MeiosisMeiosisFormation of Formation of

GametesGametes (Eggs & (Eggs & Sperm)Sperm)

Facts About MeiosisFacts About Meiosis

Preceded by interphase which Preceded by interphase which includesincludes chromosome replicationchromosome replication

TwoTwo meiotic divisions ---meiotic divisions --- Meiosis I and Meiosis IIMeiosis I and Meiosis II

Original cell is diploid (2n)Original cell is diploid (2n)Four daughter cellsFour daughter cells produced produced

that arethat are monoploid or haploid monoploid or haploid (1n)(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 theOccurs in the testestestes in malesin males

((SpermatogenesisSpermatogenesis))Occurs in theOccurs in the ovariesovaries in femalesin females

((OogenesisOogenesis))

Start withStart 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 ourOccurs 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 basisIt is the fundamental basis of of sexualsexual reproductionreproduction

Two haploid (1n) gametes are Two haploid (1n) gametes are brought together throughbrought together through fertilizationfertilization to form a diploidto form a diploid (2n) (2n) zygote zygote

Fertilization – “Putting Fertilization – “Putting it all together”it all together”

1n =31n =3

2n = 62n = 6

Replication of Replication of ChromosomesChromosomes

ReplicationReplication is the is the process of process of duplicating a duplicating a chromosomechromosome

OccursOccurs prior to prior to divisiondivision

Replicated copies Replicated copies are calledare called sister sister chromatidschromatids

Held together atHeld 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 mustMeiosis must reduce the chromosome number by halfreduce the chromosome number by half Fertilization then restoresFertilization then restores the 2n numberthe 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

DiploidDiploid

MeiosisMeiosis I I

MeiosisMeiosisIIII

DiploidDiploid

HaploiHaploidd

Meiosis IMeiosis I

NucleusNucleus SpindleSpindlefibersfibers NuclearNuclear

envelopeenvelopeEarly Early

Prophase IProphase I(Chromoso(Chromosome number me number

doubled)doubled)

Late Late ProphasProphas

e Ie IMetaphaMetapha

se Ise I AnaphasAnaphase Ie I

Telophase Telophase II (diploid)(diploid)

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 tetrada tetrad cross cross over each otherover each other

Pieces of Pieces of chromosomeschromosomes oror genes are genes are exchangedexchanged

Produces Produces Genetic Genetic recombinationrecombination in the offspringin the offspring

Homologous Homologous Chromosomes During Chromosomes During

Crossing-OverCrossing-Over

Metaphase IMetaphase I

Homologous Homologous pairspairs of of

chromosomes chromosomes align along thealign 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 chromatidsSister chromatids remainremain attachedattached at theirat their centromerescentromeres.

Telophase ITelophase I

Nuclear envelopesNuclear envelopes reassemble.reassemble.

Spindle disappears.Spindle disappears.

CytokinesisCytokinesis 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 Identical 4 Identical

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

SisterSister 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

Meiosis AnimationMeiosis Animation

http://www.johnkyrk.com/meiosis.html

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