1. Cell Division for Reproduction
Meiosis
Cell Division for Reproduction

2. Meiosis Terms to Know:
Meiosis: Cell division that produces gametes (sex cells/sperm & egg). These cells are used for sexual reproduction. They have half the number of chromosomes as the parent cell.
Diploid Number: the number of chromosomes found in the body cells of an organism. These chromosomes occur in pairs.
Haploid Number: the number of chromosomes found in the gametes (sex cells). This is half the number in the body cells.
Homologous Chromosomes: “look” “alike” chromosomes. Pairs of chromosomes in a diploid cell. They are the same size and carry similar genetic information.
Zygote: a fertilized egg cell (when sperm fertilizes the egg, it forms a zygote)

3. More Information about Meiosis:
The diploid number for humans is 46, for dogs it’s 78, for onion plants it’s 16. Notice the pattern? That’s right—they are all even numbers. That’s
because chromosomes occur in pairs in
diploid cells.
“Di”=2; Diploid cells have chromosomes
in 2’s
“Ha”= ½; Haploid cells have half the number of chromosomes as a normal diploid body cell. What is the haploid number for humans? For dogs?

4. Homologous Chromosomes
Homologous chromosomes have genes for the same traits at the same locations. One of each pair comes from the mother, the other comes from the father.
This karyotype shows the chromosomes of a human male.

5. “C” Words:
Chromatin-Strands of DNA in a cell that is not dividing—unwound and indistinct.
Chromosomes-Strands of DNA in a cell that is dividing—wound up and distinct.
Chromatin is unwound and indistinct in the nucleus of the cell.

6. More “C” Words:
Chromatids—one of 2 identical strands of DNA in a chromosome (called sister chromatids)
Centromere—the structure that holds together the 2 sister chromatids.
Histones-protein “beads” that keep the strand of DNA from tangling up.

7. More “C” Words
Centrosome/Centriole-Structure found in animal cells that anchors the spindle fibers
Cytokinesis—the division of the cytoplasm following mitosis (the dividing of the nucleus)

8. Interphase:
Cell Membrane
This is a normal diploid body cell located in the testis (of a male) or the ovary (of a female).
Nuclear Membrane
Chromatin
Centrioles
DNA Strands have replicated.

9. Prophase I
When the DNA coils up in Prophase I, homologous chromosomes move together and become intertwined.
Centrioles migrate to the poles; Spindle forms
Nuclear membrane disappears

10. Synapsis and Crossing Over
Synapsis: The pairing together of homologous chromosomes during Prophase I. (Note: This does NOT happen in mitosis!)
Crossing Over: During Synapsis, homologous chromosomes can exchange pieces of DNA with each other. This leads to greater genetic diversity.

11. Metaphase I
Homologous pairs of chromosomes move to the equator of the cell.
Each chromosome pair is attached to a spindle fiber.
Homologous pairs of chromosomes are still intertwined.

12. Anaphase I
Homologous pairs of chromosomes separate. One chromosome from each pair moves to one pole, the other moves to the opposite pole.
NOTE: Chromosomes do not split in half here. Each chromosome still has 2 sister chromatids.
Note the effects of crossing over from Prophase I—each chromatid is slightly different from the other.

13. Telophase I
In Telophase I, the chromosomes unwind to form chromatin again. New nuclear membranes form and the cell divides into two.
After Telophase I, the cells rest. This is called Interkinesis. (NOTE: this is not Interphase—DNA strands do not replicate here!)

14. Meiosis I Animation

15. Prophase II
Prophase II is very similar to Prophase in Mitosis except that it is occurring in both cells at the same time.
Chromatin winds up to form distinct chromosomes.
Nuclear membranes disappear.
Centrioles migrate to the poles.

16. Metaphase II
In both cells, the chromosomes line up at the equator.

17. Anaphase II
In Anaphase II, the chromosomes split in half at the centromeres and move to opposite poles.
This is just like what happens in Anaphase of Mitosis.

18. Telophase II
In Telophase II, the chromosomes unwind to form chromatin again. New nuclear membranes form. The cells each divide into two cells—making four haploid cells.

19. Meiosis II Animation

20. Genetic Variability
One way meiosis generates genetic variability is through the different ways in which maternal and paternal chromosomes are combined in the daughter cells. For humans, with 23 chromosome pairs, there are over 8 million metaphase arrangements!

21. Spermatogenesis vs. Oogenesis

22. Mitosis vs. Meiosis Produces body cells
One division producing 2 daughter cells
Cells are genetically identical
Produces diploid cells
These cells make up the body of the organism
No Synapsis and No Crossing Over
Produces sex cells/gametes
Two divisions producing 4 cells
Cells are genetically different from each other
Produces haploid cells
These cells are used for sexual reproduction
Synapsis and Crossing Over occurs

23. Mitosis vs. Meiosis

24. Nondisjunction
Nondisjunction is an error that occurs when chromosomes fail to separate correctly in Meiosis.
The result is that one gamete receives too many of one kind of chromosome, and another receives none of that chromosome.

25. Nondisjunction can occur in Meiosis I or Meiosis II:

26. Karyotypes:
Trisomy 13: Patau Syndrome—Causes serious eye, brain and circulatory defects.
Trisomy 21: Down’s Syndrome- Causes mental retardation, heart defects, short stature, and characteristic facial features.

27. Other Syndromes:
XXY—Klinefelter’s Syndrome: Causes infertility, males show some feminine characteristics.
XO—Turner’s Syndrome:
Causes sterile females who possess sex organs that fail to mature at puberty; generally very short stature.