1. Meiosis

3. Meiosis Overview
Meiosis - cell division in which gametes are made (eggs and sperm).
Occurs at the start of puberty
The cells made will have half the number of chromosomes (haploid n)

4. Haploid –half the total of the number of the chromosomes(sperm and egg cells are haploid)
Diploid – a cell that contains full set of chromosomes(body cells)

5. Meiosis I

6. Interphase I Cell starts out as a DIPLOID cell.
Nucleus
Interphase I
Cell starts out as a DIPLOID cell.
DNA in chromosomes replicates.
Cell Membrane

7. Prophase I Nuclear membrane disappears and the nucleolus disappears
Homologous Chromosomes pair up to form tetrads,
(2 chromosomes together).
4. Crossing-over can occur during
5. Centrioles move to the poles

8. Crossing Over
Crossing over: the swapping of DNA which occurs between homologs.
It gives us genetic variation or differences.
This is one reason why you are your own unique person.

9. Metaphase I Spindle Fibers appear.
Tetrads line up along the middle of the spindle.
Fibers attach to centromeres.
Spindle Fibers

10. Anaphase I
Spindle fibers pull the tetrads apart, with sister chromatids remaining together.
2. Chromosomes are pulled to the centrioles.

11. Telophase I Spindle Fibers and centrioles disappear.
Nucleus reappears around each of the new sets of chromosomes.
Cell starts to form a break at the middle and starts to pinch in at the sides.

12. Telophase I (continued)
Cytokinesis occurs

13. Continues with both cells, but this will only show one
Meiosis II
Continues with both cells, but this will only show one
Movie

14. Prophase II 1. Chromosomes are already visible 2. Nucleus disappears
3. Centrioles move to the poles

15. Metaphase II 1. Spindle Fibers appear
2. Chromosomes are lined up along middle of cell

16. Anaphase II
Spindle fibers pull sister chromatids to each pole.

17. Telophase II 1. Spindle fibers and centrioles disappear
2. A nucleus appears around each set of chromosomes
3. Cell starts to pinch in at the center

18. Telophase II (Continued)
1. Both cells divide (cytokinesis), leaving 4 new cells
2. Each with half the number of chromosomes the original cell had.
3. Each cell is now HAPLOID, called a gamete.
4. Gametes can be sperms or eggs.

19. Spermatogenesis- making sperm cells by meiosis. Makes 4 sperm
Movie

20. Oogenesis - making egg cells by meiosis. Makes 1 egg and 3 polar bodies
Movie

21. Comparing Mitosis and Meiosis
Happens all over body
PMAT
Makes 2 cells
Diploid cells—full set of chromosomes
Body cells (Somatic cells)
1 set of divisions

22. Comparing Mitosis and Meiosis
Reproductive organs
PMAT PMAT
Makes 4 cells
Haploid—1/2 set of chromosomes
Sex cells (Gametes)
2 sets of divisions
Crossing over