1. Meiosis
Section 7-1

2. Meiosis Forms Haploid (n) Cells
Some organisms reproduce by joining gametes to form the first cell of a new individual
Gametes are haploid which means they contain 1 set of chromosomes
Meiosis – form of cell division that halves the number of chromosomes when forming specialized reproductive cells (gametes or spores)

3. Meiosis involves 2 divisions of the nucleus – meiosis I and meiosis II – with each divided into prophase, metaphase, anaphase and telophase.
Before meiosis begins, the DNA in the original cell is replicated.
Meiosis starts with homologous chromosomes.

5. Stages of Meiosis Meiosis is two rounds of mitosis essentially
PI – prophase I PII – prophase II
MI – metaphase I MII – metaphase II
AI – anaphase I AII – anaphase II
TI – telophase I TII – telophase II

7. Prophase I Chromosomes condense Nuclear envelope breaks down
Homologous chromosomes pair all along their length and then cross-over
Crossing-over – occurs when portions of a chromatid on one homologous chromosome are broken and pair with the corresponding portion on one of the chromatids on the other homologous chromosome

10. Metaphase I
Pairs of the homologous chromosomes moved by the spindle to the equator of the cell
Homologous chromosomes (2 chromatids) remain together

12. Anaphase I Homologous chromosomes separate
Chromosomes of each pair pulled to opposite poles of cell by spindle fibers
Chromosomes DO NOT separate at their centromeres – 1 chromosome is still 2 chromatids

14. Telophase I Individual chromosomes gather at each pole
Cytoplasm divides – now 2 cells

16. Prophase II
New spindle forms around chromosomes

18. Metaphase II Chromosomes line up along equator
Chromosomes attached to spindle fibers at their centromeres

20. Anaphase II Centromeres divide
Chromatids (now called chromosomes) move to opposite poles of cell.

22. Telophase II Nuclear envelope forms around each set of chromosomes
Spindle breaks down
Cells undergo cytokinesis

24. After all eight phases are complete, will have FOUR haploid (n) cells.

25. Meiosis Contributes to Genetic Variation
Meiosis allows for the rapid generation of new genetic combinations
Three Mechanisms for genetic variation
1. independent assortment
2. crossing-over
3. random fertilization

26. Independent Assortment
Chromosome pairs separate independently
Defined as the random distribution of homologous chromosomes during meiosis

27. Crossing-over and Independent Assortment
DNA exchange that occurs during crossing-over adds even more recombination to the independent assortment of chromosomes that occurs later in meiosis
Unlimited combinations
Joining of two gametes, and each gamete produced independently gives approximately 64 trillion possibilities

28. Importance of Genetic Variation
Meiosis and joining of gametes is essential to evolution
No genetic process generates variation more quickly

29. Gamete Formation in Male and Female Animals Involves Meiosis
Gametogenesis – meiosis is the primary event in the formation of gametes
Spermatogenesis – process by which sperm are produced in male animals. Occurs in testes.
Oogenesis – process by which gametes are produced in female animals . Occurs in ovaries.

30. Oogenesis
After meiosis I, cytoplasm divides unevenly and forms a polar body. May or may not divide again, but WILL NOT survive.
Larger cell – will undergo meiosis II.
Larger cell develops into an ovum or egg. Polar bodies die.