1. Mitosis, Meiosis and Gametogenesis
Prof. Abdulameer Al-Nuaimi
E. mail:

2. Chromosomes
Chromosomes are structures located in nucleus of the cell. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.
Chromosomes are long thin threads inside the nucleus, they are called chromatin until cell division occurs, then they become visible as rod-like chromosomes.
Chromosomes are composed of genes.
All the genes on chromosomes make up the cell’s genome, Genes contain genetic information vital for proper cell function.
Genes control the physical characteristics of a species.
All organisms of the same species contain the same number of chromosomes in their nuclei.

4. Diploid cells contain 2 sets of chromosomes in their nuclei.
Diploid and Haploid CELLS
Diploid cells contain 2 sets of chromosomes in their nuclei.
The human species contain 46 chromosomes in their nuclei i.e. 23 sets of two copies of autosomal chromosomes, it is a diploid (2n) number.
Haploid cells contain 1 set of chromosomes in their nuclei.
Human sex cells contain 23 chromosomes in their nuclei. This is the haploid (n) number.
When fertilisation takes place the 23 chromosomes (n) from the father (called Paternal chromosomes) and the 23 chromosomes (n) from the mother (called the Maternal chromosomes) combine to form the diploid (2n=46) number of chromosomes in the fertilised egg cell.

6. (two identical parts of a chromosome after S phase)
Scheme of a replicated
Chromosome.
(two identical parts of a
chromosome after S phase)
Chromatid
Centromere
Chromatid: One of the two identical parts of the chromosome after S phase (synthesis phase) .
(2) Centromere: The point where the two chromatids touch, and where the microtubules attach.
(3) Short arm
(4) Long arm. In accordance with the display rules in Cytogenetics, the short arm is on top

7. Meiosis  
Meiosis is the program used by sexually reproducing organisms to produce haploids from diploid precursors
It is a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells,
In Meiosis  DNA replication is followed by two rounds of cell division to produce four potential daughter cells, each with half the number of chromosomes as the original parent cell
Phases of meiosis
1-Premeiotic S-phase (synthesis phase): the DNA of each chromosome is replicated so that it consists of two identical sister chromatids, which remain held together through cohesion.

8. 2-Meiotic prophase: homologous chromosomes can swap different parts of their DNA by “crossing over” to create new genetic combinations. In this stage, DNA is cut and then repaired, which allow homologous chromosomes to exchange some of their genetic information
3-Meiosis I:  Each pair of homologous chromosomes segregate away from other pair
4-Meiosis II: the cohesion between sister chromatids is released and they segregate from one another

9. Meiosis (1) (2) (3) premeiotic S-phase meiotic prophase (4)Meiosis II

10. Gametogenesis
Gametogenesis is a process of formation and development of male(sperm) and female (ovum)gametes.
During this process a reduction of the the number of chromosomes takes place from diploid number (46) of primary germ cells to the haploid number (23) in the mature germ cell.
Copyright © 2006/07 The University of Sharjah

11. Copyright © 2006/07 The University of Sharjah
Gametogenesis
Male
Mature sperm
The large rounded primary male germ cells, lose most of their cytoplasm and develop head, tail and neck to assist in the movement of the sperms.
The rounded female germ cells become larger as a result of increase in the a mount of their cytoplasm and called ovum.
Mature oocyte
Copyright © 2006/07 The University of Sharjah

12. Introduction
Mature Male Genital Organ

13. Cross section in the Testis
Seminiferous tubules get canalised at puberty
Cross section in the Testis

14. Testis Testis and Sperm Cross section in the seminiferous tubule Sperm

15. Spermatogenesis
is the process in which spermatozoa are produced from spermatogonial stem cells by way of mitosis and meiosis
The initial cells in this pathway are called spermatogonia, it divides mitotically into two primary spermatocytes . The primary spermatocyte divides meiotically (Meiosis I) into two secondary spermatocytes; each secondary spermatocyte divides into two spermatids by Meiosis II. Spermatids develop into mature spermatozoa, also known as sperm cells.
Thus, the primary spermatocyte gives rise to two secondary spermatocytes.
Secondary spermatocytes by their subdivision produce four spermatozoa

16. Seminiferous
Tubule in the
Testis

18. Introduction
(Uterine tube)
Mature Female Genital Organ

19. During Embryonic life ovaries appear as a pair of longitudinal Genital ridges on each side of the midline.
Primordial (primitive) Germ Cells originate in the Epiblast migrate to the genital ridges, invade them and producing the ovaries
If Germ cells fail to reach the Ridge, the gonads do not develop
Inside these ovaries, clusters of cells begin to surround each Germ cell, Oogonium (oocyte) with a layer of epithelial cells called Follicular cells Producing ovarian follicles.
Between 16 and 20 weeks of pregnancy, the ovaries of a female fetus contain 6 to 7 million oocytes.  Most of the oocytes gradually degenerate, leaving about 1 to 2 million present at birth. 

20. The many thousand of oocytes that do not mature degenerate.
At puberty, only about 300,000 are present and only a small percentage of oocytes mature into eggs (ova). 
The many thousand of oocytes that do not mature degenerate. 
Degeneration progresses more rapidly in the 10 to 15 years before menopause. 
All oocytes are gone by menopause.
Only about 400 ova are released during a women's reproductive life, usually one during each menstrual cycle. 
Until released, an ovum remains dormant in its follicle. Thus, the ovum is one of the longest lived cells in the body

21. Migration of Primordial germ cells to the Gonadal ridge
Allantois
(Langman’s Medical Embryology)

22. Ovary primitive Mesonephric duct = Follicular cells + Germ cell
(Langman’s Medical Embryology)

23. Primary Oocyte (diploid)-(Meiosis I)
Oogenesis
Oogonium divides mitotically giving rise to Primary Oocyte this develops into primary follicle, the functional unit of the ovary.
Primary Oocyte (diploid)-(Meiosis I)
First Polar Body (Discarded afterward) + Secondary oocyte
  (haploid)
Secondary oocyte—(Meiosis II)—>
Second Polar Body (Discarded afterward) + Ovum (haploid)
Oogonium
Mitosis

24. Copyright © 2006/07 The University of Sharjah
Gametogenesis
Female
Mature sperm
The large rounded primary male germ cells, lose most of their cytoplasm and develop head, tail and neck to assist in the movement of the sperms.
The rounded female germ cells become larger as a result of increase in the a mount of their cytoplasm and called ovum.
Mature oocyte
Copyright © 2006/07 The University of Sharjah

25. Mitosis
Meiosis I
Meiosis II

26. Fertilization
The fertilization occurs when sperm contacts an ovum to form a zygote in the uterine tube.
The zygote, which contains 46 chromosomes, rapidly divide by mitosis to form mass of cells as 2,4, 8, 16, 32….
This mass of cells continue dividing in order to give the different tissues then organs of the human body.
Copyright © 2006/07 The University of Sharjah

27. Sun Set Malaysia
16 Feb. 2015
Thank You