1. Meiosis
Gamete Formation

2. Prophase I

3. What’s happening in Prophase I?
Nuclear envelope begins to dissolve
Chromatin condenses into chromosomes
Homologous chromosomes pair (“synapsis”)
Already duplicated in S phase of Interphase  “tetrads”
Centrosomes begin migrating to opposite poles
Centrosome = amorphous material around centrioles
Spindle fibers begin to form from centrosomes
Spindle fibers made of microtubules & proteins
Create “mitotic spindle”
“Kinetochore fibers” attach to centromere
“Polar fibers” link centrosomes but don’t attach to chromosomes

4. Metaphase I

5. What’s happening in Metaphase I?
Tetrads (4 sister chromatids/ 2 homologous chromosomes) are pulled to middle of cell by kinetochore fibers
Chromosomes’ kinetochores are ½ way between the 2 poles & kept in place there
Homologues align so same genes are adjacent
Crossing over (sister chromatids) occurs
Karyotype from photomicrographs made here

6. Anaphase I

7. What’s happening in Anaphase I?
Homologues (2 sister chromatids) are pulled to opposite poles by kinetochores
“Independent Assortment” = random separation of homologous chromosomes
Independent assortment  genetic variation
Earlier crossing over  genetic recombination
Genetic diversity improves species’ survival

8. Telophase I

9. What’s happening in Telophase I?
Chromosomes reach opposite poles of cell
1 chromosome (½ of the homologue pair) to each
Each chromosome still is paired sister chromatids
Cytokinesis begins
Splits cytoplasm between the cells
Half of organelles will go to each cell
Nuclear envelope reassembles in some cells but not in others
This is species-dependent

10. From Metaphase I to Cytokinesis

14. Meiosis II: Telophase II

15. Meiosis II in Lily Anthers

16. Summary of Meiosis Chromosomes replicated in Interphase
Meiosis I separates homologues
Meiosis II separates sister chromatids of each homologue
Four haploid cells form
Each has only 1 copy of the chromosomes (1 n)
Human ovum/ sperm has 23 chromosomes each
Haploids join in fertilization  23 pairs in diploid