1. Meiosis

2. Meiosis
A process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell.

3. Formation of Haploid Cells
Meiosis produces gametes, which are haploid reproductive cells.
Cells preparing to divide by meiosis undergo the G1, S, and G2 phases of interphase.
Cells begin meiosis with a duplicate set of chromosomes, just like mitosis.
Cells undergoing meiosis divide twice, resulting in 4 haploid (1n) cells.

4. Two Stages of Meiosis First cell division = Meiosis I
Prophase I, Metaphase I, Anaphase I, Telophase I and Cytokinesis I
Second cell division = Meiosis II
Prophase II, Metaphase II, Anaphase II, Telophase II and Cytokinesis II

5. Telophase I and Cytokinesis
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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6. Prophase I DNA coils tightly into chromosomes. Spindle fibers appear.
Nuclear membrane disassembles.
Synapsis occurs - homologous chromosomes line up next to each other.
Each pair of homologous chromosomes is called a tetrad because there are 4 chromatids in a tetrad.
Tetrad

7. Tetrads Form in Prophase I
Homologous chromosomes (each with two chromatids)  
Join to form a TETRAD
Called Synapsis

8. Prophase I
During synapsis, the chromatids within a homologous pair twist around one another.
Portions of chromatids may break off and attach to adjacent chromatids–a process called crossing-over.
Genetic recombination results – genetic material between maternal and paternal chromosomes is exchanged.

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Crossing-over
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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10. Metaphase I Tetrads line up randomly along the middle of the cell.
Spindle fibers attach to the centromere of each homologous chromosome.

11. Anaphase I
Each homologous chromosome moves to an opposite end of the cell.
Note: Each homologous chromosome still consists of two chromatids attached by a centromere.
Random separation of homologous chromosomes is called independent assortment and results in genetic variation.

12. Telophase I and Cytokinesis I
Chromosomes reach opposite ends of the cell and cytokinesis begins.
A nuclear membrane forms around each set of chromosomes.
Two new cells are produced, each containing one chromosome from each homologous pair (haploid – 1n).

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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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14. Telophase I and Cytokinesis I Telophase II and Cytokinesis
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
Prophase II
Metaphase II
Anaphase II
Telophase II and Cytokinesis
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15. Prophase II
The nuclear membrane breaks down and a new spindle forms.

16. Metaphase II
The chromosomes line up in the center of cell.

17. Anaphase II
Chromatids separate and move toward opposite ends of the cell.

18. Telophase II and Cytokinesis II
Telophase II – a nuclear membrane forms around the chromosomes in each of the four new cells.
Cytokinesis II – 4 new cells are formed, each with half of the original cell’s number of chromosomes.

19. Stages Of Meiosis: Meiosis I
Interphase
Mother cell
Prophase I:
Condensing Chromosomes
Prophase I:
Tetrad formation/
crossing over
Metaphase I
Anaphase I
Telophase I
Meiosis II

20. Stages Of Meiosis: Meiosis II
The products of meiosis are 4 haploid cells each with a unique set of chromosomes.
Telophase I
Prophase II
Metaphase II
The products of mitosis are 2 diploid cells with identical chromosomes.
Anaphase II
Telophase II

21. Development of Gametes
In animals, the only cells that divide by meiosis are those that produce gametes within the reproductive organs.
In humans – testes (males) and ovaries (females)

22. Development of Gametes
SPERMATOGENESIS
In the testes – male gametes known as sperm cells or spermatozoa are produced.
One diploid cell divides meiotically to form four haploid cells called spermatids – each develops into a mature sperm cell.

23. Development of Gametes
OOGENESIS
Production of mature egg cells, or ova.
A diploid reproductive cell divides meiotically to produce one mature egg cell (ovum) and three polar bodies which will degenerate – cytoplasm is not evenly distributed in cytokinesis.

24. Sexual Reproduction
Production of offspring through meiosis and the union of a sperm and an egg.
Offspring are genetically different because genes are combined in new ways.
Advantage: enables species to adapt rapidly to new conditions.

25. Comparing Mitosis and Meiosis Mitosis Meiosis
Number of Cell Divisions
Number of Cells
Produced
Homologous Chromosomes pair up (yes/no)
Mother Cell Haploid (1n) or Diploid (2n)
Daughter Cells Haploid (1n) or Diploid (2n)
Type of cell being produced
Daughter cells have identical DNA (yes/no) 1 2 2 4 No
Yes
Diploid (2n)
Diploid (2n)
Diploid (2n)
Haploid (1n)
Body Cells
Sex Cells
Yes No