1. Intro to Meiosis

2. Meiosis What is it? Who does it? Where does it happen?
Why does it happen?
Why is a different process needed?
Making gametes (sperm & eggs)
All sexually reproducing organisms
In males- in the testes
In females- in the ovaries
To allow organisms to reproduce
Chromosome # needs to be cut in half
Creates genetic diversity (can help evolution of species)

3. Meiosis allows sex cells to join to form a complete
set of chromosomes/instructions to make a new organism.
Diploid adult
MITOSIS
FERTILIZATION
MEIOSIS
Haploid gametes
Diploid zygote

4. Remember, humans have 23 pairs of chromosomes
Pairs 1-22 are said to be
Homologous chromosomes
(they go together because
they have same traits but are
not identical)
Gene for eye color
Gene for hair color

5. Now let’s look at how sex cells are made…

6. Eight Stages of Meiosis
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II

7. Prophase I:
1.) Homologous chromosomes pair up
2.) Crossing over occurs between replicated
homologous chromosomes
Crossing over
(exchanging parts)

8. Crossing over

9. Note: chromosomes in
daughter cells are all different

10. Metaphase I: Homologous chromosomes get next to
each other in the cell, and line up in pairs
Along the middle of the cell.
Question: How is
metaphase I of meiosis
different from
metaphase of mitosis?
Note: “X’s” are next to each other,
not above each other

11. Anaphase I: homologous chromosomes separate

12. Telophase I: sister chromatids still need to
be separated
still looks like
an “X”

13. Now Meiosis II occurs
(looks like mitosis)

14. Meiosis

15. Meiosis animation

16. Practice
1.) If a haploid cell has 7 chromosomes, how many chromosomes does a diploid cell have?

18. 2.) If a gamete has 23 chromosomes, how many chromosomes are in a body cell of that organism?

19. 3.) How are the cells at the end of meiosis I different from the cells at the end of meiosis II?

20. 4.) What is the same and what is different about mitosis and meiosis?

21. 5.)Why is crossing over important to the survival of a species?

22. And now let’s see a more technical version…
Meiosis technical

24. One final detail…

25. The End!