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Meiosis Meiosis FactsFacts

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MeiosisMeiosis• The form of cell divisioncell division to

produce gametesgametes- cells - cells with halfhalf the number of chromosomeschromosomes

• DiploidDiploid (2n) (2n) haploidhaploid (n) (n)• MeiosisMeiosis is sexualsexual

reproductionreproduction• Two divisionsTwo divisions (meiosis I meiosis I

and meiosis IImeiosis II)

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MeiosisMeiosis

• Sex cells divide to produce gametes (sperm or egg)

• Meiosis-similar to mitosis with some differences– Requires 2 cell divisions– Makes 4 haploid daughter

cells

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MeiosisMeiosis

•Occurs only in gonads (testes or ovaries)

•Male: Spermatogenesis

•Female: Oogenesis

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SpermatogenesiSpermatogenesiss

2n=46

Humansex cell

Diploid (2n)

n=23

n=23

Meiosis I

n=23

n=23

n=23

n=23

Sperm

Haploid (n)

Meiosis II

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Interphase IInterphase I

• Similar to mitosismitosis interphase

• ChromosomesChromosomes replicate (S (S phase)phase)

• Duplicated chromosomechromosomes have two identical sister chromatidschromatids attached at the centromerescentromeres

• CentrioleCentriole pair also replicates.

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Interphase IInterphase I

• NucleusNucleus and nucleolusnucleolus visible.

Nuclear membrane

Nucleolus

Cell membrane

Chromatin

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Meiosis IMeiosis I

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Stages of Meiosis IStages of Meiosis I• Meiosis I- Cell division Meiosis I- Cell division to

reduce chromosomechromosome number by one-half one-half (reduction division)(reduction division)

• FourFour phases phases:

a.a. Prophase IProphase I

b.b. Metaphase IMetaphase I

c.c. Anaphase IAnaphase I

d.d. Telophase ITelophase I

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Prophase IProphase I• Longest and most complexLongest and most complex

phase (90%)phase (90%)• ChromosomesChromosomes condense• SynapsisSynapsis occurs: homologous homologous

chromosomes chromosomes come together forming the tetradtetrad

• TetradTetrad- two replicated chromosomes chromosomes or 4 chromatidschromatids (sister and nonsister)

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Prophase IProphase I - -

SynapsisSynapsis

Homologous chromosomes

Sister chromatids Sister chromatidsTetrad

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Homologous Homologous ChromosomesChromosomes

• Pair of chromosomeschromosomes (maternalmaternal and paternalpaternal) similar in shape size, & genes carried

• Homologous pairs (tetrads)(tetrads) carry genes controlling same inherited traits

• Each locuslocus (position of a gene)(position of a gene) holds same position on both homologues

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Homologous Homologous ChromosomesChromosomes

• Humans- 23 pairs of homologous chromosomeshomologous chromosomes

• First 22 pairs- called autosomes, autosomes, look alikelook alike

• Last pair- called sex sex chromosomeschromosomes

• XX female or XY maleXX female or XY male• (Y much smaller than X)(Y much smaller than X)

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Homologous Homologous ChromosomesChromosomes

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

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Crossing OverCrossing Over• Synapsis- Homologues join Synapsis- Homologues join

togethertogether• Crossing overCrossing over (variation) (variation) can

occur btwn nonsister chromatidschromatids – segments of nonsister chromatidschromatids

break and reattach to the other chromatidchromatid

– Chiasmata (chiasma)-Chiasmata (chiasma)- sites of crossing overcrossing over

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Crossing Over - Crossing Over - VariationVariation

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

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Sex ChromosomesSex Chromosomes

XX chromosome - female XY chromosome - male

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Prophase IProphase I

CentriolesSpindle fiber

Asterfibers

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Metaphase IMetaphase I

• ShortestShortest phase phase• TetradsTetrads align on the metaphase metaphase

plateplate• Independent assortment occursIndependent assortment occurs• Chromosomes separate Chromosomes separate

randomlyrandomly to the poles of the to the poles of the cellscells

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Metaphase IMetaphase I• Independent assortment causes

variation in the forming cells• Orientation of homologous pair

to poles is random• Formula for determining

variation: Formula: 2n

ExampleExample: 2n = 4: 2n = 4 then n = 2then n = 2

thus 2thus 22 2 = 4 = 4 combinationscombinations

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Metaphase IMetaphase I

Metaphase plate

OR

Metaphase plate

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Question:Question:

• In terms of In terms of Independent Independent Assortment, Assortment, how many how many different combinations different combinations of sperm could a of sperm could a human male produce? human male produce?

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AnswerAnswer

• Formula: 2Formula: 2nn

• Human chromosomes:Human chromosomes:

2n = 462n = 46

n = 23n = 23

222323 = ~8 million = ~8 million combinationscombinations

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Anaphase IAnaphase I

• Homologous Homologous chromosomes chromosomes separate and move towards the poles

• Sister chromatids Sister chromatids remain attached at their centromerescentromeres

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Anaphase IAnaphase I

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Telophase ITelophase I

•Each pole now has haploidhaploid set of chromosomeschromosomes

• 1n = 23 (human)1n = 23 (human)• CytokinesisCytokinesis occurs and

two haploid daughter cells are formed

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Telophase ITelophase I

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Meiosis IIMeiosis II

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Meiosis IIMeiosis II• Short or NoShort or No interphase II interphase II • DNA NOT replicatedDNA NOT replicated

againagain• Remember: Meiosis II Remember: Meiosis II is

similar to mitosismitosis• Prophase I, Metaphase II, Prophase I, Metaphase II,

Anaphase II, and Anaphase II, and Telophase IITelophase II

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Prophase IIProphase II• Same as prophaseprophase in

mitosismitosis• Nuclear envelope breaks Nuclear envelope breaks

down and the spindle down and the spindle apparatus formsapparatus forms

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Metaphase IIMetaphase II• Same as metaphasemetaphase in

mitosismitosis• Chromatids lined up at Chromatids lined up at

equatorequator

Metaphase plateMetaphase plate

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Anaphase IIAnaphase II

• Same as anaphaseanaphase in mitosismitosis

• Sister chromatids Sister chromatids separateseparate

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Telophase IITelophase II

• Same as telophasetelophase in mitosismitosis

• Nuclei reform• CytokinesisCytokinesis occurs• Remember: Remember: Four haploid Four haploid

daughter cellsdaughter cells produced produced• GametesGametes = sperm or egg = sperm or egg

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Telophase IITelophase II

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MeiosisMeiosis

2n=4

sex cell

diploid (2n)

n=2

n=2

Meiosis I

n=2

n=2

n=2

n=2

Sperm

Haploid (n)

Meiosis II

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Genetic Genetic VariationVariation

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VariationVariation

• Important to the Important to the survival of populationssurvival of populations

• Aids in natural Aids in natural selectionselection

• Strongest individuals Strongest individuals are able to survive are able to survive and reproduceand reproduce

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QuestionQuestion

•What are the three What are the three sources of genetic sources of genetic variation in sexual variation in sexual reproduction?reproduction?

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Answer:Answer:

1.1. Crossing OverCrossing Over (Prophase I)(Prophase I)

2. 2. Independent Independent AssortmentAssortment (Metaphase (Metaphase I)I)

3. 3. Random FertilizationRandom Fertilization(sperm joins with egg)(sperm joins with egg)

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Remember: Variation is

helpful to the survival of a

species!

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Question:Question:

•A diploid cell containing 20 chromosomes20 chromosomes (2n = (2n = 20)20) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

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Answer:Answer:

•10 chromosomes 10 chromosomes (haploid)(haploid)

•1n = 101n = 10

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KaryotypeKaryotype• A method of organizing the chromosomes

of a cell in relation to number, size, and type.

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FertilizationFertilization• The fusion of a spermsperm and eggegg to

form a zygotezygote.• A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

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Question:Question:

•A cell containing 40 40 chromatidschromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

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Answer:Answer:

•Four cells with 10 Four cells with 10 chromosomes chromosomes eacheach