1. Meiosis
Chapter 8

2. What you need to know!
The role of meiosis and fertilization in sexually reproducing organisms
The importance of homologous chromosomes to meiosis
How the chromosome number is reduced from diploid to haploid through the stages of meiosis
Important differences between mitosis and meiosis
The importance of crossing over, independent assortment, and random fertilization to increasing genetic variability

3. Meiosis
A process by which an organism produces haploid (n) sex cells from diploid (2n) germ cells
Meiosis is divided into meiosis 1 and meiosis 2
Meiosis 1 = separation of homologous chromosomes
Meiosis 2 = separation of sister chromatids

4. Interphase G1, S, G2 Assembly of centrioles
S Phase: Duplication of DNA 2n (diploid)  4n (diploid)
Assembly of centrioles

5. Prophase I 90% of meiotic time Condensation of chromosomes
Nuclear envolope disappears
Spindles form
Synapsis:
Homologous chromosomes pair up as tetrads
The chiasmata are where crossing-over takes place

6. Crossing Over Leads to the creation of novel chromosomes
Genes from homologous maternal and paternal chromosomes mix
Synapsis: homologous chromosomes form tetrads
Chiasmata: homologous chromosomes overlap on multiple points
Crossing Over: identical parts break off and reattach

7. Metaphase 1 Homologous chromosomes line up at the equator side by side
One spindle attaches to each chromosome
4n (diploid) = 92 chromosomes in humans

8. Anaphase 1
Homologous chromosomes separate (sister chromatids remain together)

9. Telophase 1 (cytokinesis)
1 cell separates into 2
Each contains one homologous chromosome with two sister chromatids 2n (haploid)
Often no nuclei form and the DNA remains condensed 2 x (n) = 46 (no pairs)

10. Prophase 2 Spindle fibers form
kinetochore spindle fibers attach to each chromosome

11. Metaphase II
Chromosomes line up at the equator of the cell

12. Anaphase 2 Sister chromatids are pulled apart
Sister chromatids are not identical (crossing over)

13. Telophase 2 and Cytokinesis
4 new cells form
All are genetically different
All are haploid with 1n chromosomes
Humans 23

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