Dr. Siham Gritly 1
8- How cell dividedcell division 11
Meiosis; Reduction division
Dr. Siham Gritly
University of Bahri
Dr. Siham Gritly 2
Meiosis; Reduction division
That Occurs in Sexually Reproducing Organisms
• Part of the process of gamete formation,
• consisting of chromosome conjugation and two cell
divisions, in the course of which the diploid
chromosome number becomes reduced to the haploid.
• Diploid cells have two homologous copies of each
chromosome.
Dr. Siham Gritly 3
Dr. Siham Gritly 4
• Meiosis is used only for the production of
sperm and eggs
Dr. Siham Gritly 5
Meiosis has two unique features: synapsis and reduction division.
• 1- Synapsis
• * The process of pairing throughout the length
of the homologous chromosomes and
exchanging genetic fragments is called
synapsis.
Dr. Siham Gritly 6
Dr. Siham Gritly 7
• 2-Reduction Division• Reduction division: The first cell division in
meiosis, the process by which germ cells are formed.
• In reduction division, the chromosome number is reduced from diploid (46 chromosomes) to haploid (23 chromosomes)
Dr. Siham Gritly 8
Dr. Siham Gritly 9
Meiosis features
• -yields genetically unique haploid daughter
cells (cells with only one member of each
homologous chromosome pair).
• -involves 2 nuclear and cytoplasmic divisions
• -produces 4 haploid cells
Dr. Siham Gritly 10
The Sexual Life Cycle
1. In the sexual life cycle, there is an alternation of diploid and haploid generations.
2. Normal diploid body cells are called somatic cells.
3. Somatic cells arise from the zygote and are all genetically identical.
Dr. Siham Gritly 11
Germ-Line Tissues
1. Cells that produce gametes are called germ-line tissues.
2. Germ-line cells will undergo meiosis to produce haploid gametes.
Dr. Siham Gritly 12
The stages of meiosis can be broken down into two main stages, Meiosis I and Meiosis II
• A. Meiosis I can be broken down into four substages:
• Prophase I,• Metaphase I, • Anaphase I • and Telophase I• Meiosis I serves to divide the two versions of
each chromosome;
Dr. Siham Gritly 13
• B. Meiosis II can be broken down into four substages:
• Prophase II,• Metaphase II, • Anaphase II• and Telophase II• meiosis II separates the two replicas of each
chromosome.
Dr. Siham Gritly 14
Meiosis I
• The first cell division in Meiosis is known as Meiosis I.
• The function of Meiosis I is very different from that of either Mitosis or Meiosis II.
• In both Mitosis 1 and Meiosis II sister
chromatids are separated during anaphase
to produce identical daughter cells.
Dr. Siham Gritly 15
Dr. Siham Gritly 16
• In Meiosis I members of homologous chromosome pairs are separated.
• This results in the segregation of genes into the two gametes.
Dr. Siham Gritly 17
stages of Meiosis I :
• 1-prophase I occurs as the chromosomes condense, homologues pair, and crossing over occurs;
• Prophase; Main event in prophase of M1 is
“crossing over”, also called
“recombination”.
Dr. Siham Gritly 18
• In crossing over, homologous chromosomes pair up, and exchange segments by breaking and rejoining at identical locations.
• ( Crossing over allows for combinations of genes that may never have existed previously).
Dr. Siham Gritly 19
Crossing over allows for combinations of genes that may never have existed previously
Dr. Siham Gritly 20
Dr. Siham Gritly 21
• 2-metaphase I involves formation of the spindle apparatus and alignment of chromosome pairs along the center of the cell;
Dr. Siham Gritly 22
• 3-anaphase I, The centromeres break and homologous chromosomes separate and pulled toward the poles (note that the sister chromatids are still attached)
• Cytokinesis begins
Dr. Siham Gritly 23
• 4-telophase I, the chromosomes gather at each pole and prepare for the second division.
• cleavage furrow formed• Cytokinesis reaches completion, creating two
haploid daughter cells
Dr. Siham Gritly 24
Meiosis II
• Meiosis II, which also has four stages, follows
after meiosis I,
• This result is the separation of the sister
chromatids to form four haploid gametes.
Dr. Siham Gritly 25
4 stages of meiosis11
• * prophase 11, the chromosomes condense and the spindle forms.
• Centrioles form and move toward the poles• The nuclear membrane dissolves
Dr. Siham Gritly 26
• *Metaphase 11; chromosomes line up singly on the cell equator.
• Microtubules grow from the centrioles and attach to the centromeres
• The sister chromatids line up along the cell equator
Dr. Siham Gritly 27
• *Anaphase 11; centromeres divide and pulled to opposite poles.
• The centromeres break and sister chromatids separate
• Cytokinesis begins
Dr. Siham Gritly 28
• *telophase 11; cytoplasm divided into 2 cells.• The chromosomes may decondense • Cytokinesis reaches completion, creating four
haploid daughter cell
Dr. Siham Gritly 29
Cytokinesis
• the cytoplasmic division of a cell at the end of mitosis or meiosis, bringing about the separation into two daughter cells.
• The division of the cytoplasm and the plasma membrane following the division of the nucleus resulting into two cells, each having its own nucleus and cytoplasm surrounded by a plasma membrane.
• This process begins during the late stage of mitosis (Telophase) and meiosis (Telophase II).
Dr. Siham Gritly 30
Dr. Siham Gritly 31
Dr. Siham Gritly 32