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Meiosis
biology
Sexual Reproduction
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2. Sperm and ova (egg) are created! Sperm and ova are haploid
Phases of Meiosis
What happens during the process of meiosis?
Sperm and ova (egg) are created!
Sperm and ova are haploid
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Chromosome Number
Chromosome Number
All organisms have different numbers of chromosomes.
Human = 46
Carrot = 18
Dog = 78
Goldfish = 94
These chromosomes are from a fruit fly. Each of the fruit fly’s body cells has 8 chromosomes.
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Chromosome Number
*These two sets of chromosomes are homologous.
One chromosome from mom and one chromosome from dad.
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6. Homologous Chromosomes
*Def: Pair of chromosomes that contain the same alleles for traits and phenotypes
EX: chromosome with eye color allele from mom is homologous to chromosome with eye color allele from dad
Dad (2,4)
Mom (1,3)

7. Chromosome Number QUIZ
How many chromosomes are in the picture? 46
How many homologous chromosomes are in the picture? 23

8. Telophase I and Cytokinesis
Phases of Meiosis I
Meiosis I
Meiosis I
Interphase I
During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.
Prophase I
Metaphase I
Anaphase I
Telophase I and Cytokinesis
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Interphase I
centrioles
DNA copies itself
Interphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes.
nucleus
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Prophase I
Nuclear membrane disappears.
Homologous chromosomes pair up to form a tetrad.
Tetrad = 4 sister chromatids
Crossing over occurs
MEIOSIS I Prophase I - Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
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11. Which pair up to make homologous chromosomes?
Pink with pink;
yellow with yellow;
blue with blue
How many pairs of homologous chromosomes are shown? 3
How many chromosomes are shown? 6

12. CROSSING OVER *Homologous chromosomes pair up
One sister chromatid breaks off and exchanges itself with a sister chromatid from the other homologous chromosome.
Results in variation! Nature likes this because some will be more fit than others. Occurs randomly at different spots on sister chromatid each time of meiosis. That is why you don’t look exactly like your siblings.

13. Does crossing over always occur at the same allele?
When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over.
Crossing-over produces new combinations of alleles.
Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged.
Does crossing over always occur at the same allele?
NO! It is random
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Metaphase I
Homologous chromosomes line up in middle of cell
Spindle fibers attach to the chromosomes.
MEIOSIS I Metaphase I - Spindle fibers attach to the chromosomes.
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Anaphase I
The fibers pull the homologous chromosomes apart toward opposite ends of the cell.
MEIOSIS I Anaphase I - The fibers pull the homologous chromosomes toward opposite ends of the cell.
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Telophase I
Nuclear membranes form.
The cell separates into two cells.
MEIOSIS I Telophase I and Cytokinesis - Nuclear membranes form. The cell separates into two cells.
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Phases of Meiosis
Meiosis involves two divisions
*meiosis I
*meiosis II
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18. Telophase II and Cytokinesis
Phases of Meiosis II
Meiosis II
During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.
Meiosis II
Telophase I and Cytokinesis I
Metaphase II
Anaphase II
Telophase II and Cytokinesis
Prophase II
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Prophase II
Nuclear membrane disappears.
MEIOSIS II Prophase II - Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
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Metaphase II
Individual chromosomes line up in the middle of cell.
MEIOSIS II Metaphase II - The chromosomes line up in a similar way to the metaphase state of mitosis.
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Anaphase II
The sister chromatids separate and move toward opposite ends of the cell.
MEIOSIS II Anaphase II - The sister chromatids separate and move toward opposite ends of the cell.
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Telophase II
Meiosis II results in four non-identical haploid cells.
MEIOSIS II Telophase II and Cytokinesis - Meiosis II results in four haploid (N) daughter cells.
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Gamete Formation
Gamete Formation
In male animals, meiosis results in four equal-sized gametes called sperm.
Meiosis produces four genetically different haploid cells. In males, meiosis results in four equal-sized gametes called sperm.
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Gamete Formation
In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction.
Meiosis produces four genetically different haploid cells. In females, only one large egg cell results from meiosis. The other three cells, called polar bodies, usually are not involved in reproduction.
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Meiosis
Why is meiosis important?
Creates gametes that are HAPLOID.
Due to crossing over, meiosis also creates variation.
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