1. MEIOSIS

2. Meiosis: hereditary material is exchanged (mixed and transferred)
Each species has a specific number of chromosomes
For example humans total number of chromosomes is 46
diploid number is 46 or 2n = 46
the number of chromosomes in the gametes (sex cells bb egg and sperm) is the haploid number or n = 23
Other examples: crayfish 2n = 126 fruit flies 2n = 4 2

3. Meiosis Meiosis animation (1:49 min):
Beginning of third class 3

4. Vocabulary
Diploid -containing 2 complete sets of chromosomes, one from each parent
Haploid - cell has half the usual number of chromosomes
Germ/Sex cell
Somatic/Body cell

5. Each resulting cell still has chromosomes from mom & dad

7. Early Interphase Meiosis is preceded by interphase
Chromosomes have not yet condensed

8. Late Interphase
chromosomes have replicated, and the chromatin begins to condense

9. Meiosis Prophase I the chromatin shortens to form chromosomes
homologous chromosomes pair up or synapse to form tetrads
Crossing over occurs in late prophase I

10. Crossing Over/Genetic Recombination Occurs in late Prophase I
Homologous chromosomes exchange portions of DNA resulting in offspring that are genetically different from their parents and from each other
Animation

11. Synapsis in Prophase I of Meiosis
Animation

12. Meiosis Metaphase I
chromosomes line up across the middle, or equator, of the cell
centromeres of chromosomes attach to spindle fibres
random assortment of homologous chromosomes happens here

13. Meiosis Anaphase I
homologous chromosomes separate and move along spindle fibres towards the poles or ends of the cell

14. Meiosis Telophase I & Cytokinesis
cell begins to divide into two daughter cells
each daughter cell can get any combination of maternal and paternal chromosomes
cell has divided into two daughter cells

15. Meiosis Prophase II
nuclear membrane dissolves and the spindle fibres begin to form

16. Meiosis Metaphase II
as in Meiosis I, the chromosomes line up on the spindle fibers
sister chromatids remain attached at their centromeres

17. Meiosis Anaphase II
centromeres split and sister chromatids separate and move to opposite poles of the cell
chromatids are now called chromosomes

18. Telophase II & Cytokinesis
With the formation of four cells, meiosis is over.
Each of these prospective germ cells carries half the number of chromosomes of somatic cells.

19. Comparing Mitosis and Meiosis

20. Comparing Chromosome Numbers in Mitosis and Meiosis