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THE CELL CYCLE
STAGES OF THE CELL CYCLE
Animal development from a single-cell zygote to fertile adult requires many rounds of cell
division. Cell cycle is a complement of genetic, biochemical and morphological events from cell
birth till its division, differentiation or death.
The cell cycle is an ordered set of periods that includes
interphase and the division, culminating in cell growth and
separation into two daughter cells. The interphase stages are
G1-S-G2-M, where the G1 stage stands for "GAP 1"; the S
stage stands for "Synthesis" (when DNA replication occurs),
the G2 stage stands for "GAP 2". The M stage stands for
"mitosis", and is the period when nuclear division
(karyokinesis) and cytoplasmic division (cytokinesis)
occur. Mitosis is further divided into the following phases:
prophase, prometaphase, metaphase, anaphase and
telophase.
INTERPHASE
Interphase generally lasts at least 12 to 24 hours in
mammalian tissue. During this period, the cell is constantly
synthesizing RNA, producing protein and growing in size. There are a lot of activities during this
period: the cell obtains nutrients, makes ATP, grows, reads
its DNA, and conducts other "normal" cell functions
including preparation for cell division.
G1 (presynthetic or postmitotic) stage of interphase is
a period of cell growth in which proteins, carbohydrates,
and lipids typical for a given cell type are synthesized, but
DNA is not replicated (the "G" refers to the "gap" or break
in DNA synthesis during this stage). So, chromosomes are
single-chromatid (in a diploid cell the number of
chromosomes (n) is equal to the number of chromatids (c):
2n=2c), decondensed, with transcriptionally active parts
(euchromatin) and inactive parts (heterochromatin).
At some point, if the cell is going to divide, DNA
replication begins. The initiation of DNA replication ends
G1 and begins the S period of interphase (from S=DNA synthesis). During S phase, the entire
nuclear complement of DNA is duplicated. The chromosomes thus become bichromatid (each
chromosome consists of 2 identical DNA that make 2 sister chromatids, united at centromere
(2n=4c). Histone and nonhistone chromosomal proteins associated with DNA, as well as proteins
required for a successful replication and repair are synthesized.
This period is also marked by duplication of
centrosomes. Doubling of a centrosome is similar to DNA
replication in two respects: the semiconservative nature of
the process and the action of cdk2 (cyclin-dependent
kinase) as a regulator of the process. But the processes are
essentially different in that centrosome doubling does not
occur by template reading and assembly. The mother
centriole just aids in the accumulation of materials required
for the assembly of the daughter centriole. Aberrant numbers of centrosomes in a cell have been
associated with cancer.
As replication is completed, S ends and the cell enters the final stage of interphase, G2. G2
(postsynthetic or premitotic) is characterized by control of replication quality, DNA repair,
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synthesis and accumulation of a group of proteins necessary for progress through mitosis (tubulin,
factors for cell division). Most cell types remain in G2 only briefly; and at the end of G2, which
marks the end of interphase, mitosis begins. After mitotic division is complete, the two cell
products each enter G1 of the next cell cycle.
G0 is a state of cell cycle arrest. This is the time when a cell quits dividing. It is a period of
proliferative rest. Some cells may stay in G0 permanently. This is the case of highly differentiated
cells, such as cells of nervous system, muscles, crystalline, which have lost their proliferation
properties. Other cells may be temporally resting in G0: liver cells, lymphocytes – differentiated
cells with low proliferation activity.
The main genetic events performed during interphase and absent during division are: gene
expression (transcription, translation), replication and repair.
MITOSIS
Mitosis is the type of division of somatic cells, which assures the growth of the organism and
tissue regeneration. Cell growth and protein production stop at this stage in the cell cycle. All of the
cell's energy is focused on the complex and orderly division into two similar daughter cells. Mitosis
is much shorter than interphase, lasting perhaps only one to two hours.
Prophase
Chromatin in the nucleus
begins to condense and becomes
visible in the light microscope.
Chromatin condensation is mediated
by the condensin complex, which
reorganizes chromosomes into their
highly compact mitotic structure.
The
nucleolus
disappears.
Centrioles begin moving to opposite
ends of the cell. The mitotic spindle
is formed.
Since the genetic material has
been duplicated in an earlier phase
of the cell cycle, the chromosome
consists of 2 identical copies of
DNA, called sister chromatids,
which are attached to each other at
the centromere. The replicated sister
chromatids are “glued” together by
a specific proteic complex called
cohesin, which maintain chromatids
together till they split at anaphase.
Prophase accounts for approximately 3% of the cell cycle's duration.
An important organelle in mitosis is the centrosome, the microtubule
organizing center (MTOC) in animal cells. The centrosome is copied only
once per cell cycle so that each daughter cell inherits one
centrosome, containing two centrioles. The centrosome
replicates during the S phase. During prophase, the two
centrosomes have their microtubule-activity increased due to
the recruitment of γ-tubulin. The centrosomes are pushed
apart to opposite ends of the cell nucleus by the action of molecular motors (motor
proteins: dyneins and kinesins that use microtubules and hydrolysis of ATP for
movement). The nuclear envelope breaks down to allow the microtubules to reach
the kinetochores (a DNA–protein complex required for attachment of mitotic
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spindle at centromere) on the chromosomes, marking the end of prophase. During prometaphase the
chromosomes are captured by the microtubules.
Prometaphase
The nuclear membrane dissolves completely, marking the beginning of prometaphase.
Proteins attach to the centromeres creating the kinetochores. Microtubules attach at the kinetochores
and the chromosomes begin moving to the cell equator.
Metaphase
During this the stage of mitosis in which condensed and
highly coiled chromosomes align in the middle of the cell before
being separated into each of the two daughter cells. Metaphase
accounts for approximately 4% of the cell cycle's duration. The
centromeres of the chromosomes arrange themselves on the
metaphase plate (or equatorial plate), an imaginary line that is
equidistant from the two centrosome poles. This even alignment is
due to the counterbalance of the pulling powers generated by
the opposing kinetochores.
Anaphase
During anaphase the longitudinal cleavage of centromeres occurs. As a result of chromatids
disjunction the single-chromatid chromosomes separate. Each chromatid moves to opposite poles of
the cell, the opposite ends of the mitotic spindle, near the microtubule organizing centers. During
this stage, errors (such as chromatids nondisjunction, transversal cleavage or anaphase lag) could
happen, resulting in abnormal number or structure of chromosomes in daughter cells.
Anaphase begins abruptly and accounts for approximately 1% of the cell cycle's duration. At
this point, a protease known as separase cleaves cohesin, a protein responsible for holding sister
chromatids together. The movement of chromatids is achieved by the shortening of spindle
microtubules from kinetochores sites by depolymerization at their plus ends. No motor protein is
involved in chromatid movement. Molecular motors are however required for microtubule
reorganization during late anaphase.
Telophase
During telophase, the effects of prophase and prometaphase events are reversed. Two
daughter nuclei form in the cell. The nuclear envelopes of the daughter cells are formed from the
fragments of the nuclear envelope of the parent cell. As the nuclear envelope forms around the
chromosomes, the nucleoli reappear. The DNA decondensation processes begin. Telophase
accounts for approximately 2% of the cell cycle's duration.
Cytokinesis (distribution of the cytoplasm) usually
occurs at the same time that the nuclear envelope is
reforming, yet they are distinct processes.
In animal cells, a contractile ring, made of non-muscle
myosin II and microfilaments, assembles equatorially.
Myosin II uses the ATP energy to move along the actin filaments, constricting the cell membrane to
form a cleavage furrow. The cleavage furrow thus develops where the metaphase plate used to be,
pinching off the separated nuclei. Each daughter cell has a complete copy of the genome of its
parent cell (2n=2c), and mitosis is complete.
REGULATION OF THE CELL CYCLE
Cell external signals and cell intrinsic (internal) information together determine whether cells
enter a division cycle. In general, external signals affect this decision only until cells commit to go
through the entire cycle, at a time in G1 known as "Restriction point" in mammals. From there on,
progression through the cell cycle is controlled intrinsically by the cell-cycle machinery.
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The collective results from studies in various eukaryotes have demonstrated that progression
through the cell-division cycle is driven by activation and inactivation of cyclin-dependent kinases
(cdk), which trigger the transition to subsequent phases of the cycle. CDKs are small protein
kinases that require association with a cyclin for their activation. Members of the cyclin family of
proteins are thus key regulators of the cell cycle.
Cyclins are grouped into several classes:
Cyclin D family members are G1 phase cyclins that regulate the entry of cells into G1 from
Go. Cyclin D is upregulated by growth factor and external signals. Cyclin D couples with Cdk4 and
Cdk6. Cyclin D-Cdk4 facilitates the expression of cyclin E. Cyclin E and Cyclin A are able to bind
Cdk2 and promote the cell cycle progression through G1/S transition. Cyclin E stimulates
replication complex assembly. Cyclin A activates DNA synthesis. Cyclins B1 and B2 are M-phase
cyclins. Cyclin B1 and cyclin B2 and their catalytic partner, Cdk1 are components of the MPF (M
phase/maturation promoting) factor that regulates processes that lead to assembly of the mitotic
spindle and sister-chromatid pair alignment on the spindle.
Period
G1
S-phase
Mitosis
Cyclins
D cyclins
cyclins E and A
mitotic cyclins (B cyclins)
Cyclin-dependent kinases (Cdks)
Cdk4, Cdk6
Cdk2
Cdk1
These are short-life proteins. Their levels Their levels in the cell remain fairly stable, but
in the cell rise and fall
each must bind the appropriate cyclin (whose
with the stages of the cell cycle.
levels fluctuate) in order to be activated.
Cell Cycle Checkpoints
The cell has several systems for interrupting the cell cycle if something goes wrong. Since
DNA is the main cell component that assures the molecular processes and cell life, DNA integrity is
controlled at specific DNA damage checkpoints, which arrest the cell cycle till the DNA is
repaired. DNA damage checkpoints sense DNA damage both before the cell enters S phase (a G1
checkpoint, assuring that everything is ready for DNA synthesis: DNA is not damaged, the internal
and external cell environments are favorable, cell grows normally), as well as after S phase (a G2
checkpoint, to determine if the DNA is properly replicated, cell is grown enough and can now
proceed to enter M mitosis). If the damage is so severe that it cannot be repaired, the cell selfdestructs by apoptosis.
One of the cell cycle checkpoints occurs during prometaphase and metaphase – spindle
checkpoint. Only after all chromosomes have become aligned at the metaphase plate, when every
kinetochore is properly attached to a bundle of microtubules, does the cell enter anaphase. It is
thought that unattached or improperly attached kinetochores generate a signal to prevent premature
progression to anaphase, even if most of the kinetochores have been attached and most of the
chromosomes have been aligned.
DNA replication and chromosome distribution are indispensable events in the cell cycle
control. Cells must accurately copy their chromosomes, and through the process of mitosis,
segregate them to daughter cells. The checkpoints are surveillance mechanism and quality control
of the genome to maintain genomic integrity. Checkpoint failure often causes mutations and
genomic arrangements resulting in genetic instability. Genetic instability is a major factor of birth
defects and in the development of many diseases, most notably cancer.
All the checkpoints examined require the services of a complex of proteins. Mutations in the
genes encoding some of these have been associated with cancer; that is, they are oncogenes. This
should not be surprising since checkpoint failures allow the cell to continue dividing despite
damage to its integrity.
Examples of checkpoint proteins:
The p53 protein senses DNA damage and can stop progression of the cell cycle in G1 (by
blocking the activity of Cdk2). The p53 protein prevents a cell from completing the cell cycle if its
DNA is damaged or the cell has suffered other types of damage. When the damage is minor, p53
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arrests the cell cycle until the damage is repaired. If the damage is major and cannot be repaired,
p53 triggers the cell to commit suicide by apoptosis. These functions make p53 a key player in
protecting against cancer; that is, it is an important tumor suppressor gene. More than half of all
human cancers do, in fact, harbor p53 mutations and have no functioning p53 protein. Cells with
functional p53 arrest in G1 or G2 when exposed to γ-irradiation.
The Rb (retinoblastoma) protein integrates the signals reaching the cell to determine whether
it is safe for the cell to complete the passage from G1 of the cell cycle to mitosis. The Rb protein
also plays a role in mitosis itself: it is needed for proper chromosome condensation starting in
prophase, as well as their proper attachment to the spindle. Failure of Rb function during mitosis
can lead to aneuploidy (abnormal number of chromosomes) and chromosome breakage.
Cells that fail to replicate all their chromosomes do not enter mitosis (G2 checkpoint arrest).
Operation of this checkpoint control involves the recognition of unreplicated DNA and inhibition of
MPF activation. Although the ability of unreplicated DNA to inhibit entry into mitosis is well
documented, little is yet known about the proteins that mediate this checkpoint control.
Restriction Points
The eukaryotic restriction point is a particular checkpoint where it is proposed that cells are arrested
under growth limiting conditions and DNA damage. Progression through the cell cycle restriction
point makes the cell independent of external stimuli. The 2 main restriction points are: R1 - G1/S
(control of the readiness for DNA synthesis) and R2 - G2/M (control of replication quality and
readiness for cell division).
APOPTOSIS
Apoptosis, programmed or physiological cell death, is a
normal process of the development and health of multicellular
organisms. Cells die in response to a variety of stimuli and during
apoptosis they do so in a controlled, regulated fashion. This makes
apoptosis distinct from another form of cell death called necrosis, in
which uncontrolled cell death leads to lysis of cells, inflammatory
responses and, potentially, to serious health problems. Apoptosis, by
contrast, is a process in which cells play an active role in their own
death (which is why apoptosis is often called as cell suicide).
The following cells undergo apoptosis: mutant, transformed cells; old cells; cells with
abnormal receptors, recognized as foreign; cells that have lost contacts with neighbor cells;
excessive number of cells.
Upon receiving specific signals instructing the cells to undergo apoptosis a number of
distinctive changes occur in the cell. Proteins known as caspases are typically activated in the early
stages of apoptosis. These proteins cleave key cellular components that are required for normal
cellular function including structural proteins in the cytoskeleton and nuclear proteins such as DNA
repair enzymes. The caspases also activate other degradative enzymes such as DNases, which begin
to cleave the DNA in the nucleus, causing its fragmentation. The breakdown of chromatin in the
nucleus often leads to nuclear condensation and in many cases the nuclei of apoptotic cells take on a
"horse-shoe" like appearance. Further proteolytic processes lead to cell fragmentation and formation
of apoptotic bodies (small vesicles), which undergo phagocytosis by neighbor cells.
There are a number of mechanisms through which apoptosis can be induced in cells. In some
cases the apoptotic stimuli comprise extrinsic signals such as the binding of death inducing ligands
to cell surface receptors called death receptors. In other cases apoptosis can be initiated following
intrinsic signals that are produced following cellular stress. Cellular stress may occur from exposure
to radiation or chemicals or to viral infection. It might also be a consequence of growth factor
deprivation or oxidative stress caused by free radicals. In general intrinsic signals initiate apoptosis
via the involvement of the mitochondria.
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