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THE CELL CYCLE
The cell cycle is the interval between two
mitotic divisions resulting in two daughter cells
The cell cycle is divided into two major phases:
1-Interphase and 2-Mitosis
Interphase -the period between mitoses is
divided into two gap (G1 and G2) phases
and one synthesis (S) phase.
Interphase is the time during which both cell
growth and DNA replication occur in
preparation for cell division.
During the first gap phase (G1), the
cell keeps on synthesis of proteins,
lipids, and carbohydrates following
mitosis.
If growth factors are not existing
during G1, the cells enter a quiescent stage
of the cycle called G0
Early embryonic cell cycles
The cells do not grow during these
cycles, which lack G1 and G2.
In mammalian cells, DNA damage leads to
rapid increases in p53 levels.
The protein p53 signals cell cycle arrest at
the G1 checkpoint.
Chromosomes are replicated during
S phase.
Many proteins are synthesized during
this phase.
3H-thymidine
or BrdU-labeled cells are
investigated in S phase.
The second gap phase, G2, happens
before mitosis begins. A cell in this phase
synthesizes more proteins.
INTERPHASE
NUCLEUS
NUCLEOLUS
The cells of an onion root that is stained by H+E
Mitosis and cytokinesis last about
an hour, approximately 95% of the
cell cycle is spent in interphase
CELL DIVISION:
MITOSIS
The division of a somatic cell
into two daughter cells is called
mitosis
The body grows and regenerates
by mitosis.
Mitotic events are divided into
six phases: prophase,
prometaphase, metaphase,
anaphase, telophase and
cytokinesis.
PROPHASE
During prophase, the cell of an onion root that is stained by H+E
Mitosis is preceded by the duplication of
a pair of centrioles.
Most of the cytoskeleton reorganizes
during prophase.
Phosphorylation of various nucleolar
proteins disassembles the nucleolus.
Cohesins cross-link two sister chromatids,
gluing them together.
Condensins play a role in the process of
chromosome condensation.
Prophase begins with condensation of the
chromosomes.
The onset of condensation associates with
phosphorylation of Histon1 by Cdk1-cyclin B-p9
and Histon3 by Aurora-B protein kinase.
DNA topoisomerase II also uses for chromosome
condensation.
PROMETAPHASE
During prometaphase, the cell of an onion root that is stained by H+E
Prometaphase is begun by the breakdown of
the nuclear envelope.
The nuclear envelope is absorbed into the
endoplasmic reticulum.
Choromosomes attach to spindle.Mitotic
spindle is conducted by the action of different
types of kinesins and cytoplasmic dynein.
METAPHASE
During metaphase, the cell of an onion root that is stained by H+E
At metaphase, chromosomes align on equator.
Both motor proteins and osscillates in the length of
kinetochore microtubules contribute chromosome
motions during metaphase.
Mitotic spindle consists of four kinds of
microtubules: Astral, chromosomal, kinetochore
and overlap=polar microtubules.
ANAPHASE A
During early anaphase,the cell of an onion root that is stained by H+E
Anaphase begins with degradation of the cohesin
linkage holding sister chromatids together. Sister
chromatids separate and approach poles (Anaphase A).
The two spindle poles move apart (Anaphase B)
Three factors regulate sister chromatid
separation: a protein complex known as
cohesin, a protease known as separase,
and an inhibitor of the protease known as
securin.
ANAPHASE B
During late anaphase,the cell of an onion root that is stained by H+E
Nuclear envelope reassembles during anaphase and
is completed during telophase.
The dephosphorylated lamins and nuclear pore
complexes re-form the nuclear lamina.
TELOPHASE
During telophase, the cell of an onion root that is stained by H+E
Cytokinesis divides the cell into two daughter
cells. In animals, daughter cells are separated
by a contractile ring. Centrosomes are absent
in plants. So daughter cells are separated by a
the phragmoplast.
The contractile ring contains many proteins
along with actin and myosin II.
The phragmoplast consists actin filaments,
microtubules and vesicles derived from the
Golgi and endoplasmic reticulum.
REFERENCES
1-Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. :
Molecular Biology of The Cell. Fourth edition Garland Science
Taylor & Francis Group New York 2008.
2-Pollard T.D. and Earnshaw W.C.: Cell Biology. Elsevier
Science USA 2008.
3- Cooper G.M. and Hausman R.E.: The Cell: A molecular
Approach 5th ed. ASM Press Sinauer Associates Inc. USA 2009.
4- Lodish H, Berk A, Kaiser C A, Krieger M, Scott M P,
Bretscher A, Ploegh H, Matsudaira P. : Molecular Cell Biology.
Sixth Edition. W.H. Freeman and Company New York 2008.
5-Lewis R. Human Genetics. Concepts and Applications. Fifth
Editon McGraw-Hill New York 2003.