1. LESSON # 19
MEIOSIS

2. TWO TYPES OF CELL DIVISION
1- MITOSIS
The number of chromosomes in the daughter cells is identical to that of the mother cells. It takes place in most cells (somatic cells)
2- MEIOSIS
The number of chromosomes in the daughter cells is the half of that of the mother cells. It takes place only in sex cells (gametes)

3. Reproductive Cells or Gametes are Haploid Cells
Somatic Cells
are Diploid Cells
Reproductive Cells or Gametes
Sperm
Egg
are Haploid Cells

4. DIPLOID: Two sets of chromosomes.
Somatic human cells are DIPLOID cells because they have 23 pairs of chromosomes ( 46 chromosomes ).
(23)
DIPLOID: Two sets of chromosomes.
Diploid cells: 2n cells. In this case, 2(23) cells.
Chromosomes in every pair have the same structure and function and are called homologous chromosomes.
(23)
Reproductive human cells are HAPLOID cells because they have 23 chromosomes.
HAPLOID: One set of chromosomes
Haploid cells: 1n cells. In this case, 1(23) cells.

5. (23)
(23)
MITOSIS
Haploid
Haploid
During MITOSIS, the number of chromosomes does NOT change. Therefore, haploid cells will produce haploid cells.
Haploid

6. MITOSIS
Diploid
Diploid
During MITOSIS, the number of chromosomes does NOT change. Therefore, diploid cells will produce diploid cells.
Diploid

7. Diploid (2n cell) MEIOSIS I
During MEIOSIS I, a single diploid cell divides to produce two haploid repro-ductive cells. The number of chromo-somes is reduced to the half. Sister chromatids remain together.
Gametogonium
They are the cells that produce the gametes (oocyte and sperm).
(2n cell)
Diploid
(23)
MEIOSIS I
Gamete (Haploid)
(23)

8. During MEIOSIS II, each haploid gamete produces two haploid gametes
During MEIOSIS II, each haploid gamete produces two haploid gametes. Sister chromatids separate like in mitosis.
(23)
(23)
MEIOSIS II
Haploid
Haploid
During MEIOSIS II, the number of chromosomes does NOT change. Therefore, haploid cells will produce haploid cells.
Haploid

9. + Diploid Haploids (2n cell) Fertilization
Sperm
Egg
Haploids +
Fertilization
1n cell
1n cell
During FERTILIZATION, two haploid gametes combine to produce a diploid embryo.
(2n cell)
Diploid

10. + + Somatic Cells undergo MITOSIS and cell division.
Gametogonia: They are cells, which undergo meiosis to produce haploid reproductive cells or gametes.
Reproductive Cells or Gametes: They are specialized cells, which come together to produce offspring.
Reproduction: It is the process by which the reproductive cells come together to produce offspring.
Haploid
Diploid +
chromosomes 46 + 23
chromosomes
chromosomes

11. The cells that give rise to sperm and egg are diploid cells.
When the cells that give rise to sperm and egg cells divide, the result is sperm or egg cells that have only half of the usual somatic number of chromosomes.
Diploid (2n)
Spermatogonium
Oogonium
Haploid (1n)

12. Sex Human Cells (haploid) Somatic Human Cells (diploid)
Sperma-
togonia
Meiosis
Haploid
Haploid + Haploid
(Diploid)
Meiosis
Haploid
Oogonia
(Diploid)
Diploid zygote
Diploid
Somatic Human Cells (diploid)
Mitosis

13. Meiosis
It is the process by which a single diploid cell divides to produce haploid reproductive cells. 1n 2n
Homologous means same in size and function.

14. The Steps in Meiosis

15. Homologous chromosomes
Meiosis
Homologous chromosomes 2n 1n
One diploid cell gives rise two haploid cells.
MEIOSIS-I
MEIOSIS-II 1n
Each haploid cell gives rise two haploid cells.

16. Paternal chromosome
Maternal chromosome
DNA I
Metaphase
Metaphase

17. Homologous chromosomes2n
MITOSIS 2n
MEIOSIS I
Homologous chromosomes
Homologous chromosomes 2n 1n

18. 2n
MEIOSIS-I 1n
MEIOSIS-II 1n

19. MEIOSIS I
Prophase I
1- Homologous chromosomes links as they condense forming tetrads.
2- Crossing over occurs.
Maternal chromosomes
Paternal chromosomes 2n 2n 2n

20. Crossing over is the first important source of genetic variation.
MEIOSIS I
It is the process by which non-sister chromatids exchange genetic material.
CROSSING OVER:
Crossing over is the first important source of genetic variation.

21. Metaphase I or 1- Homologous chromosomes align at metaphase plate.
MEIOSIS I
Metaphase I
1- Homologous chromosomes align at metaphase plate.
2- Independent assortment occurs.
Prophase I or

22. Independent Assortment:
MEIOSIS I
It is the random distribution of homologous chromosome pairs during the metaphase of meiosis I.
Independent Assortment:
Green eyes & blond hair
Black eyes & black hair
Green eyes & black hair
Independent Assortment is the second important source of genetic variation.
Black eyes & blond hair

23. SOURCES OF GENETIC VARIATION
MEIOSIS I
SOURCES OF GENETIC VARIATION
Crossing over is the first important source of genetic variation.
Independent assortment is the second important source of genetic variation.

24. MEIOSIS I
Anaphase I
Microtubules separate the homologous chromosomes (sister chromatids remain together). 1n 2n

25. MEIOSIS I
Telophase I
Two haploid cells result from Cytokinesis 2n 1n

26. MEIOSIS I
GENETIC VARIATION

27. MEIOSIS I Prophase I Metaphase I Anaphase I Telophase I
1- Homologous chromosomes links as they condense forming tetrads.
2- Crossing over occurs.
Prophase I
Crossing over is the process by which non-sister chromatids exchange genetic material Crossing over is the first important source of genetic variation.
Metaphase I
1- Homologous chromosomes align at metaphase plate.
2- Independent assortment occurs. Independent assortment is the second important source of genetic variation.
Anaphase I
1- Microtubules separate the homologous chromosomes (sister chromatids remain together).
Telophase I
1-Two haploid cells result from Cytokinesis.

29. Meiosis and Sex Outcome
Diploid
DNA replication
Diploid
Haploid
Haploid

30. The X chromosome has about 1500 genes.
The Y chromosome has about 50 genes.

31. Meiosis and Sex OutcomeXX
GAMETOGONIA XY
MOTHER
FATHER
Diploid X
MEIOSIS I X Y
Haploid
MEIOSIS II X X Y X X X Y
FERTILIZATION XX XY
Female
Diploid
Male