1. Meiosis and Reproduction
California Science Standards #1c, 2a, 2b, 2c, 2d, 2e, 2f, 2g

2. Meiosis
Only certain cells in a multicellular organism undergo meiosis.
Special diploid cells (spermatogonia) in the testis of the male
Produce the haploid sperm
Special diploid cells (oogonia) in the female ovary
Produce the haploid eggs

4. Meiosis
One cell (oogonia or spermatogonia) undergoes meiosis and is divided into four daughter cells

5. Spermatogenesis
Movie: “repro_sperm.mov,” © Glencoe, 2000

6. Oogenesis

7. Mitosis vs. Meiosis

8. Mitosis vs. Meiosis Mitosis makes exact copies of cells
Meiosis makes gametes (egg, sperm)
Mitosis makes daughter cells with 100% of the parent cell’s DNA
Meiosis makes daughter cells with 50% of the parent cell’s DNA

9. Egg (50%) + sperm (50%) = zygote (100% of DNA)
Movie: “repro_fertilization.mov,” © Glencoe, 2000

10. Man or Woman?
Only 1 chromosome makes us different!

11. Meiosis PMAT PMAT 8 phases: Meiosis I Meiosis II Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II
PMAT
Meiosis I
PMAT
Meiosis II

12. Prophase I Movie: “Meiosis.mov,” © Wadsworth, 1997
DNA is already duplicated (from interphase)
DNA condenses into chromosomes
Spindle fibers appear (“fishing line”)
Nucleus breaks down
Homologous chromo-
somes are paired
(synapsis)
Crossing-over
Trade pieces of DNA

13. Prophase I: Crossing Over
Between a sister chromatid from one chromosome and a sister chromatid from the other homologous chromosome
Results in genetic recombination
“Shuffle the DNA deck”

14. Metaphase I Tetrads (homologous chromosome pairs) line up randomly
Spindle fibers attach
to centromeres

15. Anaphase I / Telophase I
A1- Independent assortment
Each homologous chromosome migrates (“gets reeled in”) to an opposite pole (but which pole?)
T1- Cytokinesis begins (pinch off)
Each of the cells are haploid- ½ # chromosomes
But, they are in the duplicated state

16. A Closer Look…

17. 2nd “Shuffle” Results in genetic recombination
“Shuffle the DNA deck” again

18. Prophase II / Metaphase II
P2- Spindle fibers form
M2- Chromosomes move to the midline (equator)

19. Anaphase II / Telophase II
Movie Review: “Meiosis_g.mov,” © Glencoe, 2000
Anaphase II / Telophase II
A2- Chromatids separate and migrate toward opposite poles
T2-
A nucleus re-forms around chromosomes in each of the FOUR new cells
Cytokinesis
Each of the cells are haploid- ½ # chromosomes
Now they are back to the unduplicated state

20. Review Q’s
Track the amount of DNA from interphase, through Meiosis I and II, to interphase again (use the terms haploid, diploid, duplicated, and unduplicated).
Speculate: Why is it advantageous for a species to use genetic recombination (shuffle the DNA)?

21. Gestation
From One to a Trillion

22. Fertilization Gestation=the length of a normal pregnancy
Begins after fertilization
Sperm and egg fuse to form a zygote
1n + 1n = 2n
Haploid + haploid = diploid
½ chromosomes (mom’s 23) + ½ chromosomes (dad’s 23) = 1 whole set of chromosomes (baby’s 46)
50% DNA + 50% DNA = 100% DNA

23. Cleavage & Implantation
Zygote divides by mitosis into a ball of cells called a morula (cleavage)
Morula divides into a blastocyst, which is a ball of cells filled with a fluid cavity
Blastocyst burrows and embeds itself into the lining of the uterus (implantation)

24. Development of Embryo
1st trimester: most dramatic (from one cell into 3 types of cells: endoderm, ectoderm, mesoderm); heartbeat, brain activity

25. Development of Embryo
2nd trimester: heartbeat heard, skeleton formed, can suck thumb, and kick

26. Development of Embryo
3rd trimester: rapid growth, viable outside mother, responds to music, sound, and light

27. Development of Embryo Ready for birth (~270 days after fertilization)
Now trillions of cells big