1. Chromosomes and Chromosome Number
Human body cells have 46 chromosomes
Each parent contributes 23 chromosomes
Homologous chromosomes -one of two paired chromosomes, one from each parent

2. Homologous Chromosomes
Same length
Same centromere position
Carry genes that control the same inherited traits
One came from Dad, &
one came from Mom!

3. Why Meiosis?
An organism produces gametes to maintain the same number of chromosomes from generation to generation, and also introduce and maintain variation within the population.
We will discuss later, what happens when there is an error in meiosis. Mutations and Evolutionary events

4. Compare diploid and haploid number
Diploid: cell that contains two of each kind of chromosome (2N)
Body cells(somatic cells) are diploid
Haploid: cell with only one kind of chromosome (N)
Gametes (sex cells) are haploid

5. Meiosis I The sexual life cycle in animals involves meiosis.
Meiosis produces
gametes.
When gametes combine in fertilization,
the number of chromosomes is restored.
This is often called alternation of generations, the cycling between haploid and diploid organisms (cells)

6. Meiosis I
Prophase I- each chromosome pairs with its corresponding homologous chromosome to form a tetrad(4 chromatids)
The nuclear envelope breaks
down.
Spindles form.
*Crossing-over- chromosomes will switch some genes. This gives us genetic variation. Crossing-over is more common the farther it is located from the centromere. Remember map units in Sordaria

7. Crossing Over

8. Metaphase I
Homologous pairs of chromosomes, still in their tetrad, line up together at the middle of the cell
Chromosome centromeres attach to spindle fibers.

9. Anaphase I
homologous chromosomes separate (the tetrad seperates) & move to opposite ends. (centromeres do not split)

10. Telophase I & Cytokinesis
spindle disappears and the cell divides
Meiosis I results in 2 daughter cells, each with the same number of chromosomes as the original cell however sister chromatids are still attached to each other

11. Meiosis II spindle apparatus reforms and the
During the second meiotic division, nothing is replicated, everything is going to be just divide
Prophase II
spindle apparatus reforms and the
chromosomes condense, while the nuclear membrane dissolves again

12. Metaphase II
chromosomes still attached at the centromere line up at the equator (middle)

13. Anaphase II
sister chromatids are pulled apart at the centromere & move to opposite poles

14. Telophase II (includes cytokinesis)
when the 2 daughter cells divide into 4 new daughter cells, each new daughter cell has 23 chromosomes. This is half the number of the original parent cell

15. Difference in meiosis In males: spermatogenesis In females: oogenesis
4 mature sperm
Males begin to produce sperm after puberty, produced constantly until death; meiosis II occurs immediately after meiosis I
Much smaller than egg
May have X or Y chromosomes
Have flagella to move
In females: oogenesis
1 mature egg, 3 polar bodies which break down
Women born with all eggs they will have, meiosis I occurs before birth, meiosis II occurs once a month
Much larger
Have all X chromosomes
Has no method of movement

16. Meiosis/gamete production
Females have XX as their 23rd pair of chromosomes
Males have XY which are nonhomologous
Since all eggs are X, father determines the sex of the child since the sperm may be X or Y

17. Meiosis 23 + 23 = 46 chromosomes
There are 2 divisions of meiosis- Meiosis I and Meiosis II. It must be this way in order to end up with the sex cells (sperm & egg) only having 23 chromosome.
Meiosis occurs in the testes of the male and is called spermatogenesis (4 viable sperm)
And in the ovaries of the female it is called oogenesis. ( produces 1 egg and 3 polar bodies)
= chromosomes

18. Mitosis/Meiosis Mitosis
Cell division producing identical somatic cells
Results in 2 daughter cells
One division
Daughter cells have same number of chromosomes (2N)
Meiosis
Cell division producing unique gametic cells
Results in 4 daughter cells
Two divisions
Daughter cells have half the number of chromosomes (N)

19. How cells divide:Mitosis vs. meiosis
Cell division cell type # of daughter # of
cells chromosomes
Mitosis somatic(body) (diploid)
Meiosis gametes(sex) (haploid)
How cells divide:Mitosis vs. meiosis