Download Lecture 6: Cell division

Lecture 6: Cell division
 The continuity of life from one cell to another
is based on ‫ بناءاً على‬the 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 (damage repair).
- Nerve cells and muscle cells do not divide at all.
 Cell division distributes identical sets of chromosomes to daughter
cells.
 A cell’s genetic information (genome ‫ )البنك الـجينى‬is 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 3 m 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 ‫الخلية الجسدية‬.
 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 nonsex 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 ‫ثنائي‬
‫ المجموعة الكروموسومية‬cells.
2- Meiosis ‫االنقسام االختزالي أو الميوزي‬:
•
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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.
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Chromatid
Chromatin
+ 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.
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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