1. Meiosis

2. Homologous Chromosomes
Square 1
Homologous Chromosomes
1 chromosome from parent
1 chromosome from parent
Tetrad

3. Square 2 Diploid 2 sets (chromosomes) 2n Somatic cells Body cells
ex.: liver, skin, etc.
Human 2n = 46

4. Square 3 Haploid 1 set (chromosomes) 1n Sex cells Sperm/Egg Gamete
Human 1n = 23

5. Square 4 WHY? maintain chromosome # from gen. 2 gen. 2n -> 1n
# of chromosomes cut in ½ by separating homologous chromosomes
Creates genetic variation
Used in sexual reproduction

6. Do not write
“Putting It All Together”
Fertilization

7. Do not write Goal: reduce genetic material by half
Why? n (mom) + n (dad) = 2n (offspring)
from mom
from dad
child
too
much!
meiosis reduces
genetic content
Just right!

8. 2 rounds of cell division
Do not write
Meiosis Overview
2 rounds of cell division
Meiosis 1 called reduction division. (diploid cell becomes 2 haploid cells.)
Meiosis 2 divides 2 cells into 4 cells.

9. Meiosis I : the reduction division
Do not write
Meiosis I : the reduction division
Prophase I
(early)
(diploid)
(late)
Metaphase I
Anaphase I
Telophase I
Nucleus
Spindle
fibers
Nuclear
envelope

10. Square 5 MEIOSIS I: PROPHASE I Prophase I Interphase
2n: Homologs pair up
Interphase

11. MEIOSIS I: PROPHASE I Crossing over
Top 1/3 of squares 6/7
MEIOSIS I: PROPHASE I
Crossing over
Portions of the homologous chromosomes exchange – more genetic variation

12. Square 6
METAPHASE I
2n: Homologs line up at Metaphase Plate
Law #1

13. ANAPHASE I Square 7 Homologs separate; move to opposite poles.
Sister chromatids remain attached

14. TELOPHASE I & CYTOKINESIS
Square 8
TELOPHASE I & CYTOKINESIS
Cytokinesis divides cell in two
(Reduction Division…2n 1n)

15. Meiosis II Do not write Gene X Sister chromatids carry
Meiosis II produces gametes with
one copy of each chromosome and thus one copy of each gene.
Sister chromatids carry
identical genetic
information.

16. Meiosis II Do not write Prophase II (haploid) Metaphase II Anaphase II
Telophase II
Four
Non-identical
haploid
daughter cells

17. Square 9
PROPHASE II
Similar to mitosis… 1n

18. Square 10
METAPHASE II
Sister chromatids line up 1n

19. Square 11
ANAPHASE II
Sister chromatids separate 1n
Law #2

20. TELOPHASE II & CYTOKINESIS
Square 12
TELOPHASE II & CYTOKINESIS
Four haploid cells
One copy of each gene

21. Meiosis: cell division in two parts
Do not write
Meiosis: cell division in two parts
Sister
chromatids
separate
Haploid
Diploid
Meiosis I
(reduction
division)
Meiosis II
Chromatids
separate
(equational
division) 2n n n
Result: one copy of each chromosome in a gamete.

22. #1 REPRODUCTION #2 # OF DIVISIONS Square 13 MITOSIS MEIOSIS
Mitosis – Asexual
Meiosis - Sexual
#2 # OF DIVISIONS
Mitosis = 1
2 identical 2n cells
Meiosis = 2
4 unique 1n cells
VS.

23. #3 change in CHROMOSOME #
Square 14
MITOSIS
MEIOSIS
#3 change in CHROMOSOME #
Mitosis – no change
– 1 diploid :2 diploid
Meiosis - change occurs (1/2 of original)
– 1 diploid: 4 haploid
#4 SEPERATION OF GENETIC MATERIAL
Mitosis – each daughter cell gets complete set (2n)
Meiosis – each gamete gets 1 copy (1n)
VS.

25. LAW #1: INDEPENDENT ASSORTMENT
Square 15
LAW #1: INDEPENDENT ASSORTMENT
Homologous chromosomes do not affect how others sort
Gives even more genetic variation

26. LAW #2: SEGREGATION Square 16 Separation of alleles
Needed so gametes have only 1 set of each gene