Reduction-DivisionGenetic

Recombination

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Creation of GAMETESGAMETES, , with HALFHALF the number of CHROMOSOMESCHROMOSOMES, (, (HAPLOID)

MeiosisMeiosis is SEXUALSEXUAL reproduction.reproduction.

TWOTWO divisions divisions (MEIOSIS IMEIOSIS I and MEIOSIS IIMEIOSIS II).

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Similar to mitosismitosis interphase. CHROMOSOMESCHROMOSOMES (DNA)(DNA) replicate in the S S

phasephase

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Cell division Cell division that reduces the chromosome number chromosome number by one-half.one-half.

Four phasesFour phases:a.a. Prophase IProphase Ib.b. Metaphase IMetaphase Ic.c. Anaphase IAnaphase Id.d. Telophase ITelophase I

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Prophase IProphase I

• ChromosomesChromosomes condense.

• Nuclear membrane Nuclear membrane disappears

• SpindlesSpindles forms SynapsisSynapsis occurs -

Homologous Homologous chromosomeschromosomes come togetherto form a tetradtetrad.

• Crossing over Crossing over Occurs

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Homologous chromosomes

sister chromatids sister chromatidsTetrad

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Crossing over Crossing over may occur between non-sister chromatidschromatids at sites called chiasmatachiasmata.

Crossing overCrossing over: segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid.

Chiasmata (chiasma) Chiasmata (chiasma) are where chromosomes touch each other and exchange genes exchange genes (crossing overcrossing over.)

Causes Genetic RecombinationGenetic Recombination

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nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

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Tetrads align on the equator. Independent assortment occurs –

chromosomes separate randomly causing GENETIC RECOMBINATION

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Homologs line up at equator or metaphase plate

OR

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In terms of In terms of Independent Independent AssortmentAssortment -how many -how many different combinations of different combinations of sperm could a sperm could a humanhuman malemale produce? produce?

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Formula: 2Formula: 2nn

Human chromosomes:Human chromosomes:2n = 462n = 46

n = 23n = 23 222323 = ~8 million = ~8 million combinationscombinations

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Homologous chromosomes Homologous chromosomes separate and move towards the poles.

Sister chromatids Sister chromatids remain attached at their centromerescentromeres.

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Nuclear membrane Nuclear membrane reforms around each group of chromosomes

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CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

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cytokinesis

Interphase II Interphase II or very shortor very short No DNA ReplicationNo DNA Replication Remember: Meiosis II is Remember: Meiosis II is

similar to mitosissimilar to mitosis

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Same as Same as ProphaseProphase in in mitosismitosis

Nucleus & nucleolus disappearNucleus & nucleolus disappear Chromosomes condenseChromosomes condense Spindle formsSpindle forms

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Same as Metaphase in mitosis

Chromosomes line up at equator

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Same as AnaphaseAnaphase in mitosismitosis

SISTER CHROMATIDS SISTER CHROMATIDS separateseparate

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Same as Same as TelophaseTelophase in mitosis. in mitosis. Nuclear membrane reformsNuclear membrane reforms, ,

spindle disappearsspindle disappears

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The cells splitThe cells split FOURFOUR HAPLOID HAPLOID

DAUGHTER cells are produced.DAUGHTER cells are produced. Called Called GAMETESGAMETES (eggs and sperm) (eggs and sperm)

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2n=46

humansex cell

diploid (2n)

n=46

n=46

Meiosis I

n=23

n=23

n=23

n=23

sperm

haploid (n)

Meiosis II25

2n=46

humansex cell

diploid (2n)

n=23

n=23

Meiosis I

n=23egg

Haploid (1n)

Meiosis II26

Polar Bodies (die)

Also known as Also known as GENETIC GENETIC RECOMBINATIONRECOMBINATION

Meiosis allows for much variation Meiosis allows for much variation due to:due to:

1. 1. Independent assortment Independent assortment 2. Crossing over2. Crossing over 3. Random fertilization3. Random fertilization

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A cell containing 20 20 chromosomeschromosomes (diploid)(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

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1010 chromosomes (haploid chromosomes (haploid or 1n)or 1n)

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AAn organized organized picture picture of the chromosomes of a human arranged in pairs arranged in pairs by size by size from largest to smallest.

PPairs 1-221-22 called AUTOSOMESAUTOSOMES

LLast pair are SEX SEX CHROMOSOMESCHROMOSOMES

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Male - XYMale - XY

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Female - XXFemale - XX

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Female - XXFemale - XXDown Syndrome – Trisomy 21

The fusion of a spermsperm and eggegg to form a zygotezygote.

A zygote is a FERTILIZED EGGFERTILIZED EGG 64 trillion combinations for the 64 trillion combinations for the

zygotezygote

n=23egg

sperm n=23 2n=46

zygote

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