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Meiosis• To solve this problem, the cells in the gametes (eggs

and sperm), undergo Meiosis instead of Mitosis, when they divide.

• Why Meiosis?

• Most cells in our body are Diploid (“di” = 2)• Diploid = have 2 sets of chromosomes, 2n

(one from mom, one from dad)• Sex cells, or gametes, are Haploid • Haploid = have 1 set of chromosomes, 1n

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Meiosis results in cells with ½ the # of chromosomes, so they can combine in sexual

reproduction, which results in an embryo with the normal amount of chromosomes.

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Chromosome Number

• An example.

• Fruit Flies (Drosophila), have 8 chromosomes total – 4 from dad fruit fly, 4 from mom fruit fly.

Diploid number?2n = 8

Haploid number?

1n = 4

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Homologous chromosomes= 2 sets of similar chromosomes, one from mom, one from dad

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Phases of Meiosis

• How are haploid (1n) gamete cells produced from diploid (2n) cells?

• By Meiosis (my-OH-sis)

• Meiosis = process of reduction cell division where the # of chromosomes per cell is cut in ½ by the separation of homologous chromosomes

Homologous = same chromosome, it just comes from different parents (mom or dad)

Meiosis involves 2 distinct divisions, 1. Meiosis I 2. Meiosis II Results of Meiosis? = cells with a haploid (1n)

number of chromosomes

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Meiosis I

Chromosome duplicationChromosome duplication

2 sister chromatids2 sister chromatids

4 sister chromatids 4 sister chromatids form a Tetradform a Tetrad

Interphase I Prophase I

Interphase I = Cells undergo a round of DNA replication, forming duplicate chromosomes

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Prophase I = Each chromosome pairs up with its matching homologous chromosome forming a tetrad

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Tetrads= in prophase I, chromosomes duplicate and form a structure with 4 chromosomes (tetrad)

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During prophase I of Meiosis, when the tetrads form, a phenomenon called “Crossing Over” can occur.

Owww! My

arm!

Sorry!

Who me?

Hey hot stuff, wanna dance?

I think he likes

me!

You got chocolate in my peanut

butter!

You got peanut butter

in my chocolate!

Help!Crossing Over = results in the exchange of genetic information between homologous chromosomes

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Meiosis I

Interphase I Prophase I Metaphase I

Interphase I = Cells undergo a round of DNA replication, forming duplicate chromosomes

Prophase I = Each chromosome pairs up with its corresponding homologous chromosome to form a tetrad

Anaphase I = Fibers pull the homologous chromosomes towards opposite ends of the cell (“A” for apart)

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Metaphase I = Spindle fibers attach to the chromosomes, then they line up in the middle (“M” for middle)

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Anaphase I

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Meiosis I

Interphase I Prophase I Metaphase I Anaphase I Telophase and Cytokinesis

Prophase I = Each chromosome pairs up with its corresponding homologous chromosome to form a tetrad

Interphase I = Cells undergo a round of DNA replication, forming duplicate chromosomes

Anaphase I = Fibers pull the homologous chromosomes towards opposite ends of the cell (“A” for apart)

Metaphase I = Spindle fibers attach to the chromosomes, then they line up in the middle (“M” for middle)

Telophase I and Cytokinesis = Nuclear membranes form, the cell separates into 2 new cells

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Meiosis IIThe 2 new cells now enter a 2nd round of meiotic division

No more chromosome duplication

prophase II

Prophase II = of meiosis I results in 2 haploid daughter cells, each with ½ the # of chromosomes as the original cell Box

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Meiosis II

prophase II metaphase II

Prophase II = of meiosis I results in 2 haploid daughter cells, each with ½ the # of chromosomes as the original cell

Metaphase II = chromosomes line up in the middle Box

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Meiosis II

anaphase II

1

2

3

4

telophase IIprophase II metaphase II

Metaphase II = chromosomes line up in the middle

Prophase II = of meiosis I results in 2 haploid daughter cells, each with ½ the # of chromosomes as the original cell

Anaphase II = sister chromatids separate and move to opposite ends of the cell

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Telophase II and Cytokinesis = of meiosis II results in 4 haploid (1n) daughter cells

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Prophase I

Anaphase I

Telophase I&

Cytokinesis

Prophase II

IP

MA

T

P

MAT

I then …PMAT…..PMATI then …PMAT…..PMAT

Telophase II&

CytokinesisAnaphase II Metaphase II

4 daughter cells4 daughter cellswith haploid # (1n)with haploid # (1n)

Humans = 2n = 46

9292

92 92

46

46

464623

Flow Chart for MeiosisFlow Chart for Meiosis

Interphase I

Metaphase I

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Gamete Formation

• Why do we even have Meiosis?• To make gametes (eggs and sperm).• Male gametes = sperm• Female gametes = eggs

Why do we need eggs and sperm?Why do we need eggs and sperm?

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End of Chapter 10 and Chapter 11 Section 11-4

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For For Sexual ReproductionSexual Reproduction, where egg , where egg and sperm unite to make an embryoand sperm unite to make an embryo

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