THE CELL CYCLE
***
The life of a cell from its origin in the division
of a parent cell until its own division into
two.
The major roles (functions) of cell division
• 1. The continuity of life from one cell to another is
based on the reproduction of cells via cell division.
• 2. Cell division is also central to the development of
a multicellular organism that begins as a fertilized
egg or zygote.
3. Multicellular organisms also use cell division to
repair and renew cells that die normally or by
accidents.
***
This division process occurs as part of the cell
cycle.
Cell division distributes the genetic
material (DNA) to two daughter cells.
Division is differ among cells:
-Skin cells divide frequently.
-Liver cells divide when needed (damage repair).
-Nerve cells and muscle cells do not divide at all.
How does Cell division distribute identical sets of chromosomes to
daughter cells?
• A cell’s genetic information (genome ‫ )البنك الـﭽينى‬is packaged as DNA.
• DNA molecules are packaged into chromosomes.
• A human cell must duplicate chromosomes so each daughter cell ends up with
a complete genome.
– Every eukaryotic species has a characteristic
chromosomes in the nucleus.
– Human somatic cells (body cells) have
chromosomes.
– Human gametes ‫( أمشاج‬sperm or eggs) have
chromosomes, half the number in
a somatic cell ‫الخلية الجسدية‬.
number of
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***
• Each DNA molecule has hundreds or thousands of genes.
• This DNA-protein complex (chromatin) is organized into a long thin fiber.
• After the DNA duplication, chromatin
coiling and folding to make chromosome
smaller package
• Each duplicated chromosome consists of
two sister chromatids which contain
identical copies of the chromosome’s DNA.
• The narrow region where the chromatids
connect is called centromere.
• Later, the sister chromatids are pulled apart
and repackaged into two new nuclei.
• Cytokinesis (division of cytoplasm)
It occurs in somatic cells ‫الخاليا الجسدية‬
condenses,
a
Definitions
Genes:
***
The units that specify the inherited characters.
Chromatin: A DNA-protein complex which is organized into a long
thin fiber
Chromosome: The package that formed of a condensed, coiled and
folded chromatin (chromatids).
Chromatids: Two sister chromatids are formed of duplicated
chromosome. They contain identical copies of the
chromosome’s DNA
Centromere: The narrow region at which the two chromatids are
connected together.
Mitosis (somatic cells): Is the division process which forms two daughter cells
, each with the same number of parent cell.
Meiosis (In the gonads): is the division process which forms four daughter cells
, each with half the chromosomes of the parent.
Cytokinesis: is the division stage of the cytoplasm which usually follow mitosis
The Mitotic Cell Cycle (division)
•
•
A.
***
The mitotic (M) phase of the cell cycle alternates ‫ تتبادل‬with the much
longer interphase ‫المرحلة البينية‬.
–
The M phase includes mitosis and
cytokinesis ‫إنشطار السيتوبالزم‬.
–
Interphase accounts for 90% of the
cell
cycle.
During interphase the cell prepares for
division by producing cytoplasmic
organelles and copying its chromosomes.
Interphase has three subphases:
1.
The G1 phase (“first gap”):
the cell
grows.
B.
2.
The S phase (“synthesis”): the chromosomes are copied.
3.
The G2 phase (“second gap”): the cell completes preparations for cell division.
Division phase (M). The cell starts the division process.
***
Cell Cycle
Interphase
Division process (M
phase)
Cytokinesis
Mitosis
G1
S
Prophase
‫التمهيدية‬
G2
Prometaphase
‫قبل اإلستوائية‬
Metaphase
‫اإلستوائية‬
Anaphase
‫اإلنفصالية‬
Telophase
‫اإلنتهائية‬
***
A. Mitosis: is usually include five sub-phases ‫مراحل فرعية‬:




Prophase, ‫التمهيدية‬
Prometaphase, ‫قبل اإلستوائية‬
Metaphase, ‫اإلستوائية‬
Anaphase, ‫اإلنفصالية‬

Telophase. ‫اإلنتهائية‬
• 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 chromosomes or joined sister chromatids are
tightly coiled.
- The mitotic spindle begins to form
appears to push the centrosomes away from each
other towards opposite ends (poles) of the cell.
2)
Prometaphase
- the nuclear envelope fragments.
- spindle fibers from each pole attach to
centromeres.
3)
Metaphase
the spindle fibers push the sister chromatids until
they are all arranged at the imaginary equator plane
between the poles.
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•
Anaphase
***
- the centromeres divide, result in separating
the sister chromatids into 2 chromosomes.
- each chromosome is then pulled toward the
pole by spindle fibers.
- by the end, the two poles have equivalent
chromosomes.
•
Telophase
the cell continues to elongate however each
centrosome pushs off each other.
1)
2)
3)
Two nuclei begin to form, surrounded by the
fragments of the parent’s nuclear envelope.
Chromatin becomes slightly uncoiled.
Cytokinesis, begins as the division of the
cytoplasm occurs.
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B. The cytokinesis: ‫ إنشطار السيتوبالزم‬divides the cytoplasm:
• Cytokinesis (division of the cytoplasm)
typically follows mitosis.
• In animals, the first sign of cytokinesis
(cleavage ‫)إنغماد‬
is the appearance of a cleavage furrow
in the cell surface near the old
metaphase plate.
• Contraction ‫ إنقباض‬of the ring pinches
the cell into two
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MEIOSIS AND SEXUAL LIFE CYCLES
The Role of Meiosis in Sexual Life Cycles
***
Meiosis reduces chromosome number from diploid to haploid
***
(Reduction Division)
 It occurs mainly in sex gonads to form
Gametes (sperms and ova)
 Many steps of meiosis resemble steps in
mitosis.
 Meiosis reduces chromosome number by
copying the chromosomes once, but dividing
twice (meiosis I and meiosis II).
 The first division (meiosis I) separates
homologous chromosomes.
 The second (meiosis II) separates sister
chromatids.
 Each final daughter cell has only half
chromosomes number (haploid ‫(فردى‬.
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Meiosis
***
A)- Meiosis I
interphase
the chromosomes
replicated to form sister chromatids.
are
1) Prophase I,
- the chromosomes condense and homologous
chromosomes pair up to form tetrads.
Homologous
(synapsis ‫)التشابك‬.
chromosomes
attached
together
- (internal) Chromatids of homologous chromosomes are
crossed (chiasmata) and segments of the chromosomes are
exchanged (Crossing Over).
2) Metaphase I
- the tetrads are all arranged at the metaphase plate.
- Microtubules (spindle) from one pole are attached to the centromere of one
***
chromosome of each tetrad, while those from the other pole are attached to the other
centromere.
3) Anaphase I,
the homologous chromosomes
separate and are pulled toward
opposite poles.
***
4) Telophase I
movement of homologous chromosomes
continues to each pole.
– Each chromosome consists of linked sister
chromatids.
Cytokinesis
• by the same mechanisms as mitosis usually
occurs simultaneously.
B)- Meiosis II
***
. 1) Prophase II:
spindle fibers attaches to centromers of each
sister chromatids, and moves them around.
• 2) Metaphase II: the sister chromatids are arranged at the
metaphase plate.
• 3) Anaphase II:
the sister chromatids separate into 2 chromosomes
each one travels toward opposite poles.
and
***
4) Telophase II
separated sister chromatids arrive at opposite poles.
– Nuclear envelop forms around the chromatids.
• Cytokinesis
• Divide of the cytoplasm.
• At the end of meiosis, there are four haploid
daughter cells.