1. MEIOSIS

2. 1. The division of the nucleus of a diploid germ cells
D. Meiosis
1. The division of the nucleus of a diploid germ cells
* Produces haploid gametes = egg
or sperm
* Reduces the chromosomes in
the gametes to half of what is
found in the somatic cells of the
organism. (= reduction division)

3. 2. Why is meiosis necessary?
* For SEXUAL REPRODUCTION! * =
egg sperm zygote
* The union of egg and sperm is called
Fertilization.

4. * Without meiosis, the chromosome number would double with each generation.

5. 3. Meiosis Overview
1st Division
2nd Division
2nd Division

6. Meiosis
Bozeman - MEIOSIS

7. A Germ Cell In Interphase

8. 4. Germ Cell Life Cycle: Interphase = Meiosis Stage I begins:
*G1 = diploid germ cell is growing in
the testes or ovaries
*S = DNA Replication occurs (the only time
this happens during the entire process)
*G2 = Cell prepares to divide
Meiosis Stage I begins:
* Homologous pairs of chromosomes
will be separated into different
cells.

9. Prophase I

10. MEIOSIS STAGE I 1. PROPHASE I: * Chromosomes become visible
* Homologous chromosomes pair = Synapsis
* Crossing over occurs= a portion of a
chromatid breaks off and is exchanged
with corresponding chromatid portions on
the other homologous chromosome.

11. Crossing -Over

12. Metaphase I

13. MEIOSIS STAGE I METAPHASE I * Homologous pairs of chromosomes
line up on the equator of the cell.
* Spindles attach to the centromeres

14. Anaphase I

15. MEIOSIS STAGE I 3. ANAPHASE I * Homologous pairs separate and
move to opposite centrioles
* Nondisjunction could occur.
(The failure of homologous chromosomes to separate during Anaphase I or failure of sister chromatids to separate during Anaphase II of meiosis which can result in a gamete having an abnormal chromosome number)

17. Telophase I followed by Cytokinesis I

18. MEIOSIS STAGE I TELOPHASE I then Cytokinesis
* Chromosomes gather at the
centrioles.
* Cytokinesis = The cytoplasm
divides.

19. Prophase II

20. MEIOSIS STAGE II PROPHASE II * New spindles form in each of
the two new cells.

21. Metaphase II

22. MEIOSIS STAGE II METAPHASE II * Individual chromosomes line
up on the equator in each of
the two cells.
* Spindles attach to each
chromosome in each cell.

23. Anaphase II

24. MEIOSIS STAGE II ANAPHASE II * Chromatids (copies of the DNA) are
pulled apart and move to opposite
centrioles in each of the two cells.
* Nondisjunction could occur.

26. Telophase II and Cytokinesis IIOR

27. MEIOSIS STAGE II TELOPHASE II and cytokinesis
* A nuclear envelope forms around each
set of chromosomes in each of the two
cells
*Chromosomes return to chromatin
*Nucleoli reappear
*Cytokinesis = The cytoplasm divides
*Gametes are formed (1 egg females;
4 sperm in males)
*Each gamete is haploid.

28. F. Spermatogenesis vs Oogenesis
Diploid Germ Cells
Meiosis I
Meiosis II
* Uneven distribution of the cytoplasm during Meiosis I and Meiosis II results in the formation of 1 haploid egg cell and 3 polar bodies that will be destroyed by the body and not used for reproduction.
*Results in the formation of 4 haploid sperm cells.

29. Comparison of Mitosis to Meiosis
MEIOSIS ANIMATION
Bozeman Biology Meiosis Video
Bozeman animation – meiosis

30. Begins with a diploid cell
Allows for growth and to maintain structure
A cell with 16 chromosomes divides to produce 2 cells each having 16 chromosomes.
Is preceded by DNA Replication
Synapsis occurs
Starts with a somatic cell
Forms gametes.
Results in 2 diploid cells
A cell with 8 chromosomes divides to produce 4 cells each having 4 chromosomes
Starts with a germ cell
Occurs only in the testes or ovaries
Crossing over occurs
Results in haploid cells
Produces somatic cells