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Cell division
Is a complex phenomenon by which cellular material is divided
equally between daughter cell? During embryonic development most
cells are undergoing repeated division as the body grows in size and
complexity .As a particular cell matures it becomes differentiated with
respect to its structure and function and may eventually lose the ability to
divide as a neurons and skeletal muscle cells ,do not divide at all ;others
,such as liver cells normally divide only once every year or two while
certain epithelial cells in the gut and skin divide more than twice a day in
order to renew ,or it may persist as a stem cell (capable and dividing
throughout life ).
Cell division is important in all organisms for renewal and repair ,for
example : skin cells, new red blood cells to replace these that are worn
out or damaged .As the cell grow i.e. the cell-increase in size ,the surface
area of the cell is not adequate to meet the needs of the cell ,so the cell
beings to divide .Certain factors are needed before and during
cell
division such as hormones ,enzymes and surface area to volume .
The rate of cell division varies considerably in different tissues ,it
may be rapid as in the crypts between intestinal villi ,or it may vary
according to demand as in healing of wounded skin .Division in the
unicellular (single – cell )organisms produce two new individuals that are
like the parent (asexual reproduction ) ,while in the multicellular animals
,cell division culminates in the development and growth of the organism.
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There are two types of cell division: Direct (Amitosis) cell division: in which the nucleus elongates
and constricts into two portions without either chromosome
formation or spindle formation, followed by constriction of the
cytoplasm.
 Indirect cell division: also called somatic cell division, in this
type nuclear division include formation of asters, spindle
fibers, and division of chromosomes.
Cell are contains full complement of DNA ,the genetic material
,simple cell division distributes the chromosomes from parent cell to
daughter cells .Each cell of the human body ,except for sex cells ,contain
46 chromosomes are called a diploid number of chromosomes (2n )
(include two chromosomes of each kind ),while the haploid (n) number
contain only one of kind (sperm ,egg).
Cell cycle
A growing cell undergoes cell cycles that comprise essentially two
periods:
1-The interphase (period of non apparent division) of the period of
division.
2-Division may take place by mitosis, meiosis, or the mechanisms of cell
replication.
The duration of the cell cycle varies from one cell type to another.
The shortest cell cycle of fly embryo which lasting as little as 8
minutes.The more typical example in mammalian cell with a cycle time
of about 24 hours .
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The cell cycle is divided into several phases or stages ,of which the
most important is mitosis (the process nuclear division ).In most cell the
whole of (M) phase take only about an hour ,which is only a small
traction of the total cycle line .The longer period that eclipse between one
(M) phase and the next is known as interphase .Under the microscope ;the
cell simply appears grows in size .Replication of the nuclear DNA
occupies apportion of interphase ,called the (S) phase of the cell cycle
(S= synthesis ) ,the completion of mitosis of the beginning of mitosis I
called G2 phases .
G1 and G2 provide additional time for growth :if interphase lasted
only long enough for DNA synthesis (replication ) ,the cell would not
have time to double its mass before it divided ,so during G1 the cell
monitors its environment and its own size .The G2 phase provides a
safety gap ,allowing the cell to ensure that DNA replication is complete
before it plunges into mitosis .
The cell cycle control system is cyclically operating device
constructed from a set of interacting proteins that induce and coordinate
the essential downstream processes that duplicate and divide the cell's
contents, also it is regulated by stimulatory substance that cause the cell
to proceed through two critical checkpoints:
1- G1 stage --- S stage.
2- 2- G2 stage ---- M stage.
Two factors affected the checkpoints these are:
Feedback and environmental signals conveying information about the
downstream processes and act on the control system at one or other of
two major checkpoints in its cycle (ex. G1 stage just before entry into S
phase ,other G2 to mitosis ).
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This system (cell-cycle) is a set of protein complexes formed from two
types of components:
1- Cyclin-dependent protein kinases (Cdk) which induced the process by
phosphorylating selected proteins on serines and threonines .
2- Activating proteins called cyclins ( so called because they undergo a
cycle of synthesis and degradation in each cell division cycle ) that
binds Cdk molecules and control their ability to phosphorylate
appropriate target proteins ,the cyclin –Cdk complexes regulate the
normal cell cycle .
There are two main classes of cyclins : Mitotic cyclins which bind to
Cdk molecules during G2 (required for entry into mitosis ) ,and G1
cyclins ,which binds to Cdk molecules during G1 (required for entry into
S phase ) , then S-cyclin is destroyed and Cdk is no longer active .In
mammalian cells there are at least two different Cdk proteins ,one for
each checkpoint ,while in yeast cells it provides the same member at both
checkpoints .
In other word, mitotic cyclin accumulates gradually during G2 and
bind to Cdk to form M-phase –promoting factor complex (MPF), this
complex is at first inactive, but its convert to an active form by the action
of other enzymes that phosphorylate and dephosphorylate it.
The ultimate activation of |(MPF) is almost explosive ,this is
believed to be due to a positive feedback mechanism ,this is the
concentration of active (MPF) builds up at an accelerating pace until a
critical point is reached ,then (MPF) is inactivated suddenly by the
degradation of mitotic cyclin at the metaphase –anaphase boundary
,enabling the cell to exit from mitosis .
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