1. Meiosis

2. Cell Division Cell Division occurs in two ways: Mitosis
produces genetically identical daughter cells
Meiosis
produces genetically different offspring

3. Chromosome Changes
Most normal cells start with “n” number of pairs of chromosomes
n = number of kinds of chromosomes
2n = total number of chromosomes
Examples: n 2n
Onion 4 8
Fly
Human
How Many Chromosomes Does It Have?
Fathers donate sex cell with half a set of chromosomes = 23
Mothers donate sex cell with half a set of chromosomes = 23
Offspring receives full set of chromosomes = 46/23 pairs
Diploid cells – 2n chromosomes
Haploid cells – n chromosomes, ½ set

4. Quick Quiz!! If a certain cell has 10 total chromosomes, what number of types of chromosomes does it have? 10 5
2.5 20 1 2 3 4 5

5. Quick Quiz!! If the cells of a sexually reproducing organism have 16 chromosomes, how many were likely to be donated from it’s father? 32 16 8 4 1 2 3 4 5

6. Quick Quiz!! If you have a diploid cell with 12 kinds of chromosomes, how many total chromosomes are in the cell? 24 12 13 6 1 2 3 4 5

7. Quick Quiz!! If you have a haploid cell with 12 kinds of chromosomes, how many total chromosomes are in the cell? 24 12 13 6 1 2 3 4 5

8. Meiosis Sperm Ovum/Egg
Only used to create sex cells cells have n number of chromosomes
Human = 23
Onion = 4
Sperm
Ovum/Egg

9. They result in new species They have half the number of chromosomes
Quick Quiz!! How are sex cells genetically different from other body cells?
They are not different
They result in new species
They have half the number of chromosomes 1 2 3 4 5

10. Stages of Meiosis
Meiosis occurs over two separate sets of stages, Meiosis 1 and Meiosis 2.
Let’s examine how this works with a cell that has 6 chromosomes

11. Prophase I
Homologous chromosomes form, find each other and become tetrads
Spindle forms
Nucleus breaks up

12. Metaphase I Tetrads line up in the middle of the cell
Spindle fibers attach to centromeres

13. Anaphase I Tetrads split
Homologous chromosomes move to opposite ends of the cell
Cell begins cytokinesis

14. Telophase I Two haploid daughter cells are created
Meiosis 2 may begin immediately

15. Quick Quiz!! In the previous slide, what does n=?1 2 3 4 6 8 10 12 1 2 3 4 5

16. Prophase II
Spindle fibers form
Nucleus breaks apart

17. Metaphase II Individual Chromosomes line up in the center of the cell
Spindle fibers attach to centromeres

18. Quick Quiz!! How is Metaphase II different from Metaphase I of meiosis?
It isn’t, they are exactly the same
In Metaphase II, the chromosomes don’t line up
In Metaphase I, tetrads lined up, not individual chromosomes
In Metaphase I, the spindle fibers did not attach 1 2 3 4 5

19. Anaphase II Sister chromatids separate
Chromatids move to opposite ends of the cell
Spindles shorten
Cytokinesis begins

20. Telophase II Cytokinesis ends Four haploid cells are left
In most males, all four will become gametes
In females, only one will

21. Gametogenesis Gametogenesis = formation of gametes
Gametes: haploid sex cells, contain n number of chromosomes
Occurs slightly differently in males and females
Spermatogenesis = sperm
Oogenesis = egg

22. Gametogenesis Spermatogenesis Oogenesis X X x x X X x x I I I Gametes
3 nonfunctional polar bodies I I I I
Sperm
Egg or Ovum
I I
Fertilization: sperm DNA enters egg, completing the set of chromosomes

23. Quick Quiz!! After fertilization, what is the n number of the cell in the last slide?2 4 8 16 1 2 3 4 5