1. Section 11-4
Meiosis
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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 = 46
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What would happen if the egg and sperm both had 46 chromosomes?
Baby would have 92 chromosomes!! Disaster! 92
chromosomes
= death, disease

2. Mitosis = My toe sis, it happens in your toes!
Remembering Meiosis
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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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3. 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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4. Chromosome Number An example.
Fruit Flies (Drosophila), have 8 chromosomes total – 4 from dad fruit fly, 4 from mom fruit fly.
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Homologous chromosomes= 2 sets of similar chromosomes, one from mom, one from dad
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2n = 8
Diploid number?
Haploid number?
1n = 4

5. How are haploid (1n) gamete cells produced from diploid (2n) cells?
Phases of Meiosis
Homologous = same chromosome, it just comes from different parents (mom or dad)
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
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Meiosis involves 2 distinct divisions,
Meiosis I
Meiosis II
Results of Meiosis? = cells with a haploid (1n) number of chromosomes
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6. Meiosis I
Prophase I = Each chromosome pairs up with its matching homologous chromosome forming a tetrad
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Interphase I
Prophase I
Interphase I = Cells undergo a round of DNA replication, forming duplicate chromosomes
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Tetrads= in prophase I, chromosomes duplicate and form a structure with 4 chromosomes (tetrad)
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2 sister chromatids
4 sister chromatids form a Tetrad
Chromosome duplication

7. During prophase I of Meiosis, when the tetrads form, a phenomenon called “Crossing Over” can occur.
You got peanut butter in my chocolate!
You got chocolate in my peanut butter!
Sorry!
Hey hot stuff, wanna dance?
Owww! My arm!
Who me?
I think he likes me!
Help!
Crossing Over = results in the exchange of genetic information between homologous chromosomes
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8. Meiosis I
Anaphase I = Fibers pull the homologous chromosomes towards opposite ends of the cell (“A” for apart)
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Prophase I = Each chromosome pairs up with its corresponding homologous chromosome to form a tetrad
Metaphase I = Spindle fibers attach to the chromosomes, then they line up in the middle (“M” for middle)
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Interphase I
Prophase I
Metaphase I
Anaphase I
Interphase I = Cells undergo a round of DNA replication, forming duplicate chromosomes

9. Meiosis I
Anaphase I = Fibers pull the homologous chromosomes towards opposite ends of the cell (“A” for apart)
Prophase I = Each chromosome pairs up with its corresponding homologous chromosome to form a tetrad
Metaphase I = Spindle fibers attach to the chromosomes, then they line up in the middle (“M” for middle)
Interphase I
Prophase I
Metaphase I
Anaphase I
Telophase and Cytokinesis
Interphase I = Cells undergo a round of DNA replication, forming duplicate chromosomes
Telophase I and Cytokinesis = Nuclear membranes form, the cell separates into 2 new cells
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10. Meiosis II
The 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
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11. Meiosis II Metaphase II = chromosomes line up in the middle
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
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12. Anaphase II = sister chromatids separate and move to opposite ends of the cell
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Meiosis II 4 3
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
prophase II
metaphase II
anaphase II
telophase II 2
Telophase II and Cytokinesis = of meiosis II results in 4 haploid (1n) daughter cells
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13. I then …PMAT…..PMAT Flow Chart for Meiosis I P Interphase I M
Prophase I A
Metaphase I
Anaphase I T 92
Telophase I
& Cytokinesis 92 92 92
Humans = 2n = 46 46 P
Prophase II T A M
Telophase II
&
Cytokinesis
Anaphase II
Metaphase II 46 23 46 46
4 daughter cells
with haploid # (1n)
I then …PMAT…..PMAT

14. For Sexual Reproduction, where egg and sperm unite to make an embryo
Gamete Formation
Why do we even have Meiosis?
To make gametes (eggs and sperm).
Male gametes = sperm
Female gametes = eggs
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Why do we need eggs and sperm?
For Sexual Reproduction, where egg and sperm unite to make an embryo
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End of Chapter 10 and Chapter 11 Section 11-4