Download Lecture 6: Cell division

Lecture 6: Cell division
 The continuity of life from one cell to another
is based on ‫ بناءاًًعلى‬the reproduction of cells via
‫ بواسطة‬cell division.
 This division process occurs as part of the cell
cycle (the life of a cell from its origin in the
division of a parent cell until its own division
into two).
 Unicellular ‫ وحيد ًالخلية‬organisms (e.g. Amoeba)
divide to reproduce an entire organism,
increasing
the
population
(asexual
reproduction).
 Multicellular ‫ عديد ًالخلية‬organisms divide to
develop a fertilized cell, grow or repair the
damaged cells.
Fig. 12.1, Page 216
• Cell division distributes the genetic material (DNA) to two
daughter cells.
Division differs among cells:
- Skin cells divide frequently.
- Liver cells divide when needed (repair the damaged cell).
- Nerve cells and muscle cells do not divide at all.
 A cell’s genetic information (genome ‫ )البنكًالـجينى‬is packaged as DNA.
 In prokaryotes, the genome is often a single long DNA molecule.
 In eukaryotes, the genome consists of several DNA molecules.
 A human cell must duplicate about 3m of DNA and separate the two
copies such that each daughter cell ends up with a complete genome.
 DNA molecules are packaged into chromosomes.
 Human gametes ‫( أمشاج‬sperm or eggs) have 23 chromosomes, half the
number in a somatic cell ‫الخليةًالجسدية‬.
 Each eukaryotic chromosome consists of a long, linear DNA
molecule.
 Each chromosome has hundreds or thousands of genes.
 This DNA-protein complex (chromatin) is organized into a long thin
fiber.
 After the DNA duplication, chromatin
condenses to form chromosome.
 Each duplicated chromosome consists
of two sister chromatids which contains
identical copies of the chromosome’s DNA.
 The narrow region where the chromosomal
strands connect is called centromere.
 The process of the formation of the two
daughter nuclei called mitosis.
Types of cell divisions
1- Mitosis ‫االنقسام غير المباشر أو الميتوزي‬
• Mitosis is a continuous process and occurs only in eukaryotic cells in
•
•
•
non-sex cells (somatic cells).
Prokaryotic cells divide by a different process called binary fission.
Mitosis has two divisions which are karyokinesis (division of the
nucleus) is followed by cytokinesis (division of the cytoplasm).
Mitosis occurs in either haploid ‫ أحادي ًالمجموعة ًالكروموسومية‬or diploid
‫ المجموعةًالكروموسومية ثنائي‬cells.
2- Meiosis ‫االنقسام االختزالي أو الميوزي‬
• Meiosis occurs only in germ (sex) cells and is restricted to diploid
•
cells.
Meiosis takes place solely in the testes (males) and ovaries (females)
to produce haploid sex cells or gametes.
5
Chromatid
Chromatin
Protein+ DNA
Sister chromatid
Homologous
Chromosome
Centromere
Chromosome
‫الكروموسومًأوًالصبغ‬
The mitotic cell cycle
• The cell cycle consists of two basic stages:
•
•
•
a) interphase and b) mitotic (M) phase.
The M phase includes Karyokinesis ً‫االنقسامًالنووي‬
and cytokinesis ً‫االنقسامًالسيتوبالزمي‬.
Interphase accounts for 90% of the cell cycle.
During interphase the cell prepares for
division by producing cytoplasmic
organelles and copying its chromosomes.
A.Interphase has three subphases:
1. The G1 phase (first gap): the cell grows.
2. The S phase (DNA synthesis): where the DNA replication occurs
3. The G2 phase (second gap): the cell completes preparations for cell
division.
B. Mitotic phase (M). The cell starts the division process.
• The resulting daughter cells may then repeat the cycle again.
The phases of Mitotic division:
1) Prophase ‫الطورًالتمهيدي‬
2) Metaphase ‫الطورًاإلستوائي‬
3) Anaphase ‫الطور اإلنفصالي‬
4) Telophase ‫الطورًاإلنتهائي‬
• By
late
interphase
(G2),
the
chromosomes have been duplicated
‫ تضاعفت‬but are loosely packed.
• The
centrosomes
have
been
duplicated and begin to organize
microtubules into an aster (star).
1) Prophase:
 The first stage of mitosis begins with the shorting
and thickening of the chromosomes.
 The chromosomes are tightly coiled, with sister
chromatids joined together.
 The nuclear membrane fragments and the
nucleolus disappears.
2) Metaphase:
 The mitotic spindle fibers begins to form and
appears to push the centromere away from each
other towards opposite ends (poles) of the cell.
 Microtubules from one pole attach to one of two
kinetochores (special regions of the centromere)
while microtubules from the other pole attach to
the other kinetochore.
 The chromosomes align along the metaphase
plate.
9
3) Anaphase:
 The
centromeres divide, result in
separating and moving of the two sister
chromatids toward opposite poles on the
spindle fibers .
 By the end, the two poles have equivalent
collections of chromosomes.
4) Telophase:
• Two nuclei begin to form, surrounded by
the nuclear membrane.
• Chromatin becomes less tightly coiled.
 Cytokinesis takes place. In animals,
cytokinesis is first apparent by contraction
between the two poles of the cell. In plants,
a cell plate grows in the approximate location
of the metaphase plate.
Summary of the Cell Cycle
Interphase
Mitotic phase (M)
karyokinesis
G1
S
Cytokinesis
G2
Prophase
Metaphase
Anaphase
Telophase