1. Meiosis

2. Meiosis is the type of division which:
1. Reduces the chromosome # from diploid to haploid
2. Is used to produce gametes: eggs and sperm

3. Diploid number: the total # of chromosomes in a normal body cell
1. chromosomes are in pairs
2. chromosomes are in 2 sets: one from dad, one from mom
3. symbol for diploid #: 2n
Humans have 46 chromosomes in cells: diploid # = 2n = 46

4. Haploid number: # of chromosomes in an egg or sperm cell
1. chromosomes are not in pairs
2. chromosomes are in 1 set
3. symbol for haploid #: n
Humans have 23 chromosomes in gamete cells: haploid # = n = 23

5. Egg (n) + Sperm (n) = Zygote (2n)
4. When an egg (n) is fertilized by a sperm (n), the result is a zygote which is 2n
In humans:
Egg (n) Sperm (n) = Zygote (2n) 46 23 23

6. The purpose of meiosis is to divide cell chromosomes so that the diploid # is reduced to the haploid #
There are two complete divisions: meiosis I and II

7. Interphase
2n cell

8. Stages of Interphase
G1 = growth and development
S = DNA replication chromosomes become double
G2 = preparation for division

9. Interphase of meiosis is exactly like interphase of mitosis, except that it occurs only in certain cells of the ovaries or testes

10. Meiosis I – Reduction division
Prophase I:
a. Nucleolus and nuclear envelope disappear
b. Spindle forms
c. Chromosomes shorten, thicken, and become visible
d. Homologous pairs of chromosomes are together
e. Crossing over occurs

11. Prophase I
2n cell

12. Crossing over: the exchange of portions of chromosomes between homologous partners (increases variation)

13. Metaphase I:
a. Chromosomes line up at the equator
b. Spindle fibers attach to centromeres
c. Homologous chromosomes are together

14. Metaphase I
2n cell

15. Anaphase I:
a. Homologous pairs separate
b. Centromeres DO NOT divide
c. Chromosomes remain double stranded

16. Anaphase I
2n cell
2 = n = 2
n number of chromosomes go to each pole

17. Telophase I:
a. Nucleolus and nuclear membrane reappear
b. Spindle disappears
c. Nuclei are now haploid (n) but chromosomes are still double stranded (2 chromatids are joined at the centromere)

18. Telophase I
Cytokinesis may or may not occur

19. Interkinesis: the period between the divisions of meiosis I and meiosis II
May be short or long
Chromosomes are already double stranded so they DO NOT replicate again

20. Meiosis II – 2nd division
Purpose is to separate sister chromatids
Phases are very similar to mitosis

21. Prophase II:
each daughter cell is now haploid (n) with double stranded chromosomes

22. Metaphase II: Chromosomes
line up at the
equator

23. Anaphase II:
chromosomes are now single stranded and moving to opposite poles

24. Telophase II and cytokinesis:
4 haploid daughter cells are produced

25. 1 2 4 2n n Growth, asexual Sexual Yes No
Mitosis
Meiosis
# of DNA replications 1
# of divisions 2
# of daughter cells 4
n # of daughter cells 2n n
Purpose
Growth, asexual
Sexual
Daughter cells like parent?
Yes No
Daughter cells like each other?