Meiosis Humans have 46 chromosomes in every cell of our bodies except the sex cells or gametes (eggs and sperm)- they have 23 chromosomes When a baby is.

Meiosis

• Humans have 46 chromosomes in every cell of our bodies except the sex cells or gametes (eggs and sperm)- they have 23 chromosomes

• When a baby is conceived, an egg from mom and a sperm from dad, combine into the first cell of the new baby. 23+23 = 46

Section 11-4

What would happen if the What would happen if the egg and sperm both had 46 egg and sperm both had 46 chromosomes?chromosomes?Baby would have Baby would have 9292 chromosomes!! Disaster!chromosomes!! Disaster!

46 46

92chromosomes

= death, disease

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Remembering Meiosis

• Mitosis is a process that occurs in almost every cell of your body (body cells), except the sex cells, or gametes.

• Mitosis = My toe sis, it happens in your toes!

• Meiosis is a process of cell division that doesn’t happen in your toes, it only happens in your sex cells, gametes

• Meiosis = My OH! Sis (as in uh-oh!)

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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

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

67

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

Gamete Formation

• Why do we even have Meiosis?

• To make gametes (eggs and sperm).

• Male gametes = sperm

• Female gametes = eggsWhy 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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