2. Cell Division: Meiosis
What?
Division of the cell to form gametes (egg and sperm)

3. How?
DNA is replicated ONCE, but the cell divides TWICE
Results in 4 cells that are NOT identical to each other OR the parent cell

4. Why?
Gametes must have only half the chromosomes (n) as the parental cells (2n),
During fertilization, two gametes fuse… n + n = 2n

5. The Details
In Meiosis I, the homologous chromosomes pair up as a tetrad = XX, then separate (X)
This is different than mitosis!
XX X …………X
This is also where crossing over takes place (homologous chromosomes exchange genes)
In Meiosis II, the sister chromatids separate
X I………....I

6. Why is sexual reproduction important?
Asexual Reproduction
+ Efficient (time, energy)
- Results in clones (problems are inherited)
Sexual Reproduction
- Less efficient (9 months for humans!)
+ Leads to VARIATION
Random, new genetic combinations (necessary for evolution)

9. Homologous pair

10. Mitosis
Meiosis
1 division
2 divisions
Results in two 2n cells; genetically identical
Results in four n cells; genetically different
Sister chromatids separate
Homologs separate first, crossing over occurs
For growth, repair, asexual reproduction
For sexual reproduction

11. What are possible sources of genetic variation?
Warm up: do now
Think about Meiosis:
Why do Ms.Albu and her brother look different even though they have the same parents?
What are possible sources of genetic variation?