ch 7 .1 ,7 .2

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MEIOSISMEIOSISCh 7.1,7 .2Ch 7.1 ,7 .2

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MEIOSISMEIOSIS

Organisms reproduce by joining gametes

Gametes are haploid

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MeiosisMeiosis

• Form of cell division whereone cell produces fourgametes, containing half thegenetic material of theparent cell.

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Meiosis VS MitosisMeiosis VS Mitosis

1. Mitosis end result is 2identical daughter cells

Meiosis end result is 4genetically different cells

2.Mitosis occurs in somatic

cellsMeiosis occurs in gametes

3.Mitosis results in 2 diploid

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MeiosisMeiosis

 There are only twocells that undergo

Meiosis.Sperm Cells

Male

Reproductive Cells

Eggs Cells

FemaleReproductive Cells

Fertilization

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MeiosisMeiosis

Meiosis divided into twosections with a total of eightphases.

Meiosis 1

Meiosis II These phases are continuous

and flow one right after the

other.

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Meiosis IMeiosis I

Prophase 1Chromosomes

Condense and CoilUp

Nuclear Envelopedissolves

HomologousChromosomes

Line up in Tetrads Tetrad consists of 

two homologouschromosomesmade up two

sister

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Meiosis IMeiosis I

Prophase 1Crossing over

occurs

Portions of onechromatidare broken

andexchangedwith the

correspondin ortions

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http:/ /www.accessexcellen ce.org

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Meiosis IMeiosis I

Metaphase 1

Centromerebecomes attachedto spindle fibers

 Tetrads line up

along the equator

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Meiosis IMeiosis I

Anaphase 1

HomologousChromosomesmove toopposite ends of 

the cell.Chromatids do not

separate

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Meiosis IMeiosis I

 Telophase 1Spindle fiber breaks

downChromosomes uncoil

Cytokinesis takes

placeBoth cells contain one

chromosome fromeach pair of 

homologous

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MeiosisMeiosis

 There is no replication between meiosis I andmeiosis II

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Meiosis IIMeiosis II

Prophase II

Metaphase II

Anaphase II

 Telophase II

 The above steps are exactly the same as mitosis.

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Meiosis IIMeiosis II

Prophase IINew Spindle fibers form

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Meiosis IIMeiosis II

Anaphase II

Centromeres divide 

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Meiosis IIMeiosis II

 Telophase II

Nuclear Envelope appearsSpindle Fibers break downCytokinesis

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AnimationAnimationhttp://www.csuchico.edu/~ jbell/Biol207/animations/meios i s .h tmlht tp ://www.csu ch ico.e du /~ jbe ll/Biol20 7/a nim a tion s/m eios is .h tml

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MeiosisMeiosis

RESULTS-

4 Haploid cellsGenetically different cells

than the parent cell with half the genetic information.

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MeiosisMeiosis

We need half the informationso that one half comes fromthe male sex cell and theother half comes from thefemale sex cell. Theycombine to have the all thegenetic information needed.

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Formation of Formation of 

GametesGametesCalledCalled

g a m e t o g e n eg a m e t o g e n e

s iss is

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Formation of gametesFormation of gametesSpermatogenesi

s (produces 4haploid sperm)Gametes in

males testes

Diploid cellincreasesin size

Undergoes

Meiosis I

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Formation of gametesFormation of gametesProduces 2

cellsEach of these

2 cellsundergoesMeiosis II

Produces 4undifferentiatedhaploid

sperm cells

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Formation of gametesFormation of gametesOogenesis (produces

1 haploid egg and 3polar bodies)Occurs in the ovaries of 

females In cytokinesis of 

Meiosis I, the

cytoplasm doesn’tdivide equallyProduces 1 cell

with most of thecytoplasm

And 1 polar

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Formation of gametesFormation of gametesBoth undergo

Meiosis IIAgain

unequalcytokinesis

Larger = ovum(egg)Contains a

large

amount of 

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MeiosisMeiosis

 The random way that these lineup in tetrads is what gives us

the diversity in genes that makeus all different from one another.

 This is called Independent

Assortment

About 8 million different possiblecombinations

I d dI d d t

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IndependentIndependent

AssortmentAssortment

 The random distribution of homologous chromosomesduring meiosis 

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Wh t i th i tWh t i th i t

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What is the importanceWhat is the importance

of genetic variation?of genetic variation?

Increases the chances of survival.

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MitosisMitosis

1. Where it occurs Somatic cells

2. End number of Cells 2

3. Number of divisions 1

4. Number of chromosomes in resulting cell 2n (diploid) same as original cell

5. Genetic Variation? No – end cells are exactly the same as the original

cell (except in mutation)

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MeiosisMeiosis

1. Where it occurs Sex cells (gametes)

2. End number of Cells 4

3. Number of divisions 2

4. Number of chromosomes in resulting cell n (haploid) half the number of the original cell

5. Genetic Variation? yes