1. Meiosis
Chapter 10

2. Chromosome Number
Homologous: chromosomes that are passed on from parents
1 from mom/1 from dad
Diploid: cell that contains both sets of homologous chromosomes
2 complete sets of genes
# chromosomes in diploid cells written as “2N” (Human 2N = 46)

3. Chromosome Number (con’t)
Haploid: cell that contains only 1 set of chromosomes (gamete)
Written as “N” (Human N=23)
Gametes: reproductive cells (sperm & eggs)
When gametes are formed, contain only 1 set of chromosomes

4. Meiosis
Def: The process by which the # chromosomes in a cell is cut in half, and homologous chromosomes are separated
2 Stages:
Meiosis I
Meiosis II
Creates 4 haploid cells (gametes), all genetically different

5. Meiosis I Interphase I (before Meiosis): Occurs in 4 steps:
DNA replicates during S phase
Makes 2 new chromatids (copies) connected by a centromere
Occurs in 4 steps:
1) Prophase I:
Each chromosome pairs with homologous chromosome & forms a tetrad

6. Meiosis I (con’t) Sometimes when tetrads form, crossing-over can occur
Crossing Over: when homologous chromosomes exchange portions of themselves
Results in new combinations of genes, different from parents
Prophase I

7. Meiosis I (con’t) 2) Metaphase I: 3) Anaphase I: 4) Telophase I:
Spindle fibers attach and chromosomes line up
3) Anaphase I:
Fibers pull homologous chromosomes apart
4) Telophase I:
Cytokinesis occurs
Results in 2 new diploid (2N) “daughter” cells

8. Meiosis II 1) Prophase II:
Chromosomes DO NOT DUPLICATE again (no Interphase)
Occurs in 4 steps:
1) Prophase II:
From Meiosis I, have 2 diploid (2N) daughter cells

9. Meiosis II 2) Metaphase II:
Chromosomes line up in center of cell (just like Metaphase I)
3) Anaphase II:
Fibers pull chromatids apart

10. Meiosis II 4) Telophase II: Nucleus forms around each set chromatids
Cytokinesis occurs
Results in 4 haploid (N) daughter cells (gametes)

12. Mitosis vs. Meiosis
Mitosis → 2 genetically identical diploid (2N) cells
Used for cell growth and replacement
Meiosis → 4 genetically different haploid (N) cells
Used for reproduction & formation on gametes