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ONE LAST TIME :What is the difference between MITOSIS

and MEIOSIS?

HOW ARE THEY SIMILAR?HOW ARE THEY DIFFERENT?

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Mitosis and Meiosis are both forms of dividing the nucleus

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QUIZ Question 1

II

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MM

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Mitosis Meiosis

WHAT it is used for

How many daughter cells

Are the daughters identical to the parent?

If the parent is diploid with 16 chromosomes, what will the daughter be?

Does the DNA need to replicate before division?

Does crossing over occur?

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Mitosis Cell DivisionMitosis Cell Division Used for Used for growth and repairgrowth and repair

Occurs inOccurs in somatic (body) cells somatic (body) cells

Produces Produces twotwo new cells new cells identical identical to the to the parent cellparent cell

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ChromosomChromosomeses

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ChromosomesChromosomesChromosomes store genetic informationChromosomes store genetic information

Human cells Human cells have have 4646 chromosomes: 23 chromosomes: 23 pairspairs

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Chromosomes Chromosomes

Duplicated chromosomes are called chromatids

chromatids held together by centromere

Called Sister Called Sister ChromatidsChromatids

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Body cells are Body cells are diploid = 2ndiploid = 2n

The pairs are The pairs are called called

homolohomologsgs

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The Problem:

How can the cell divide and cut its chromosome number in ½?

How can we go from 2n to 1n?

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Meiosis

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Facts About MeiosisFacts About Meiosis

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

reduction- divisionreduction- division

Original cell isOriginal cell is diploid (2n) diploid (2n)

Four daughter cells areFour daughter cells are haploid haploid (1n)(1n)

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Facts About MeiosisFacts About MeiosisProducesProduces gametesgametes (eggs & (eggs &

sperm)sperm)

Occurs in theOccurs in the testestestes in malesin males (Spermatogenesis)(Spermatogenesis)

Occurs in theOccurs in the ovariesovaries in in femalesfemales ( (OogenesisOogenesis))

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• Here is the problem:

•How do you cut the chromosome number in ½?

•How do you divide and get the right chromosomes into each cell?

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• A major difference between the divisions of mitosis and meiosis:

• The HOMOLOGS pair up before dividing

http://www.pbs.org/wgbh/nova/baby/divi_flash.htmlhttp://www.pbs.org/wgbh/nova/baby/divi_flash.html

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Meiosis: Two Cell Meiosis: Two Cell DivisionsDivisions

HomologsHomologsseparateseparate

Sister Sister chromatidschromatidsseparateseparate

DiploidDiploid

MeiosisMeiosis I I

MeiosisMeiosisIIII

DiploidDiploid

HaploiHaploidd

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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.

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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

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Crossing-OverCrossing-Over Homologous Homologous

chromosomeschromosomes in a tetrad in a tetrad cross over cross over each othereach other

Pieces of Pieces of chromosomes chromosomes or or genes are genes are exchangedexchanged

Produces Produces Genetic Genetic recombinationrecombination in the offspringin the offspring

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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

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Metaphase IMetaphase I

Homologous Homologous pairspairs of of

chromosomes chromosomes align along the align along the equatorequator of the of the

cellcell

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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.

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Telophase ITelophase I

Nuclear envelopes Nuclear envelopes reassemble.reassemble.

Spindle disappears.Spindle disappears.

Cytokinesis Cytokinesis divides cell divides cell into two.into two.

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Prophase IIProphase II

Nuclear Nuclear envelope envelope fragments.fragments.

Spindle forms.Spindle forms.

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Metaphase IIMetaphase II

Chromosomes align Chromosomes align along along equatorequator of cell of cell..

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Anaphase IIAnaphase II

Sister Sister chromatidschromatids separate and separate and move to move to opposite polesopposite poles.

EquatorEquator

PolePole

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Telophase IITelophase II

Nuclear envelope Nuclear envelope assembles.assembles.

Chromosomes Chromosomes decondense.decondense.

Spindle Spindle disappears.disappears.

Cytokinesis Cytokinesis divides cell into divides cell into two.two.

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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

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GametogenesisGametogenesis

OogenesisOogenesis or or

Spermatogenesis Spermatogenesis

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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

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SpermatogenesisSpermatogenesis

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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 division of cytoplasmunequal division 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 28 daysevery 28 days

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OogenesisOogenesis

OogoniumOogonium(diploid)(diploid)

MitosisMitosis Meiosis IMeiosis I

Ovum (egg)Ovum (egg)

a

A

X

X

a

X

A X

a

X

a

X

MatureMatureeggegg

A

X

A

X

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Comparing Comparing Mitosis and Mitosis and

MeiosisMeiosis

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Mitosis Meiosis

WHAT it is used for

How many daughter cells

Are the daughters identical to the parent?

If the parent is diploid with 16 chromosomes, what will the daughter be?

Does the DNA need to replicate before division?

Does crossing over occur?

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MitosisMitosis MeiosisMeiosis

Number of Number of divisionsdivisions

Number of Number of daughter cellsdaughter cells

Genetically Genetically identical?identical?

Chromosome #Chromosome #

WhereWhere

WhenWhen

RoleRole

Comparison of DivisionsComparison of Divisions

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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 DivisionsComparison of Divisions