1. Meiosis Differs from mitosis
The process that halves the number of chromosomes in a cell
Occurs in Ovaries or Testes

2. A Variety of Sexual Lifecycles
Human Life cycle
Most fungi and some protists (including some algae)
Plants and some other species of algae

3. Figure 13.4 The human life cycle

4. Alternation of generations
Figure Three sexual life cycles differing in the timing of meiosis and fertilization (syngamy)
Alternation of generations

5. Figure 13.6 Overview of meiosis: how meiosis reduces chromosome number
Four daughter chromosomes
IMPORTANT: Homologous chromosomes are different than sister chromatids
4 Haploid (n) cells instead of 2 diploid cells (2n)

6. Meiosis I : Separates Homologous Chromosomes
Interphase
Each of the chromosomes replicate
The result is two genetically identical sister chromatids which remain attached at their centromeres

7. Prophase I Lasts longer and is more complex than prophase in mitosis
Chromosomes begin to condense and homologues, each consisting of two sister chromatids, pair up
During Synapsis: A protein structure attaches the homologous chromosomes tightly together (synaptonemal complex)

8. Prophase I
Later in prophase, when the synaptonemal complex disappears, each chromosome pair becomes visible in the microscope as a tetrad
A cluster of four chromatids
At various places along their length, chromatids of homologous chromosomes are crisscrossed
Occur at chiasmata
Hold the homologous pairs together until anaphase I

9. Prophase I
Other cellular components prepare for division of the nucleus in a manner similar to that of mitosis
Centrosomes move away from each other and spindle microtubules form between them
The nuclear envelope and nucleoli disperse
The spindle microtubules capture the kinetochores that form on the chromosomes
The chromosomes begin moving to the metaphase plate
Can last for days or longer (over 90% of meiosis)

10. Metaphase I The chromosomes are now arranged on the metaphase plate
Still in homologous pairs
Kinetochore microtubles from one pole of the cell are attached to one chromosome of each pair while microtubules from the opposite pole are attached to the homologue

11. Anaphase I
The spindle apparatus guides the movement of the chromosomes toward the poles
Sister chromatids remain attached
Move as a unit towards the same pole
The homologous chromosome moves toward the opposite pole
Contrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis)

12. Telophase I
The members of each pair of homologous chromosomes continue to move apart until they reach the poles of the cell
Each pole now has a haploid chromosome set but each chromosome still has two sister chromatids

13. Cytokinesis Occurs simultaneously with telophase I
Forms 2 daughter cells
Plant cells – cell plate
Animal cells – cleavage furrows
NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS

14. Figure 13.7 The stages of meiotic cell division: Meiosis I

15. Figure 13.7 The stages of meiotic cell division: Meiosis II

16. Meiosis II : Separates sister chromatids
Proceeds similar to mitosis
THERE IS NO INTERPHASE II !

17. Prophase II
A spindle apparatus forms and the chromosomes progress toward the metaphase II plate

18. Metaphase II
The chromosomes are positioned on the metaphase plate in a mitosis-like fashion
Kinetochores of sister chromatids of each chromosome pointing toward opposite poles

19. Anaphase II The centromers of sister chromatids finally separate
The sister chromatids of each pair move toward opposite poles
Now individual chromosomes

20. Telophase II and Cytokinesis
Nuclei form at opposite poles of the cell and cytokinesis occurs
After completion of cytokinesis there are four daughter cells
All are haploid (n)

21. Figure 13.7 The stages of meiotic cell division: Meiosis II

22. Figure 13.8 A comparison of mitosis and meiosis

23. Figure 13.8 A comparison of mitosis and meiosis: summary

24. Origins of Genetic Variation
As mentioned earlier, in species that reproduce sexually, the behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation
Independent assortment of chromosomes
Crossing Over
Random Fertilization

25. Figure 13.9 Independent Assortment

26. Figure Crossing Over

27. Random Fertilization A human ovum plus a human sperm
1 of 8 million combinations possible for each ovum and sperm
223 X 223 = over 70 billion combinations
70 trillion possible combinations with out considering crossing over
YOU REALLY ARE UNIQUE!

28. Gametogenesis
See handout for details

29. THE END OF MEIOSIS