologsHomologs (same genes, (same genes, different alleles)different alleles)
SisterSisterChromatidsChromatids(same genes,(same genes,same alleles)same alleles)
Gene XGene X
Homologs separate in meiosis I and Homologs separate in meiosis I and therefore different alleles separate.therefore different alleles separate.
Meiosis Forms Haploid Meiosis Forms Haploid GametesGametes
Meiosis must Meiosis must reduce the chromosome number by halfreduce the chromosome number by half Fertilization then restoresFertilization then restores the 2n number the 2n number
from mom from dad child
meiosis reducesgenetic content
toomuch!
The right number!
Meiosis: Two Part Cell Meiosis: Two Part Cell DivisionDivision
HomologsHomologsseparateseparate
Sister Sister chromatidschromatidsseparateseparate
HaploidHaploid
MeiosisMeiosis I I
MeiosisMeiosisIIII
DiploidDiploid
HaploiHaploidd
Meiosis I: Reduction DivisionMeiosis I: Reduction Division
NucleusNucleus SpindleSpindlefibersfibers NuclearNuclear
envelopeenvelopeEarly Early Prophase IProphase I(Chromoso(Chromosome number me number
doubleddoubled))
Late Late ProphasProphas
e Ie I
MetaphaMetaphase Ise I AnaphasAnaphas
e Ie ITelophase Telophase I I (haploid)(haploid)
Prophase IProphase I
Early Early prophaseprophaseHomologs Homologs pair.pair.Crossing over Crossing over occursoccurs.
Late prophaseLate prophaseChromosomes Chromosomes condense.condense.Spindle forms.Spindle forms.Nuclear envelope Nuclear envelope fragments.fragments.
Tetrads Form in Tetrads Form in Prophase IProphase I
Homologous Homologous chromosomeschromosomes
(each with sister chromatids) (each with sister chromatids)
Join to form aJoin to form a TETRADTETRAD
Called SynapsisCalled Synapsis
Crossing-OverCrossing-Over Homologous Homologous
chromosomeschromosomes in in a tetrad a tetrad cross cross over each otherover each other
Pieces of Pieces of chromosomes chromosomes or or genes are genes are exchangedexchanged
Produces Produces Genetic Genetic recombinationrecombination in the offspringin the offspring
Homologous Homologous Chromosomes During Chromosomes During Crossing-OverCrossing-Over
Crossing-over multiplies the already Crossing-over multiplies the already huge number of different gamete huge number of different gamete types produced by independent types produced by independent
assortmentassortment
Crossing-OverCrossing-Over
Metaphase IMetaphase I
Homologous Homologous pairspairs of of
chromosomes chromosomes align along the align along the equatorequator of the of the
cellcell
Anaphase IAnaphase I
Homologs separateHomologs separate and move to opposite and move to opposite poles.poles.
Sister chromatids Sister chromatids remain remain attached attached at theirat their centromerescentromeres.
Telophase ITelophase I
Nuclear envelopes Nuclear envelopes reassemble.reassemble.
Spindle disappears.Spindle disappears.
Cytokinesis Cytokinesis divides cell divides cell into two.into two.
Meiosis IIMeiosis II
Only one homolog of Only one homolog of each chromosome is each chromosome is present in the cellpresent in the cell.
Meiosis II produces gametes Meiosis II produces gametes with with
one copy of each chromosome one copy of each chromosome and thus one copy of each and thus one copy of each gene.gene.
Sister chromatids carry Sister chromatids carry identical genetic identical genetic
informationinformation. .
Gene XGene X
Meiosis II: Reducing Meiosis II: Reducing Chromosome NumberChromosome Number
Prophase Prophase IIII
MetaphaMetaphase IIse II
AnaphasAnaphase IIe II
TelophasTelophase IIe II 4 Different 4 Different
haploid haploid cellscells
Prophase IIProphase II
Nuclear Nuclear envelope envelope fragments.fragments.
Spindle forms.Spindle forms.
Metaphase IIMetaphase II
Chromosomes align Chromosomes align along along equatorequator of cell of cell..
Anaphase IIAnaphase II
Sister Sister chromatidschromatids separate and separate and move to move to opposite polesopposite poles.
EquatorEquator
PolePole
Telophase IITelophase IINuclear envelope Nuclear envelope assembles.assembles.
Chromosomes Chromosomes decondense.decondense.
Spindle Spindle disappears.disappears.
Cytokinesis Cytokinesis divides cell into divides cell into two.two.
Results of MeiosisResults of MeiosisGametes (egg & Gametes (egg & sperm) formsperm) form
Four haploid cells Four haploid cells with one copy of each with one copy of each chromosome chromosome
One allele of each One allele of each genegene
Different Different combinations of combinations of alleles for different alleles for different genes along the genes along the chromosomechromosome
GametogenesisGametogenesis
OogenesisOogenesis or or
Spermatogenesis Spermatogenesis
SpermatogenesisSpermatogenesisOccurs in the Occurs in the
testestestesTwo divisions Two divisions
produce 4 produce 4 spermatidsspermatids
Spermatids Spermatids mature into mature into spermsperm
Men produce Men produce about about 250,000,000 250,000,000 sperm per daysperm per day
Spermatogenesis in the Spermatogenesis in the TestesTestes
SpermatiSpermatidd
SpermatogenesisSpermatogenesis
OogenesisOogenesisOccurs in the Occurs in the ovariesovariesTwo divisions produce Two divisions produce 3 polar 3 polar
bodiesbodies that die and that die and 1 egg1 eggPolar bodies die because of Polar bodies die because of unequal unequal
division of cytoplasmdivision of cytoplasmImmature egg called Immature egg called oocyteoocyteStarting at puberty, one oocyte Starting at puberty, one oocyte
matures into an matures into an ovum (egg)ovum (egg) every every 28 days28 days
Oogenesis in the Oogenesis in the OvariesOvaries
OogenesisOogenesis
OogoniumOogonium(diploid)(diploid)
MitosisMitosis
PrimaryPrimaryoocyteoocyte(diploid)(diploid)
Meiosis IMeiosis I
SecondarySecondaryoocyteoocyte(haploid)(haploid)
Meiosis IIMeiosis II(if fertilization(if fertilizationoccurs)occurs)
First polar bodyFirst polar bodymay divide may divide (haploid)(haploid) PolarPolar
bodiesbodiesdiedie
Ovum (egg)Ovum (egg)
SecondSecondpolar bodypolar body(haploid)(haploid)
a
A
X
X
a
X
A X
a
X
a
X
MatureMatureeggegg
A
X
A
X
Comparing Comparing Mitosis and Mitosis and MeiosisMeiosis
MitosisMitosis MeiosisMeiosisNumber of Number of
divisionsdivisions 1122
Number of Number of daughter cellsdaughter cells 22 44
Genetically Genetically identical?identical? YesYes NoNo
Chromosome #Chromosome # Same as Same as parentparent Half of parentHalf of parent
WhereWhere Somatic cellsSomatic cells Germ cellsGerm cells
WhenWhen Throughout Throughout lifelife
At sexual At sexual maturitymaturity
RoleRole Growth and Growth and repairrepair
Sexual Sexual reproductionreproduction
Comparison of Comparison of DivisionsDivisions
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