1. Meiosis Process by which GAMETES are formed (sex cells)
Results in 4 haploid, genetically different cells

2. Describe meiosis Cell division that produces gametes
Reduces the number of chromosomes
Steps are similar to mitosis, but cells divide twice resulting in haploid cells
When gametes are joined in fertilization, diploid number is restored
Animation:

3. Mitosis Vs. Meiosis
The most important thing to remember in distinguishing Mitosis from Meiosis is:
Mitosis results in body cells. (SOMATIC) (Diploid, 2n, genetically identical)
Meiosis results in gametes. (SEX CELLS) (Haploid, n, genetically different)
Every sexually reproducing organism inherits a single copy of every chromosome from both its parents. Humans receive 23 chromosomes from mom and 23 from dad, 46 total.

4. Meiosis Two phases, Meiosis I and Meiosis II
Sometimes called “Reduction Division”
Similar to Mitosis:
First:
Prophase I
Metaphase I
Anaphase I
Telophase I
Then:
Prophase II
Metaphase II
Anaphase II
Telophase II

6. Prophase I of Meiosis I is where crossing over takes place!
Tetrad
Remember, these are the homologous chromosomes together.
There are 4 sister chromatids in a tetrad.
Prophase I of Meiosis I is where crossing over takes place!

7. Crossing over occurs during Prophase I of Meiosis I
Results in genetic variation

9. Tetrads are only seen in Meiosis

10. Meiosis I Interphase I Anaphase I Prophase I Metaphase I :
The fibers pull the homologous chromosomes toward the opposite ends of the cell.
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
Spindle fibers attach to the chromosomes. :

12. Meiosis. The long and short chromosomes are different chromosomes
Meiosis. The long and short chromosomes are different chromosomes. The red and grey are different versions of each. Step 1 replicated them. Step 2 is alignment and formation of the tetrad. Step 3 is crossing over. Step 4 is separation of the homologous chromosomes. Step 5 is separation of the sister chromatids.

13. Differences in meiosis
In males:
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
X or Y chromosome
Have flagella to move
In females:
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 after puberty
Much larger
X chromosome only
No method of movement

14. Gamete production Females have XX for last pair of chromosomes
Males have XY for last pair
Since all eggs are X, father determines the sex of the child since the sperm may be X or Y

15. Mitosis vs. Meiosis Cell Division Cell Type # Daughter Cells
# Chromosomes
Haploid or Diploid
MITOSIS
Somatic (body cell) 2 46
diploid
MEIOSIS
Gamete (sex cell) 4 23
haploid