1. Before the magic happens…
Meiosis
Before the magic happens…

2. Making babies!!!!
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Meiosis is the production of gametes (reproductive cells) – sperm and eggs
The gametes produced are haploid – the original cells are diploid.
Q: what are some examples of diploid cells in our body?

3. The numbers…
One diploid cell divides TWICE, resulting in four haploid gametes.
Each gamete has half the DNA of the original cell.
Q: Where does this happen?
A: The ovaries and the testes!

4. What’s so important about it?
Meiosis is a major source of genetic ‘reshuffling.’ Portions of homologous chromosomes can EXCHANGE, creating unique, never before seen chromosomes. (and therefore more genetic diversity)
In other words, your ‘moms and dads’ have one last shot at mixing it up…
Q: Why is more genetic diversity important? In the context of an entire species…
A: More diversity creates more variation which provides more ‘material’ on which natural selection can act  evolution!

5. A Tale of Two Phases Meiosis proceeds in two distinct phases called…?
Meiosis I:
Recombination!
Homologs are separated
Meiosis II:
Pretty much exactly the same as mitosis
Except no interphase!!! (instead it has interkinesis)

7. Prophase I: Where the action is
Everything ‘exciting’ about meiosis happens right at the beginning in prophase I. This is where recombination (aka crossing over) occurs.
All the usual stuff occurs also: chromosomes condensing, nuclear envelope fragmenting, etc…

8. Prophase I actually consists of 5 stages
Leptotene: chromosomes condense
Zygotene: Synapsis (homologs come together) – synaptonemal complex forms
Pachytene: chromosomes fully condense
Diplotene: synaptonemal complex begins to breaks up, except for attachment points at chiasmata
Diakinesis: synaptonemal complex gone – all four chromatids are distinct

9. Prophase I: Synapsis (zygotene)
The homologous chromosomes come together into tetrads. (homologs and their sister chromatids)

10. Prophase I: Recombination
At the points where the homologs ‘cross’ (chiasmata) they can exchange sections of their arms (recombination). This creates new combinations of genes.

11. Metaphase I
Tetrads align along the metaphase plate. (as opposed to sister chromatids in mitosis)

12. Anaphase I
Homologous chromosomes, with their sister chromatids, are pulled apart to opposite poles of the cells.
Q: What pulls them apart?
A: kinetochore microtubules

13. Telophase I and Interkinesis
The two daughter cells separate, nuclei reform.
Q: what can we say about these daughter cells?
A: They are haploid! - they have the same physical amount of DNA but only one ‘parental set’

14. Prophase II
Just like mitosis. Chromosomes condense, spindle fibers attach, etc…

15. Metaphase II
Sister chromatids align across the metaphase plate. Yay…

16. Anaphase II Sister chromatids are separated.
Q: How does this compare to mitosis and meiosis I?
A: Same as mitosis. Opposite of meiosis I.

17. Telophase II
Same ol’ story…

18. Summing it all up…
Meiosis generates gametes that can (if they’re lucky) eventually fuse with one another to create a zygote which will hopefully develop into a healthy new organism.

19. Comparing Mitosis and Meiosis
Make sure you are able to compare mitosis and meiosis in terms of what goes in, what comes out, and what happens in between.
In fact, let’s try it now!!! Use your worksheet – and your group – to compare and contrast the two processes.