1. Meiosis
A type of cell division that results in gametes(sperm and egg) being created with half the number of chromosomes(haploid-n) as the parent.

2. Purpose of Meiosis
genetic continuity
Genetic variability

3. Where does meiosis occur?
It occurs in the gonads( testes and Ovaries)

4. Two Types of Meiosis
Oogenesis
spermatogenesis

5. Meiosis

6. Meiosis
Haploid cells
Diploid cells
Somatic cells
Gametes
Role of Meiosis
To produce haploid gametes from diploid germ cells.
Occurs only in the gonads
Plays an important role in creating genetic variability through the processes of:
Segregation
Independent assortment
Crossing over

8. Meiosis Interphase S phase—replication
Cell is now diploid with all chromosomes being mitotic
Meiosis I—reductive division
Creates 2 haploid cells with mitotic chromsomes
Consists of several stages
Prophase I—Like prophase in mitosis.
Synapsis—homologous chromosomes attach to form tetrads.
Crossing over occurs.
Reshuffling of genes. Genetic recombination. Important for creating variability.

10. The stages of meiosis Meiosis 1 Meiosis 2 Four stages Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
Meiosis 2
Four stages
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2

11. Stages of meiosis 1 Meiosis one is called the reduction division
The process of reducing the number of chromosomes in a cell by half (1/2).In other words, going from diploid (2n) to haploid(n)
Ex. 46 chromosomes to 23 chromosomes

12. Prophase 1
During prophase 1 of meiosis, homologous chromosomes(similar chromosomes) line up next to each other as pairs
Pairs of homologous chromosomes are called Tetrads.
Crossing over of non-sister chromatids occurs.
Crossing over : the overlapping of non-sister chromatids resulting in an exchange of genes.

13. Prophase 1

18. Metaphase 1
During this phase homologous chromosomes line up at the middle of the cell.
tetrads are aligned in such a way that the homologous chromosomes are situated across from each other
The spindle from one end of the cell attaches to one pair of sister chromatids while a spindle fiber from the other end attaches to the other pair of sister chromatids.
Independent Assortment—maternal and paternal chromosomes are line up on the opposite sides of the metaphase plate in a random fashion. When these homologues split during anaphase I. The new nuclei consist of a mixture of maternal and paternal

19. Metaphase 1

21. Anaphase 1
Spindle fibers pull homologous chromosomes to separate ends of the cell

22. Anaphase 1

23. Telophase 1
Nuclear membrane reappears around each set of separated chromosomes
Spindle disappears
Cytokinesis (division of the cytoplasm) occurs
Two new cells are created with ½ (haploid) the chromosomes number of the original cell
46 DS chromosomes – 23 DS chromosomes per cell or 96 chromatids- 46 chromatids per cell

24. Telophase 1

25. Meiosis II Prophase II Metaphase II Anaphase II Telophase II
Almost the same as Mitosis.
2 cells are formed from each of the cells from Meiosis I.
These cells are haploid and have chromosomes formed by only one chromatid.
These cells develop into gametes
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis
Results in 4 haploid cells that will eventually form gametes

26. Stages of Meiosis II
Stages of meiosis 2 are identical to the stages of Mitosis
Prophase II
MetaphaseII
Anaphase II
Telophase II

27. Prophase II Nuclear membrane disappears Spindle fibers form
Chromosomes become visible

28. Prophase II

29. Metaphase II Chromosomes line up at the middle of the cell.
Spindle attaches to centromere of each chromosome

30. Metaphase II

31. Anaphase II
Spindle pulls one chromatid from each chromosome to opposite ends of cell

32. Anaphase II

33. Telophase II
Nuclear membrane reappears around each group of chromatids (now called SS chromosomes)
Spindle disappears
Cytokinesis occurs
two new cells are created each having the haploid number of chromosomes

34. Telophase

39. Gametogenesis In males: spermatogenesis
1 diploid germ cell goes through meiosis to form 4 haploid cells. These develop in the testes to form sperm.
In females:
Oogenesis
1 diploid germ cell split unequally into 4 haploid cells.
The large cell develops into an egg.
The small cells are know as polar bodies and usually die.
Why does this unequal division occur?

40. Oogenesis This is the meiosis that occurs in the ovaries of females
Resulr in the creation of An egg with ½ the normal number of chromosomes

41. Explanation of Oogensis
Within the ovaries of females, a diploid 2n cell called an oogonium undergoes meiosis
Before meiosis begins the oogonium replicates(doubles) its chromosomes so that it has 46 DS chromosomes or 92 sister chromatids
During the first meiotic division the Homologous (similar chromosomes are separated. During this stage cytokinesis occurs unevenly and as a result one large and one samll cell are created. The large cell is called the primary Oocyte and the small one is a polar body.
Each of these chromosomes will have 23 chromosomes or 46 SS chromatids

42. During the second metiotic division the cells divide again creating 4 cells, but this time the new cells created have 23 SS chromosomes, Again cytokinesis has resulted in an unequal distribution of the cytoplasm
Note: The three polar bodies will die because they do not have enough cytoplasm to keep them running

44. Spermatogensis This is meiosis that occurs in the testes of males
Results in the creation of 4 sperm each with ½(haploid) the number of chromosomes as the original cell

45. Explanation of Spermatogenesis
Within the testicles of a male, a spermatogonium replicates its chromosomes to contain 46 Ds chromosomes or 92 chromatids.
During the first meiotic division, the cell divides and the homologous chromosomes separate. This results in 2 new cells each with 23 DS chromosomes or 46 chromatids each
During the second division two new cells are created, each with 23 SS chromosomes. These cells are called Primary spermatids will mature into sperm
Each spermatid matures into sperm in the Epididymis

47. Definitions Diploid Haploid/ monoploid Tetrad Synapsis Crossing Over
Nondisjunction
Reduction Division
Spermatids
Primary body
Oognium

48. End Result of Meiosis 4 new cells are created
Each cell has ½ or haploid (n) the number of chromosomes as the parent /original cell
Crossing over has resulted in a new combination of genes on chromosomes leading to variety in orgasnisms

49. Meiosis / Mitosis Compared
Event
Mitosis
Meiosis
Where does it occur?
Somatic cells
Gonads
Number of cells created per complete cycle. 2 4
Number of divisions 1
Number of Chromosomes per new cell created
Same as parent
½ parent cell
Does crossing over occur? No
Yes possibly
Do homologous chromosomes line up?
Yes
Purpose
Genetic continuity, growth
Genetic variability