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11-4 Meiosis
11-4 Meiosis
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Organisms inherit a single copy of every gene from each of its “parents.”
Meiosis separates the two sets of parent genes so that each gamete ends up with just one set.
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Chromosome Number
Chromosome Number
All organisms have 2 sets of chromosomes.
These two sets of chromosomes are called homologous.
These chromosomes are from a fruit fly. Each of the fruit fly’s body cells has 8 chromosomes.
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Chromosome Number
A cell that contains both sets of homologous chromosomes is said to be diploid.
For Humans, the diploid number is 46, which means we have 23 pairs.
The sex cells contain only a single set of chromosomes.
These cells are haploid.
For Humans, the haploid number is 23.
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Phases of Meiosis
Phases of Meiosis
Meiosis is a reduction division in which the number of chromosomes per cell is cut in half by separating homologous chromosomes in a diploid cell.
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Phases of Meiosis
Meiosis involves two divisions.
The diploid cell that entered meiosis will become 4 haploid cells.
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7. Telophase I and Cytokinesis
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
Chromosomes & centrioles replicate.
Interphase I
Interphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes.
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Phases of Meiosis
Chromosome pairs come together.
Crossing over occurs.
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
Crossing-over produces new combinations of genes.
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
Chromosome pairs line up on the equator.
Each pair attaches to 1 spindle fiber.
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 separate the homologous chromosomes and move them 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 cytoplasm separates into two cells.
Meiosis I results in two with half the number of chromosomes as the original cell.
MEIOSIS I Telophase I and Cytokinesis - Nuclear membranes form. The cell separates into two cells.
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Phases of Meiosis
Meiosis II
Neither cell goes through chromosome replication.
Each of the chromosomes is still double stranded.
The double stranded chromosomes must separate.
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15. Telophase II and Cytokinesis
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
Chromosomes coil.
Centrioles move to poles.
Spindles form.
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 daughter 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 males meiosis results in four equal-sized 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 females only one egg results from meiosis.
The polar bodies die.
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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22. 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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23. 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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24. 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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11-4
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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11-4
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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11-4
The formation of a tetrad occurs during
anaphase I.
metaphase II.
prophase I.
prophase II.
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11-4
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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11-4
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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