1. Ch Meiosis
Formation of Gametes
(egg & sperm)

2. Our Chromosomes 46 Chromosomes (23 pairs)
#1-22 Pairs are Autosomes (body cell chromosomes):
Determine all traits except gender
#23 pair are Sex chromosomes
referred to as X & Y
male (XY)
female (XX)
X - Chromosome
Y-Chromosome

3. Chromosome Numbers
Vary in organisms

4. A closer look at Chromosome Pairs

5. Karyotype Picture of chromosomes arranged by size
Pairs 1-22 = autosomes
Pair 23 = sex chromo
Detects abnormalities &
XX female or XY male

6. Homologous Chromosomes (Buddy-Buddy)
Matching pair = homologous chromosomes
Homologous chromosomes: 2 chromosomes (one from mom and one from dad) that are alike in:
size,
location of centromere,
dark/light banding pattern of genes Remember: only non matching pair are sex chromosomes

7. Homologous Chromosome Pairs
Mom’s Blue eye gene
Dad’s Brown eye gene
*Allele – different versions of the same gene (traits)*

8. If a cell has all 46 chromo (23 pairs), it’s called a diploid cell
Shorthand: 2N
Homologous Pair 

9. Somatic Cell Gamete Body Cell Mitosis 2 sets of DNA 46 total chromo
Skin cell, etc.
Mitosis
2 sets of DNA
46 total chromo
1 set from each parent
DIPLOID (2N)
Sex Cell
Egg/sperm
Meiosis
1 set of DNA
23 total chromo
½ set from each parent
HAPLOID (N)

10. Important Vocab Somatic Cell vs. Gamete Diploid (2N) vs. Haploid (1N)
Body cell vs. Sex cell
Diploid (2N) vs. Haploid (1N)
2 sets of DNA vs. 1 set of DNA
Homologous Chromosomes
Matching pairs of chromo in 2N cell
Alleles
Different version of the same trait
Fertilization
When sperm meets egg and combines DNA
Zygote
Cell in growth state following fertilization
Tetrad
Structure containing 4 chromatids

11. Meiosis Info… Similar but different from Mitosis:
Sex cell division only
Involves 2 cell divisions
Results in 4 cells with half the normal genetic info
Produces gametes (egg/sperm)
Male Testes (spermatogenesis)
Female Ovaries (oogenesis)

12. Why Do we Need Meiosis? Basis of sexual reproduction
Accounts for individual genetic diversity
You are unique! You look a little bit like your mom and a little like your dad!
Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote (fertilized egg) 1N 2N
Fertilization

13. Here’s the key to your “uniqueness”
Gene X
Sister Chromatids
(same genes,
same alleles)
Homologous Pair
same genes,
different alleles
Homologous pairs separate in meiosis and therefore different alleles (versions of traits) separate. So many combos of traits are possible!

14. Meiosis Forms Haploid Gametes
Meiosis must reduce the chromosome # by half
Fertilization then restores the 2n number
23 chromo from egg + 23 chromo from sperm = you !
from mom
from dad
child
too
much!
EGG
SPERM
YOU
meiosis reduces
genetic content
The right number!
EGG
SPERM
YOU

15. Meiosis: 2-Part Cell Division
Sister
chromatids
separate
Meiosis I
Homologous
Pair
separate
Meiosis II 46
Interphase
Diploid (2x46 = 92)
Diploid (46)
Haploid (23)
1 replication of chromosomes is followed by 2 cell divisions
(aka Interphase only happens once!)

16. Meiosis: Reduction Division
2 part cell division
Meiosis I
Interphase I
PMAT I
Meiosis II
PMAT II
End result: 4 genetically different haploid cells
4 sperm or 1 egg
Interphase
Meiosis I
Meiosis II

17. Meiosis I: Reduction Division
Nucleus
Spindle
fibers
Nuclear
envelope
Chromo pair up
Early Prophase I
(Chromosome number doubled)
Late Prophase I
Metaphase I
Anaphase I
Telophase I (diploid)

18. Meiosis II: Reducing Chromo #
Prophase II
Metaphase II
Telophase II
Anaphase II
4 genetically different haploid cells

19. Interphase I
SAME as MITOSIS
Chromosomes will double
G1, S, G2

20. Prophase I
Homologs pair up and form tetrad (a pair of homologous chromosomes
Chromosomes condense.
Spindle forms.
Nuclear envelope disappears.
Crossing over occurs

21. Tetrads Form in Prophase I
Homologous chromosomes
Pair up (each with sister chromatids)  
Join to form a TETRAD
Called Synapsis

22. Crossing-Over occurs in Prophase I
Tetrad Forms
Definition: Pieces of chromosomes or genes are exchanged
Advantage of sexual reproduction = genetic variation!

23. Crossing-Over
It’s hard to predict what traits you’ll get from mom and dad because there is so many possible combinations!
**THIS IS ONE SOURCE OF GENETIC VARIABILITY!**

24. Genetic Variability is due to…
Crossing over
Independent Assortment
Random Segregation
Random Fertilization
Meaning: You are unique for these 4 reasons!

25. Independent Assortment Random Segregation
The way a pair of chromosomes lines up during metaphase is not dependent on other pairs.
Aka Mom’s chromosomes don’t necessarily all line up on one side
Random Segregation
Random chromatids separate into the newly forming eggs/sperm
Aka if you inherit mom’s hair color, you might get her brown hair trait OR her blonde hair trait

26. Independent Assortment
Random Segregation

27. Metaphase I and Anaphase I
Homologous pairs of chromosomes align along the equator
Anaphase I
-Homologs separate and move to opposite poles.
-Sister chromatids remain
attached at their centromeres.

28. Telophase I and Cytokinesis
Nuclear envelopes reassemble.
Spindle disappears.
Cytokinesis divides cell into two new diploid cells.

29. Meiosis II-occurs in 2 cells
Gene X
Meiosis II produces gametes with
one copy of each chromosome/gene.
Only one homolog of each chromosome is present in the cell.
Sister chromatids carry
identical genetic
information.

30. Meiosis II: Reducing Chromo #
Prophase II
Metaphase II
Telophase II
Anaphase II
4 genetically different haploid cells

31. Metaphase II Chromosomes align along the equator Prophase II
Nuclear envelope disappears
Spindle fibers form
No Crossing Over

32. Anaphase II Sister chromatids separate and move to opposite poles.
Equator
Pole
Sister chromatids separate and move to opposite poles.

33. Telophase II Nuclear envelope reforms.
Chromosomes loosen into chromatin.
Spindle breakdown.
Cytokinesis breaks the cells into 2 new daughter cells

34. Results of Meiosis Gametes (egg & sperm) form
Four haploid cells with one copy of each chromosome
One allele of each gene
Different combinations of alleles for different genes along the chromosome

35. Prophase II
Prophase I
Metaphase II
Metaphase I
Anaphase II
Anaphase I
Telophase II
Telophase I
Cytokinesis
Cytokinesis

36. Meiosis Animation

37. Overview of Meiosis 37

39. THE PROCESS OF MAKING EGG AND SPERM
Oogenesis & Spermatogenesis
THE PROCESS OF MAKING EGG AND SPERM

40. Spermatogenesis “Creation of sperm” Testes 2 divisions produce
4 viable haploid spermatids
Spermatids mature into sperm
Men produce about 250,000,000 sperm per day

41. Spermatogenesis in the Testes
Spermatid

42. Spermatogenesis

43. Oogenesis “Creation of Eggs” Ovary
2 divisions produce 3 polar bodies that die + 1 viable egg
Polar bodies die because of unequal division of cytoplasm
Starting at puberty, if unfertilized, one immature oocyte matures into an ovum (egg) every 28 days  MENSTRUAL CYCLE

44. Oogenesis in the Ovaries
**Egg cells are special…only one egg is made every time meiosis occurs; the other 3 cells (polar bodies) that are made are much smaller & are discarded (Remember: the egg cell is the largest cell you’ll come across)

45. Oogenesis 1st polar body may divide Polar bodies die Mitosis Meiosis I
Oogonium
(diploid)
Mitosis
Primary
oocyte
Meiosis I
Secondary
(haploid)
Meiosis II
1st polar body
may divide
Polar
bodies
die
Oocyte
polar body
(dies) a A X
Mature
Egg (ovum)

46. Meiosis is IMPORTANT… Genetic Variability
1. Independent assortment (late pro/early meta I&II) -chromosomes line up randomly 2. Law of Segregation (Late meta/ana I&II) -spindle fibers attach randomly to chromosomes and separate. 3. Crossing-over (Pro I) pieces of sister chromatids are switched 4. Random fertilization Random sperm + random egg are combined

47. What are the possibilities?
***Total possible chromosome combinations due to independent assortment = 2n [for humans = 223 = 8,388,608] ***Total possible chromosomally different zygotes due to fertilization = (223)2 = 70,368,744,000,000 ***Possible genetically different zygotes per couple if crossing-over occurs only once = (423)2 = 4,951,760,200,000,000,000,000,000,000 --advantageous b/c variability needed for evolution

48. Comparison of Divisions
Mitosis
Meiosis
# of divisions 1 2
Number of daughter cells 4
Genetically identical?
Yes No
Chromosome #
Same as parent
Half of parent
Where
Somatic cells
Gamete cells
When
Throughout life
At sexual maturity
Role
Growth and repair
Sexual reproduction

49. What’s the difference between Mitosis & Meiosis ?92
Single Chromosomes
(46 Pairs)
46 Single Chromosomes
(23 Pairs) 46
23 singles