1. Meiosis
Sex Cell Formation

2. Mitosis Versus Meiosis
We have learned that MITOSIS is the DIVISION of SOMATIC cells (Body cells)
Purpose is for organism GROWTH
Retain the same number of chromosomes (2n) through out all divisions
In order to produce GAMETES (sex cells), we must go through a different process MEIOSIS
Sometimes thought of as REDUCTION division
Why?
MITOSIS- produces two IDENTICAL daughter cells, each also identical to the original parent cell

3. Homologous Chromosomes
We inherit half of our DNA from each parent
The combination of our father’s and mother’s genes give us our own unique set of characteristics
Somatic cells contain genetic material that is half from mom and half from dad (2n)
HOMOLOGOUS chromosomes are two sets of chromosomes that control the same genes
One homolog from mom, one from dad

4. Meiosis and Chromosome Number
The number of chromosomes in any given organism’s cells is defined by the chromosome number “N”
Diploid number (2n): cells that contain both “sets” of homologous chromosomes
Aka: ALL somatic body cells
Haploid number (n): cells that contain 1 “set” of inherited DNA
AKA: ALL Sex cells (gametes)

5. Karyotype: display of chromosomes
This is a HUMAN karyotype.
There are 23 pairs of DNA
Each pair consists of one version from mom, and one version from dad (total=46 chromosomes)

6. Meiosis Sex Chromosomes – the 23rd chromosome, determines gender
Two Forms:
X – Females have XX
Y – Males have XY

7. Meiosis- reduction of chromosome number
MEIOSIS is the formation of gametes by reducing a diploid cell (2n) into FOUR genetically different haploid cells (n)
*Remember- mitosis divided one cell into 2 identical cells… this this is quite different
In males: formation of haploid sperm cells
In females: formation of haploid egg cells

8. Meiosis- 2 sets of division
Meiosis I: separation of homologous chromosomes
Each homolog contains an
attached copy (sister
chromatids)
Meiosis II: separation of sister chromatids
This stage looks identical to Mitosis

9. Meiosis I Prophase I Tetrads – the paired chromosomes Now 4 chromatids
Protein cause homologous chromosomes to stick together along with their length
Tetrads – the paired chromosomes
Now 4 chromatids
Chromosome #: 2n
1 set from mom
1 set from dad
1(mom)+1(dad)= 2n

10. Meiosis I – Prophase I
Crossing Over – 2nd “new” step; tetrads exchange genetic material
This process introduces unique, new traits.
This is one way, other than mutations, organisms can acquire NEW traits

11. Meiosis I
Metaphase I
Tetrads move to the middle of the cell and line up across the spindle.
Notice now chromosomes line up NEXT to their homologous pair (2 lanes)

12. Meiosis I
Anaphase I
Homologous chromosomes separate and migrate to opposite poles
Sister chromatids migrate together
Each chromosome is made up of two copies

13. Meiosis I Telophase I The chromosomes arrive at poles
Each pole has a haploid daughter nucleus because it only has one set of chromosomes

14. Meiosis I Cytokinesis Form two daughter cells
Chromosomes in each daughter cell are still duplicated (double in number)
Chromosome #: n
Because mom’s set and dads set were separated, now you have haploid number

15. Meiosis II Prophase II In each haploid daughter cell, spindle forms
Nuclear envelope disappears

16. Meiosis II Metaphase II Chromosomes line-up in the middle of cell
Spindle attaches to centromeres

17. Meiosis II
Anaphase II
Sister chromatids separate and move to opposite poles

18. Meiosis II Telophase II & Cytokinesis Chromatids arrive at poles
Now individual chromosomes
Nuclear envelope reforms
Cytokinesis splits cells

19. Produced four DIFFERENT haploid daughter cells
Meiosis Finished
Produced four DIFFERENT haploid daughter cells

20. Genetic Variation
How chromosomes line-up and separate at is a matter of chance
So the chromosomes that end up in the resulting cells occur randomly
Four combinations possible

21. Closure- Did you get it??
What are two sets of chromosomes that control the same genes called?
Homologous chromosomes
What are homologous chromosomes doing when they are “crossing over”?
Exchanging genetic material
What is the chromosome number at the beginning of meiosis I? at the END of meiosis I? At the end of Meiosis II?
2n (diploid), n (haploid), n (haploid)

22. Bellringer- Name that phase
Name each phase that is either being described or the picture depicts Ready??

23. Metaphase I

24. Prophase I
Homologous chromosomes exchange genetic material by crossing over and tetrads form

25. What is separating here???
Anaphase II
What is separating here???
Sister Chromatids

26. Telophase I
Two nuclei form, each containing a haploid set of replicated chromosomes

27. Homologous pairs separate and migrate to opposite ends of the cell
Anaphase I
Homologous pairs separate and migrate to opposite ends of the cell

28. Comparing mitosis and Meiosis

29. Mitosis vs Meiosis: Cell division processes
Occurs in all growing tissue of organisms
Purpose is for organism GROWTH
Division of BODY cells (Somatic cells)
Examples: hair, blood, muscle, skin, etc.
Occurs in the Reproductive Organs of organisms
Purpose is to create GAMETES (sex cells) for sexual reproduction
Examples:
Egg cells
Sperm cells

30. Prophase vs Prophase I Mitosis Meiosis Spindle forms
Nuclear membrane breaks down
Chromosomes condense
Chromosome #: 2n (diploid)
Spindle forms
Nuclear membrane breaks down
Homologous chromosomes cross over and form tetrads
Chromosome #: 2n (diploid)

31. Metaphase vs Metaphase I
Mitosis
Meiosis
Chromosomes line up single file
Spindle attaches to centromeres
Chromosome #: 2n (diploid)
Homologous pairs line up side-by-side
Spindle attach to centromeres
Chromosome #: 2n (diploid)

32. Anaphase vs. Anaphase I Mitosis Meiosis
Sister Chromatids are pulled apart
Cell elongates
Chromosome #: 2n (diploid)
Homologous pairs are pulled apart
Cell elongates
Chromosome #: 2n (diploid)

33. Telophase vs Telophase I (And Cytokinesis)
Mitosis
Meiosis
Cytoplasm divides
Nucleus forms
Two genetically IDENTICAL daughter cells form
Chromosome #: 2n (diploid)
Cytoplasm divides
Nucleus forms
2 different daughter cells form
Nuclei is still REPLICATED
Chromosome #: n (haploid)

34. Meiosis II
Mitosis
Meiosis
DONE
Meiosis II begins

35. Prophase II
Nuclear Envelope breaks down
Spindle Forms

36. Metaphase II Sister chromatids Line up single file
Spindle attaches to centromeres

37. Anaphase II
Sister Chromatids Separate
Cell Elongates

38. Telophase II Nuclear Membrane reforms Spindle disappears
Cytoplasm begins to divide via cytokinesis

39. Notice how none are the same
END of Meiosis
Cytokinesis divides both cells
Now, 4 genetically different daughter cells
All reduced to haploid (n # of chromosomes)
Notice how none are the same

40. We Do Activity Visualizing meiosis
Take out a piece of paper and number it 1-8
Take out 3 sheets of blank paper
Using scrap paper, cut 8 small strips of paper
Color 4 strips one color
Color 4 another color
Follow Ms. Hamadeh’s directions
We Do Activity
With your table partner, take out 2 different colored highlighters

41. Human chromosomes
There are 46 chromosomes (23 homologous pairs) in each somatic cell
22 pairs of autosomes
1 pair of sex chromosomes
XX = Female, XY = Male
Karyotype - chromosomes are arranged according to shape and size

42. Nondisjunction and chromosomal disorders
Nondisjunction – failure of chromosomes to separate and segregate into daughter cells
Nondisjunction may occur during meiosis 1 or meiosis 2
Abnormal number of chromosomes may result

43. Results of crossing-over not shown
Normal meiosis
MEIOSIS I
Replicate DNA
Results of crossing-over not shown
MEIOSIS II
Normal monosomic gametes

44. Nondisjunction during meiosis I
Replicate DNA
Non-disjunction
MEIOSIS II
Disomic gametes
Nullisomic gametes

45. Nondisjunction during meiosis II
Replicate DNA
MEIOSIS II
Non-disjunction
Disomic
Nullisomic
Monosomic
Monosomic
gametes

46. EXAMPLES OF NON-DISJUNCTION

47. AN EXTRA COPY OF CHROMOSOME 21 CAUSES DOWN SYNDROME
This condition is called trisomy 21.
Person with this condition suffers from
the Down syndrome.
Characteristic facial features:
Round face
Flattened nose bridge
Small, irregular teeth
Short stature
Heart defects

48. Klinefelter's syndrome, 47, XXY
It is the most common sex chromosome disorder and the second most common condition caused by the presence of extra chromosomes
Symptoms:
Language impairment
Lanky, youthful build or rounded body type
Low levels of Testosterone and small testicles / Infertile

49. Turner’s Syndrome (X) Common symptoms: Short stature
swelling of the hands and feet
Broad chest and widely spaced nipples
Low hairline
Low-set ears
Reproductive sterility
Increased weight, obesity
Small fingernails
Characteristic facial features
Webbed neck

50. What genetic disorder is this?
They may have never known…

51. Down Syndrome
47, XY, +21

52. Down Syndrome 1 in 1,250 births 47 chromosomes XY or XX
#21 Trisomy Nondisjunction