1. Meiosis (Ch 11.4)

2. Background Chromosome = package form of DNA in a cell
Genes = genetically inherited traits
Genes normally have two alleles per gene (there are some exceptions like ABO blood type)
Genes are carried on chromosomes from the parents to the offspring that ride on gametes (sex cells: sperm or egg)
Genes can be found in specific locations on the chromosomes

3. Chromosome Number
Homologous Chromosomes: each set of chromosomes that come from the parents
Example: Fruit Flies → 8 chromosomes (4 from dad and 4 from mom)
Diploid: a cell that contains both sets of homologous chromosomes from the parents (2N)
N = the single set of chromosomes coming from one parent
2N = the complete set of homologous chromosomes from mom and dad
Haploid: a single set of chromosomes (N)
Example: Fruit Flies → 2N = 8 chromosomes (4 from mom and 4 from dad)
N = 4 chromosomes coming from each parent

4. Phases of Meiosis
Meiosis: the process during sexual reproduction where chromosomes are split in half through the separation of homologous chromosomes
The two divisions creates the four haploid cells → one haploid cell from mom (23 chromosomes = 11 pairs → egg) will join with a haploid cell from dad (23 chromosomes= 11 pairs → sperm) in the offspring (46 chromosomes = 23 pairs)
2 divisions → Meiosis I / Meiosis II

5. Meiosis I
Before Meiosis, the cell goes through interphase (G1 / S / G2)
Prophase I
Each replicated chromosome pairs with the corresponding homologous chromosome (tetrad)
Each tetrad contains 4 chromatids
Crossing-over: the chromatids cross over one another -- then the chromatids (contain alleles) are exchanged
Spindle fibers form and attach to the tetrads

6. Meiosis I (Cont.) Metaphase I
Spindle fibers pull the homologous chromosomes to line up across the center
Anaphase I
Spindle fibers pull the homologous chromosome pairs towards opposite ends of the cell
Telophase I
The nuclear membrane forms, spindle fibers disappear, and the cells begin to pinch off
Cytokinesis
One cell becomes two cells

7. Meiosis I

8. Meiosis II
The two cells now undergo a second cell division to get from 2N → N
Prophase II
Chromosomes become visible -- chromosomes do not need to form tetrads because they already crossed over in Prophase I
Spindle fibers form from centrioles
Metaphase II
Spindle fibers pull the chromosomes to line up at the center of the cell

9. Meiosis II Anaphase II Spindle fibers pull the sister chromatids apart
Telophase II
The nuclear envelope begins to form, spindle fibers disappear, and the two cells begin to pinch off
Cytokinesis
The two cells pinch off completely to form four haploid cells (N)

10. Meiosis II

11. Gametes → Zygotes Meiosis II = 4 haploid cell (N) = gametes
male gametes = sperm (23 chromosomes  11 pairs)
female gametes = eggs (23 chromosomes  11 pairs)
When the female gamete and the male gamete come together, fertilization begins to form the zygote
Zygote → mitosis → forms a new organism

12. Mitosis vs. Meiosis Mitosis Both Meiosis asexual reproduction
similar chromosomes in the daughter cells
2N parent cell → 2N daughter cells (8 chromosomes)
2 daughter cells = identical
1 division
Interphase
sexual reproduction
homologous chromosomes move to separate daughter cells = genetic variation
2N parent cells → N daughter cells (4 chromosomes)
4 daughter cells = genetically different
2 divisions

13. Gene Linkage
Mendel → independent assortment: genes located on different chromosomes separate independently of each other
Morgan
Drosophilia = fruit flies
Each chromosome is a group of linked genes
Mendel’s principle of independent assortment holds true
Gene linkage: alleles of different genes tend to be inherited together when the genes are located on the same chromosome