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Chapter 11-4 
What is Meiosis
How is it different than Mitosis
11-4 Meiosis
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Each organism must inherit a single copy of every gene from each of its “parents.”
Gametes are formed by a process that separates the two sets of genes so that each gamete ends up with just one set.
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Chromosome Number
Chromosome Number
All organisms have different numbers of chromosomes.
A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent.
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 (one from mom and one from dad) are homologous.
In other words, each of the chromosomes that came from the male parent has a corresponding chromosome from the female parent.
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Chromosome Number
A cell that contains both sets of homologous chromosomes is said to be diploid.
The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N.
For Drosophila, the diploid number is 8, which can be written as 2N=8.
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Chromosome Number
The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes.
These cells are haploid. Haploid cells are represented by the symbol N.
For Drosophila, the haploid number is 4, which can be written as N=4.
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Phases of Meiosis
Phases of Meiosis
Meiosis is a process of cell division where the number of chromosomes per cell is cut in half by separating the homologous chromosomes in a diploid cell.
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Phases of Meiosis
Meiosis involves two divisions, meiosis I and meiosis II.
By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.
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Phases of Meiosis
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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Phases of Meiosis
DNA is replicated, and duplicate chromosomes are formed.
Interphase I
Interphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes.
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Phases of Meiosis
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
There are 4 chromatids in a tetrad.
MEIOSIS I
Prophase I
MEIOSIS I Prophase I - Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
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Phases of Meiosis
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.
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Phases of Meiosis
Spindle fibers attach to the chromosomes.
MEIOSIS I
Metaphase I
MEIOSIS I Metaphase I - Spindle fibers attach to the chromosomes.
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Phases of Meiosis
MEIOSIS I
Anaphase I
The fibers pull the homologous chromosomes 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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Phases of Meiosis
MEIOSIS I
Telophase I and
Cytokinesis
Nuclear membranes form.
The cell separates into two cells.
The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.
MEIOSIS I Telophase I and Cytokinesis - Nuclear membranes form. The cell separates into two cells.
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Phases of Meiosis
Meiosis II
The two cells produced by meiosis I now enter a second meiotic division.
Unlike meiosis I, neither cell goes through chromosome replication.
Each of the cell’s chromosomes has 2 chromatids.
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Phases of Meiosis
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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Phases of Meiosis
MEIOSIS II
Prophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
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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Phases of Meiosis
MEIOSIS II
Metaphase II
The chromosomes line up in the center of cell.
MEIOSIS II Metaphase II - The chromosomes line up in a similar way to the metaphase state of mitosis.
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Phases of Meiosis
MEIOSIS II
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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Phases of Meiosis
MEIOSIS II
Telophase II and Cytokinesis
Meiosis II results in four haploid (N) daughter cells.
MEIOSIS II Telophase II and Cytokinesis - Meiosis II results in four haploid (N) daughter cells.
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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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24. Comparing Mitosis and Meiosis
How is meiosis different from mitosis?
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25. Comparing Mitosis and Meiosis
Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.
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26. Comparing Mitosis and Meiosis
Cells produced by mitosis have the same number of chromosomes and alleles as the original cell.
Mitosis allows an organism to grow and replace cells.
Some organisms reproduce asexually by mitosis.
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27. Comparing Mitosis and Meiosis
Cells produced by meiosis have half the number of chromosomes as the parent cell.
These cells are genetically different from the diploid cell and from each other.
Meiosis is how sexually-reproducing organisms produce gametes.
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If the body cells of humans contain 46 chromosomes, a single sperm cell should have
46 chromosomes.
23 chromosomes.
92 chromosomes.
between 23 and 46 chromosomes.
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During meiosis, the number of chromosomes per cell is cut in half through the separation of
daughter cells.
homologous chromosomes.
gametes.
chromatids.
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The formation of a tetrad occurs during
anaphase I.
metaphase II.
prophase I.
prophase II.
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In many female animals, meiosis results in the production of
only 1 egg.
1 egg and 3 polar bodies.
4 eggs.
1 egg and 2 polar bodies.
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Compared to egg cells formed during meiosis, daughter cells formed during mitosis are
genetically different, while eggs are genetically identical.
genetically different, just as egg cells are.
genetically identical, just as egg cells are.
genetically identical, while egg cells are genetically different.
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