2. Meiosis

3. In diploid cells, there are two chromosomes of each type (2n), called homologous chromosomes
Homologous chromosomes are not identical, but they do contain the same genes (different versions of the same gene are called alleles)

4. For example:
Gene: HAIR COLOR GENE
Alleles: BLONDE, BROWN, RED, BLACK
Everyone has the same gene, but we have different versions (different alleles) of that gene.

5. In humans, the diploid number is 46

6. Meiosis produces gametes that are haploid, meaning they have one of each pair (n) of homologous chromosomes.

7. Haploid or Diploid?
2n=4

8. Haploid or Diploid?
2n=8

9. Haploid or Diploid?
n=1

10. 2n=2
Haploid or Diploid?

11. Haploid or Diploid?
n=2

12. Haploid or Diploid?
2n=4

13. Haploid or Diploid?
n=2

14. Meiosis is called “REDUCTIVE DIVISION” because it begins with diploid germ cells and produces haploid gametes (egg and sperm)
Meiosis II
Haploid (1n)
GERM CELL
Diploid (2n)
Gamete
Haploid
(1n)
Meiosis I
Diploid (2n)

15. In some ways, meiosis resembles mitosis
The chromosomes are duplicated during
chromatids held together at the centromere
interphase to form sister
Chromosomes are moved by the
microtubules to the equator of the cell

16. Unlike mitosis, meiosis has two divisions – meiosis I and meiosis II
During meiosis I, homologous chromosome pairs separate
Each of the two daughter cells receives a haploid number of chromosomes
Each chromosome is still duplicated
In meiosis II, the sister chromatids separate, cytokinesis follows, resulting in 4 haploid cells

17. GERM CELL Diploid (2n) Meiosis II Haploid (1n) Meiosis I Diploid (2n)
Gamete
Haploid
(1n)
Meiosis I
Diploid (2n)

19. Stages of meiosis
Interphase:

20. Stages of meiosis
Interphase: each DNA replicates to produce two genetically identical sister chromatids which are attached at the centromere
This is DNA replication – remember helicase, single stranded binding proteins, DNA polymerase, leading strand, lagging strand, Okazaki fragments, and ligase

22. Meiosis I separates the homologous chromosome pairs
Prophase I:
Chromosomes condense
Homologous chromosome pair to form a tetrad
One member of the pair is from the mom, the other from the dad
A tetrad is the association of four chromatids (two from each homologue)
The homologous chromosomes are attached at chiasmata
Spindle microtubules form and the nuclear envelope disappears

24. Metaphase I Tetrads align at the equator of the cell
Spindle fibers attach to the centromere region of each homologous chromosome pair.

26. Anaphase I: Homologous chromosomes separate and move to the poles
Sister chromatids remain attached at their centromere and move as a single unit towards the same pole
Each pole randomly receives a mixture of maternal and paternal chromosomes

28. Telophase I: chromosomes decondense, nuclear membrane may reform
Cytokinesis occurs, forming two daughter cells (remember cleavage furrow and cell plates)

29. Interkinesis separates meiosis I and II
No new DNA synthesis occurs
It is a very short period, and may be absent in some organisms

31. Meiosis II separates the two sister chromatids of each chromosome
Prophase II:
Chromosomes recondense and spindle fibers reform
Chromosomes progress towards the equator

32. Metaphase II: chromosome line up at the equator

33. Anaphase II: Centromeres of sister chromatids separate
Sister chromatids of each pair, now individual chromosomes, move towards opposite poles of the cell

34. Telophase II: nucleus reforms
Cytokinesis results in four haploid daughter cells

36. The diploid chromosome number is restored at fertilization when two gamete nuclei fuse to form the zygote

38. Determining the Phase Is the cell haploid or diploid?
If diploid …meiosis 1
If haploid … meiosis 2
What are the chromosomes doing?
Prophase
Metaphase
Anaphase
Telophase

39. What phase?
Metaphase 1

40. What phase?
Telophase 1

41. What phase?
Prophase 1

42. What phase?
Metaphase 2

43. What phase?
Anaphase 1

44. What phase?
Anaphase 2

45. What phase?
Anaphase 2

46. What phase?
Prophase 2

47. What phase?
Metaphase 1

48. What phase?
Metaphase 2

49. What phase?
Anaphase 1

50. Make your self a study tool! Pick cards and draw corresponding cell!
Pile 1
Pile 2
Pile 3
Prophase
Metaphase
Anaphase
Telophase
Meiosis I
Meiosis II
2n=2
2n=4
2n=6
2n=8