1. MEIOSIS

2. Two types of reproduction
Asexual:
only one parent cell needed
Parent cell divides using mitosis to produce new, exactly identical offspring

3. Sexual reproduction: Two parent cells join together
Parent cells known as sex cells or reproductive cells
Different from ordinary body cells in that they have only HALF the usual amount of DNA (chromosomes)
WHY?

4. How are these sex cells made?
Through a process called MEIOSIS
Diploid (2n)  haploid (n)
(paired chromosomes) (single chromosome for each characteristic)
Meiosis is necessary for sexual reproduction.
Two divisions (meiosis I and meiosis II).

5. Meiosis Sex cells have half the # of chromosomes.
Similar to mitosis with some chromosomal differences.
Below is model of mitosis.

6. So how do we get half the number of chromosomes?
Chromosomes are copied ONCE (replication), but the nucleus divides TWICE.
Two stages: Meiosis I and Meiosis II
Each stage still has prophase, metaphase, anaphase and telophase, where the same things occur as in mitosis

7. Remember… Prophase: Prepare: Chromosomes form, nucleus disappears
Metaphase: chromosomes line up in MIDDLE
Anaphase: Chromosomes move APART
Telophase and Cytokinesis : Cell TEARS in two

8. This is a model of Meiosis

9. Interphase
DNA is Replicated (copied).

10. Prophase I

11. Metaphase I

12. Anaphase I
Note that the sister chromatids are still attached at the centromere (different from mitosis)

13. Telophase I
Cytokinesis I

14. Meiosis 1 Review

15. Meiosis II
No interphase II
(no more DNA replication)

16. Prophase II
same as prophase in mitosis

17. Metaphase II
same as metaphase in mitosis

18. Anaphase II
same as anaphase in mitosis
sister chromatids separate

19. Telophase II Same as telophase in mitosis. Nuclei form.
Afterwards, Cytokinesis occurs.
Remember: Four haploid daughter cells are produced.

20. Telophase II
Cytokinesis II

21. Meiosis 2 Review

22. Meiosis

23. Fertilization n=23 egg 2n=46 zygote
The fusion of a sperm (gamete or “S” Cell) and egg (gamete or “S” Cell) to form a zygote.
A zygote is a fertilized egg
sperm
n=23
human
n=23
egg
2n=46
zygote
human
human