IntroductionIntroductionThe continuity of life from one cell to The continuity of life from one cell to

another is based on the reproduction another is based on the reproduction of cells of cells viavia cell divisioncell division..

This division process occurs as part of This division process occurs as part of the the cell cyclecell cycle ( (the life of a cell from its the life of a cell from its origin in the division of a parent cell until origin in the division of a parent cell until its own division into twoits own division into two).).

The division of a unicellular The division of a unicellular وحيد الخليةوحيد الخلية organism (organism (e.g. Amoebae.g. Amoeba) reproduces an ) reproduces an entire organism, increasing the entire organism, increasing the population.population.

Cell division is also central to the Cell division is also central to the development of a multicellular development of a multicellular عديد الخليةعديد الخلية organism that begins as a organism that begins as a fertilized fertilized egg or zygoteegg or zygote..

Fig. 12.1, Page 216

Lecture 6: Cell division Lecture 6: Cell division

Division is differ among cells:.Division is differ among cells:.- Skin cells divide frequently.Skin cells divide frequently.

- Liver cells divide when needed (damage repair).Liver cells divide when needed (damage repair).

- Nerve cells and muscle cells do not divide at all.Nerve cells and muscle cells do not divide at all.

Cell division distributes the genetic material (DNA) to two Cell division distributes the genetic material (DNA) to two daughter cells.daughter cells.

Cell division distributes identical sets of chromosomes to daughter cellsCell division distributes identical sets of chromosomes to daughter cells

A cell’s genetic information (A cell’s genetic information (genome genome ينىچالبنك الـ ) is packaged ) is packaged as DNA.as DNA.

In prokaryotes, the genome is often a single long DNA In prokaryotes, the genome is often a single long DNA molecule.molecule. In eukaryotes, the genome consists of several DNA moleculesIn eukaryotes, the genome consists of several DNA molecules..

A human cell must duplicate about 3 m of DNA and A human cell must duplicate about 3 m of DNA and separate the two copies such that each daughter cell ends separate the two copies such that each daughter cell ends up with a complete genome.up with a complete genome.

DNA molecules are packaged into chromosomes.DNA molecules are packaged into chromosomes. Every eukaryotic species has a characteristic number of chromosomes Every eukaryotic species has a characteristic number of chromosomes

in the nucleus.in the nucleus. Human somatic cells (Human somatic cells (body cellsbody cells) have ) have

46 chromosomes46 chromosomes.. Human gametes Human gametes أمشاج (sperm or eggs) have (sperm or eggs) have

23 chromosomes23 chromosomes, half the number in , half the number in a somatic cell a somatic cell الخلية الجسدية..

Each eukaryotic chromosome consists Each eukaryotic chromosome consists of a long, linear DNA molecule. of a long, linear DNA molecule.

Each chromosome has hundreds or thousands of genes (Each chromosome has hundreds or thousands of genes (the the units that specify an organism’s inherited characters units that specify an organism’s inherited characters الصفات الصفات.(.(الوراثيةالوراثية

This DNA-protein complex (This DNA-protein complex (chromatinchromatin) is organized into a long ) is organized into a long thin fiber.thin fiber.

After the DNA duplication, chromatin After the DNA duplication, chromatin condensescondenses form (form (chromosomechromosome). ). • Each duplicated chromosome consists Each duplicated chromosome consists

of two sister of two sister chromatidschromatids which contain which contain identical copies of the chromosome’s identical copies of the chromosome’s DNA.DNA.

• The narrow region where the The narrow region where the chromosomal strands connect is the chromosomal strands connect is the called called centromerecentromere..

• Later, the sister chromatids are pulled Later, the sister chromatids are pulled apart and repackaged into two new apart and repackaged into two new nuclei at opposite ends of the parent nuclei at opposite ends of the parent cell during cell division.cell during cell division.

• The process of the formation of the two The process of the formation of the two daughter nuclei called (daughter nuclei called (mitosismitosis) and is ) and is usually followed by division of the usually followed by division of the cytoplasm (cytoplasm (cytokinesis cytokinesis الخلوىالخلوى اإلنشطاراإلنشطار ).). It occurs in somatic cells It occurs in somatic cells الجسدية الجسدية الخاليا الخاليا

ChromatidChromatidChromatin Chromatin

+ DNA+ DNA

Sister chromatidSister chromatid

Chromosome Chromosome الصبغ

CentromereCentromere

Homologous Homologous ChromosomeChromosome

The Mitotic Cell Cycle (division)The Mitotic Cell Cycle (division)• The mitotic (The mitotic (MM) phase of the cell cycle ) phase of the cell cycle alternates alternates تتبادلتتبادل with the much longerwith the much longer interphase interphase البينية البينية المرحلة ..المرحلة

– The The MM phase includes phase includes mitosismitosis and and cytokinesiscytokinesis الـخلوىالـخلوى اإلنشطاراإلنشطار ..

– Interphase accounts for Interphase accounts for 90%90% of the cell of the cell cycle.cycle.

• During interphase the cell prepares for During interphase the cell prepares for division by producing cytoplasmic organelles division by producing cytoplasmic organelles and copying its chromosomes.and copying its chromosomes.

A.A. InterphaseInterphase has three subphases: has three subphases:

1.1. The The GG11 phase (“ phase (“first gapfirst gap”): the cell grows.”): the cell grows.

2.2. The The SS phase (“ phase (“synthesissynthesis”): the chromosomes are copied. ”): the chromosomes are copied.

3.3. The The GG22 phase (“phase (“second gapsecond gap”): the cell completes preparations for cell ”): the cell completes preparations for cell division.division.

B.B. Division phaseDivision phase ( (MM). ). The cell starts the division process.The cell starts the division process.• The resulting daughter cells may then repeat the cycle again.The resulting daughter cells may then repeat the cycle again.

A.A. Mitosis:Mitosis: is usually include five sub-phases is usually include five sub-phases ::مراحل فرعيةمراحل فرعية

Prophase,Prophase, التمهيدية Prometaphase, Prometaphase, قبل اإلستوائية Metaphase, Metaphase, اإلستوائية Anaphase, Anaphase, اإلنفصاليةاإلنفصالية TelophaseTelophase.. اإلنتهائية• By late interphase (By late interphase (G2G2), the ), the

chromosomes have been chromosomes have been duplicated duplicated تضاعفتتضاعفت but are but are loosely packed.loosely packed.

• The centrosomes have been The centrosomes have been duplicated and begin to duplicated and begin to organize microtubules into organize microtubules into an aster (“star”).an aster (“star”).

1)1) ProphaseProphase, , التمهيدية the chromosomes the chromosomes are tightly coiled, with sister are tightly coiled, with sister chromatids joined together, The chromatids joined together, The nucleoli disappear. The nucleoli disappear. The mitotic spindle mitotic spindle begins to formbegins to form and appears to push the and appears to push the centrosomes away from each other centrosomes away from each other towards opposite ends (poles) of the towards opposite ends (poles) of the cell.cell.

2)2) PrometaphasePrometaphase, , قـبل اإلستوائية the nuclear the nuclear envelope fragments and microtubules envelope fragments and microtubules from one pole attach to one of two from one pole attach to one of two kinetochoreskinetochores (special regions of the (special regions of the centromere) while microtubules from centromere) while microtubules from the other pole attach to the other the other pole attach to the other kinetochore.kinetochore.

3)3) MetaphaseMetaphase,, اإلستوائية tthe spindle fibers he spindle fibers push the sister chromatids until they push the sister chromatids until they are all arranged at the imaginary plane are all arranged at the imaginary plane equidistant between the poles, defining equidistant between the poles, defining metaphase.metaphase.

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AnaphaseAnaphase, , اإلنفصاليةاإلنفصالية the centromeres the centromeres divide, result in separating the divide, result in separating the sister chromatids. Each is then sister chromatids. Each is then pulled toward the pole to which it pulled toward the pole to which it is attached by spindle fibers. By is attached by spindle fibers. By the end, the two poles have the end, the two poles have equivalent collections of equivalent collections of chromosomes.chromosomes.

Telophase,Telophase, اإلنتهائيةاإلنتهائية the cell the cell continues to elongate as free continues to elongate as free spindle fibers from each spindle fibers from each centrosome push off each other.centrosome push off each other.

1)1) Two nuclei begin to form, Two nuclei begin to form, surrounded by the fragments of surrounded by the fragments of the parent’s nuclear envelope.the parent’s nuclear envelope.

2)2) Chromatin becomes less tightly Chromatin becomes less tightly coiled.coiled.

3)3) CytokinesisCytokinesis, begins as the division , begins as the division of the cytoplasm occurs.of the cytoplasm occurs.

B. The cytokinesis:B. The cytokinesis: الخلوىالخلوى اإلنشطاراإلنشطار divides the cytoplasm:divides the cytoplasm:

Cytokinesis (Cytokinesis (division of the division of the cytoplasmcytoplasm) typically follows ) typically follows mitosis.mitosis.

Contraction Contraction إنقباضإنقباض of the cell of the cell pinches the cell into two new pinches the cell into two new cellscells

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Cell CycleCell Cycle

InterphaseInterphase

ProphaseProphase التمهيديةالتمهيدية

Prometaphase Prometaphase اإلستوائية اإلستوائية قبل قبل

Metaphase Metaphase اإلستوائيةاإلستوائية

Anaphase Anaphase اإلنفصاليةاإلنفصالية

TelophaseTelophase اإلنتهائيةاإلنتهائية

G1G1 SS G2G2

Division processDivision process

Mitosis CytokinesisCytokinesis