1. Meiosis
Dr Ahmad O Babalghith
Dr Ahmad Babalghith

2. Meiosis
Meiosis is the process of nuclear division which occurs during the final stage of gamete formation.
Meiosis is consist of two cell divisions
Meiosis I (reduction phase)
Meiosis II
Dr Ahmad Babalghith

3. Dr Ahmad Babalghith

4. Dr Ahmad Babalghith

5. Meiosis I
This is sometimes referred to as the reduction division because it is during the first meiotic division that the chromosome number is reduced from 46 to 23.
Meiosis I consist of four stages: prophase I, metaphase I, anaphase I, and telophase I.
Dr Ahmad Babalghith

6. Meiosis I: Prophase I Leptotene Zygotene Pachytene Diplotene
The prophase stage of meiosis I is relatively long and can be subdivided into five stages.
Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis
Dr Ahmad Babalghith

7. Dr Ahmad Babalghith

8. Prophase I: Leptotene
The chromosomes become visible as they start to condense.
Dr Ahmad Babalghith

9. Prophase I: Zygotene
Homologous chromosomes align directly opposite each other and are held together at several points along their length forming bivalents (is a pair of associated homologous chromosomes held together by a complex after chromosome replication).
Dr Ahmad Babalghith

10. Prophase I: Pachytene
Each pair of homologous chromosomes becomes tightly coiled.
Chiasmata begin to form. These are points at which non- homologous chromatids become associated with each other via base pairing. These become points of cross- over between the chromatids.
Dr Ahmad Babalghith

11. Prophase I: Diplotene
The homologous recombinant chromosomes now begin to separate but remain attached at the points where crossing-over has occurred.
Exchange of genetic material in chiasmata (site for crossing over).
Dr Ahmad Babalghith

12. Dr Ahmad Babalghith

13. Prophase I: Diakinesis
Separation of the homologous chromosome pairs proceeds as the chromosomes become maximally condensed.
Dr Ahmad Babalghith

14. Metaphase I
The nuclear membrane disappears and the chromosomes become aligned on the equatorial plane of the cell where they have become attached to the spindle as in metaphase of mitosis.
Dr Ahmad Babalghith

15. Anaphase I
The chromosomes now separate to opposite poles of the cell as the spindle contracts.
Dr Ahmad Babalghith

16. Telophase I
Each set of haploid chromosomes has now separated completely to opposite ends of the cell which cleaves into two new daughter gametes, so-called oocytes.
Dr Ahmad Babalghith

17. Meiosis II
This is essentially similar to an ordinary mitotic division.
Each chromosome, which exists as a pair of chromatids, becomes aligned along the center of the cell and then splits leading to the formation of two new daughter gametes, known as spermatids or ova.
Dr Ahmad Babalghith

18. Dr Ahmad Babalghith

19. Dr Ahmad Babalghith

20. Mitosis vs. Meiosis
Meiosis differs from mitosis in three fundamental ways:
1- Mitosis results in each daughter cell having a diploid chromosome complement (46). Where as in meiosis the mature gamete have a haploid complement of 23 chromosomes.
Dr Ahmad Babalghith

21. Mitosis vs. Meiosis
2- Mitosis takes place in somatic cells and during the early cell divisions in gamete formation. Meiosis occurs only at the final division of gamete maturation. 3- Mitosis occurs as a single one-step process. Meiosis can be considered as two cell divisions known as meiosis I and meiosis II, each of which can be considered as having prophase, metaphase, anaphase and telophase stages as in mitosis.
Dr Ahmad Babalghith

22. Mitosis vs. Meiosis Mitosis Meiosis Site Somatic cell Germ cell
Cell division
Single division
Two division
Chromosomes 46 23
Dr Ahmad Babalghith

23. Genetic diversity and gametogenesis
Two process in meiosis are vital in the generation of genetic diversity:
Chiasmata formation (crossing-over), which allows random exchange of genetic material between homologous chromosomes.
Independent segregation of homologous chromosomes during Anaphase I. Since human posses 23 pairs of homologous chromosomes, there are 223 (= ) different possible combinations that the chromosomes can segregates to form a haploid set.
Dr Ahmad Babalghith

24. Abnormalities of Meiosis
There are different types of abnormality which can occur and cause disease phenotype. These can be divided into:
Numerical; monosomic, disomic and nullsomic gametes
Structural
Different chromosome constitutions in two or more cell lines.
Dr Ahmad Babalghith

25. Dr Ahmad Babalghith