1. Meiosis
Making Gametes

2. Review Asexual Reproduction is also called _______________
Mitosis starts with one parent cell and ends up with __________________________________________
Genetically, the daughter cells are ________________ to the parent cell
Making an identical copy of a cell is also called _______________
What type of cells in YOUR BODY divide by Mitosis? ________________
The scientific term for Body cells is ______________
These cells are considered 2n (diploid). What does that mean? _______________________________________

3. Review Cont’d How many Chromosomes should a normal human have? _____
If a human skin cell must divide to heal a wound, how many chromosomes would each of the daughter cells have? _____
If a human bone cell must divide because a baby is growing, how many chromosomes would each of the daughter cells have? _________
What are two reasons for Mitosis? __________ & ___________
Which organelle is in control of cell division? ___________
What happens when the nucleus loses control? __________

4. Mitosis Asexual Cell Reproduction Necessary for growth & repair
Chromosomes in Somatic Cells are in pairs
2N (diploid)
Two new daughter cells are identical copies of original parent cell
Clones

6. Cell Cycle Sequence of phases in the life cycle of the cell Notice!
Mitosis is only a SMALL part of the cell cycle!
In what phase do cells spend most of their time?
_____________
Mitosis & Cell Cyle
Sequence of phases in the life cycle of the cell

7. Sex Cells are Different
Human beings have 46 chromosomes in every Somatic (Body) Cell
This is a complete set
But, if mom’s egg cell has 46…
And, if dad’s sperm cell has 46…
The Zygote (the fertilized egg cell) would have 92!
Would THAT make a human?

8. Meiosis Cell division of reproductive cells (sex cells)
Gametes or germ cells (sperm and egg)
Produced in Gonads
Each new cell is not exactly like the parent cell
Allows for variation
Important for the survival of our species!

9. Meiosis In each new daughter cell, the chromosomes are not in pairs
NOT 2n (not diploid!)
At the end of Meiosis, four daughter cells contain half of the original number of chromosomes (N)
Each is unique!
Not identical to the parent

10. Gametes (aka germ cell)
Mature egg or sperm cell
Produced in the Gonads
Each gamete has half the original number of chromosomes
1N (haploid)
HALF IS IMPORTANT!

11. Why is Half is Important?
Meiosis makes unique sex cells!
Half the number of chromosomes (haploid) is important for sexually reproducing organisms!
N = The number of chromosomes in a gamete
To create a new organism, two parents give half of their own chromosomes to make a baby with a complete set of chromosomes!
Half from mom
Half from dad
N + N = 2N (Zygote!)

12. Sexual Reproduction Sexual Reproduction requires two parents
Offspring have SOME traits inherited from each parent… but not all!
YOUR DNA is not identical to either parent
It is a mix of genes from both parents
Meiosis

13. Sexual Reproduction
Fusion of two (N) gametes to produce a single zygote (2N)
Introduces greater genetic variation, allows genetic recombination
A VERY GOOD THING!

14. Chromosome Pairing Homologous pair Homologous means similar
Each chromosome in the pair are Homologous to the other (carry genes for same trait)
Only one pair differs - sex chromosomes X or Y

15. Different!
Metaphase I in Meiosis…
Chromosomes form TETRADS (four together)
Metaphase II in Meiosis…
Chromosomes line up in the middle and then a SECOND DIVISION happens!
The two daughter cells are divided into four and each has HALF the chromosome number!

16. Two equally probable arrangements of chromosomes at metaphase I
Independent assortment of homologous chromosomes
POSSIBILITY 1
POSSIBILITY 2
Two equally probable arrangements of chromosomes at metaphase I
Metaphase II
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
Figure 8.16

17. Crossing over increases genetic variability
The exchange of genes between two homologous chromosomes happens when in Tetrad formation

18. Tetrad
Chaisma
Centromere
Figure 8.18A

19. SPERMATOGENESIS
OOGENESIS
spermatogonium
oogonium
Sperm and egg formation in humans.
In sperm formation (spermatogenesis), diploid cells called spermatogonia produce primary spermatocytes. The primary spermatocytes are the diploid cells that go through meiosis, yielding haploid secondary spermatocytes. These spermatocytes then go through meiosis II, yielding four haploid spermatids that will develop into mature sperm cells.
In egg formation (oogenesis), cells called oogonia, produced before the birth of the female, develop into primary oocytes. These diploid cells will remain in meiosis I until they mature in the female ovary, beginning at puberty. (Only one oocyte per month, on average, will complete this maturation process.) Oocytes that mature will enter meiosis II, but their development will remain arrested there until they are fertilized by sperm. An unequal meiotic division of cellular material leads to the production of three polar bodies from the original oocyte and one well-endowed egg. The egg can go on to be fertilized, but the polar bodies will be degraded.
egg
spermatids
polar bodies
(will disintegrate)

20. Figure: 10-07
Title:
Big difference in size.
Caption:
A human egg surrounded by much smaller human sperm.

21. Review Mitosis & Meiosis
Both are forms of cell division
Both involve replication
Both involve disappearance of the nucleus, and nucleolus, nuclear membrane
Both involve formation of spindle fibers