1. Meiosis (vs. Mitosis) Chapter 13 13.3 Pgs. 253-257
Objective: I can describe the events of meiosis and compare and contrast them to mitosis, noting unique events of meiosis.

2. Meiosis (in a nutshell)
Reduction Division
2 divisions (after 1 doubling)
1 diploid germ cell  4 haploid gametes
Mitosis = Division
1 division (after 1 doubling)
1 diploid somatic cell  2 diploid
somatic cells

3. Meiosis I (1st Division)
Prophase I, Metaphase I, Anaphase I, Telophase I
Start with a 1 Diploid germ cell
Separate homologues (split up pairs)
End with 2 haploid cells
Each chromosome is STILL doubled at end of Meiosis I

4. Meiosis II (2nd Division)
Prophase II, Metaphase II, Anaphase II, Telophase II
Start with a 2 Haploid cells
Separate “sister”chromatids (split up doubled)
End with 4 haploid cells
Each chromosome is singled at end of Meiosis II

5. Prophase I Events Homologues will join: called synapsis
Forms a unit called a tetrad = structure of
2 doubled chromosomes (4 chromatids)
Allows crossing over to occur
Exchange of genes between homologues
Chiasma (pl. chiasmata) = crossing over region

6. Crossing Over
Due to synapsis (close together), homologues MAY or MAY NOT exchange genes
Creates more genetic variation

7. Metaphase I Events Homologous pairs line up together
INDEPENDENTLY of each other
This is called Independent assortment
All of 1 parent’s chromosomes are not required to lined up on 1 side
More variation!

8. Anaphase I Events
Separation of homologues (splitting up tetrad) = Disjunction
Chromosomes still doubled
Telophase I events are “normal” (w/ cytokinesis)…
…except making two cells that are haploid and NOT identical

9. Between Meiosis I and Meiosis II
A brief Interphase a.k.a. Interkinesis
Chromosomes *may* briefly unravel to chromatin (short rest) – don’t show in drawings
Centrioles are duplicated in Interkinesis!
Something inconsistent in this drawing…

10. Meiosis II Operates just like Mitosis
Except have 2 cells, both haploid (chromosomes end up singled)

12. Time to Work on Meiosis Packet
Use Mitosis Packet to learn from mistakes and make meiosis packet in a similar fashion
Use the same designs from the Mitosis Packet (write down the designer’s name and redraw their designs into Meiosis Packet)
Will review Meiosis end of period (will use box o’names)

13. Meiosis: Overview Meiosis: 2 divisions
Start with 1 cell that is… Diploid (2n) Germ Cell with doubled chromsms.
Meiosis I (1st division)
Results in 2 haploid(n) cells
Chromosomes still doubled
2 chromatids each
Meiosis II (2nd division)
Still have haploid cell (n), but now 4 cells
Chromosomes now singled
1 chromatid each

14. Stages of Meiosis Meiosis stages similar to Mitosis stages
Before Meiosis, still have Interphase
During S-Phase, DNA duplicates
Meiosis I is made up of
Prophase I, Metaphase I, Anaphase I, Telophase I
Meiosis II is made up of
Prophase II, Metaphase II, Anaphase II, Telophase II

15. Meiosis: Prophase I Prophase I: similar to Prophase of Mitosis
Doubled Chromosomes appear, nucleus disappears (just like mitosis)
BUT has 2 important differences!!!
1) Homologous Chrmsms. will pair up: synapsis
Mitosis: Prophase Meiosis: Prophase I

16. Meiosis: Prophase I (cont’d)
2nd difference: Crossing-Over
Allowed by synapsis; formation of tetrads
Parts of the homologs will exchange
Allows child to have some traits of mom and some traits of dad
Only Meiosis does this – Mitosis does not!

17. Meiosis: Metaphase I Metaphase I: similar to Mitosis Metaphase
BUT has 1 important difference
Homologs line up along equator (not each chrmsm)
Formation will look different (single-file vs. pairs)
Mitosis: Metaphase Meiosis: Metaphase I

18. Meiosis: Metaphase I (cont’d)
Because homologous pairs line up, can have independent assortment
Each tetrad can have homologs flip randomly
Irrelevant for mitosis, because sister chromatids are identical
Mitosis: Metaphase Meiosis: Metaphase I

19. Meiosis: Anaphase I Anaphase I: Similar to Mitosis Anaphase
Homologues separate = Disjunction
Tetrad splits up (chromosomes still doubled)
Will result in each side with haploid (n)
Mitosis: Anaphase Meiosis: Anaphase I

20. Meiosis: Telophase I Telophase I: similar to Mitosis Telophase
Main differences
Two daughter cells are haploid (n)
Two daughter cells are different from each other and from original parent cell
Due to Crossing-Over & Independent Assortment
Chrmsmes still doubled, so can divide again
Mitosis: Telophase Meiosis: Telophase I

21. By the End of Meiosis I How many duplications? 1 How many divisions? 1
How many cells? 2
Chromosome count in each cell (Ploidy)?
Haploid (n) – chromosomes still doubled

22. Meiosis II (2nd Division)
Meiosis II = Mitosis
No differences… (Pro II, Meta II, Ana II, Telo II)
Start with haploid (doubled chromosomes)
End with haploid (singled chromosomes)
Prophase II
Metaphase II

23. Meiosis II (2nd Division) – cont’d
Meiosis II = Mitosis
Telophase II
Anaphase II

24. During (within) Meiosis II
How many duplications? 0
How many divisions? 1 (for each cell)
How many end? 4
Chromosome count (ploidy)?
Haploid (n) – chromosomes now singled

25. By the end of Meiosis (Total)
How many duplications? 1
How many divisions? 2
How many cells? 4
Chromosome count at beginning (ploidy)?
Diploid (2n) – chromosomes doubled
Chromosome count at end (ploidy)?
Haploid (n) – chromosomes singled