The Cell Cycle & Mitosis
There are so many accelerators and brakes.
The Cell Cycle
•During development from stem to fully differentiated,
cells in the body alternately divide (mitosis) and "appear"
to be resting (interphase). This sequence of activities
exhibited by cells is called the cell cycle.
•Interphase, which appears to the eye to be a resting stage
between cell divisions, is actually a period of diverse activities.
Those interphase activities are indispensible in making the
next mitosis possible.
Leland Hartwell used baker's yeast,
Saccharomyces cerevisiae, as a model system
for genetic studies of the cell cycle. In a series
of experiments 1970-71, he isolated yeast
cells, in which genes controlling the cell cycle
By this approach, he identified genes
specifically involved in cell cycle control, CDC
genes (cell division cycle genes). One of these
genes, designated CDC28, controls the first
step in the progression through the
Seattle, WA, USA.
G1 phase of the cell cycle (the function
Hartwell also identified the fundamental role
of "checkpoints" in cell cycle control. These
checkpoints monitor that all steps in the
previous phase have been correctly executed
and ensure a correct order between the cell
Leland Hartwell used baker's yeast, Saccharomyces cerevisiae (left).
Paul Nurse used another type of yeast, Schizosaccharomyces pombe
(middle). Tim Hunt used sea urchin, Arbacia (right).
Paul Nurse identified the key regulator of
the cell cycle, the gene cdc2, during the
years 1976-80. The product of this gene controls
cell division (transition from G2 to M). The gene cdc2
in the fission yeast Schizosaccharomyces pombe had the
same function as the gene CDC28 in the baker's yeast.
cdc2 controls both the transition from G1 to S and G2 to
M. In 1987. He isolated the human gene CDK1. The
CDK function has been conserved through evolution.
1949. Imperial Cancer
Lincoln's Inn Fields,
CDK1 encodes cyclin dependent kinases
(CDK). These molecules function by linking phosphate
groups to other proteins. Today half a dozen different
CDK-molecules have been found in humans.
CDK and cyclin together form an
enzyme that activates other
proteins by phosphorylation. The
amount of CDK molecules is
constant during the cell cycle,
but their activities vary because
of the regulatory function of the
CDK can be compared with an
engine and cyclin with a gear
box controlling whether the
engine will run in the idling state
or drive the cell forward in the
Tim Hunt discovered cyclins, proteins that bind to the
Cyclins regulate the CDK activity and select the
target proteins to be phosphorylated. The
proteins were named cyclins because of their
cyclic variation in amount during the cell cycle.
Fund, Clare Hall
Cyclins were degraded during mitosis turned out to be
another fundamental control mechanism in the cell
Tim Hunt discovered the first cyclin molecule in 1982,
using eggs from sea urchin, Arbacia, as a model
system. He also found that cyclins, like CDK, were
conserved during evolution. Today around ten different
cyclins have been found in humans.
Cyclins are proteins
formed and degraded
during each cell cycle.
degradation is an
mechanism of the cell
cycle. (D = cell
The fundamental molecular
mechanisms controlling the
cell cycle are highly
conserved through evolution
and operate in the same
manner in yeasts, insects,
plants, animals and humans.
Chromosomal instability in cancer cells may be the result of defective cell cycle control. Three
pairs of chromosomes (1, 3 and 14) in normal cells, compared with the cancer cells. In cancer
cells, the chromosome number may be altered (aneuploidy) and parts of chromosomes may be
染色體 - genetic information in the form of chromatin,
highly folded ribbon-like complexes of deoxyribonucleic
acid (DNA) and a class of proteins called histones.
When a cell divides,
chromatin fibers are very
highly folded, and become
visible in the light microscope
During interphase (between
divisions), chromatin is more
extended, a form used for
expression genetic information.
The DNA of chromatin is
wrapped around a complex of
histones appears in the
electron microscope as "beads
on a string" or nucleosomes.
Changes in folding between
chromatin and the mitotic
chromosomes is controlled by
the packing of the nucleosome
DNA or deoxyribonucleic acid is a large molecule structured
from chains of repeating units of the sugar deoxyribose and
phosphate linked to four different bases abbreviated A, T, G, C
The Cell Cycle
•Stages of the cell cycle The cell cycle is an ordered set of events,
culminating in cell growth and division into two daughter cells. Nondividing cells not considered to be in the cell cycle.
•The G1 stage stands for "GAP 1".
•The S stage stands for "Synthesis". This is the stage when DNA
•The G2 stage stands for "GAP 2".
•The M stage stands for "mitosis", and is when nuclear
(chromosomes separate) and cytoplasmic (cytokinesis) division occur.
Mitosis is further divided into 4 phases
Eukaryotic Cell Cycle
Regulation of the cell cycle How cell division (and thus tissue
growth) is controlled is very complex.
•Cdk (cyclin dependent kinase, adds phosphate to a protein), along
with cyclins, are major control switches for the cell cycle, causing the
cell to move from G1 to S or G2 to M.
•MPF (Maturation Promoting Factor) includes the CdK and cyclins
that triggers progression through the cell cycle.
•p53 is a protein that functions to block the cell cycle if the DNA is
damaged. If the damage is severe this protein can cause apoptosis
1. p53 levels are increased in damaged cells. This allows time to repair
DNA by blocking the cell cycle.
2. A p53 mutation is the most frequent mutation leading to cancer. An
extreme case of this is Li Fraumeni syndrome, where a genetic a
defect in p53 leads to a high frequency of cancer in affected
3. p53 protein binds DNA and stimulates another gene to produce a p21
that interacts with cdk2. Stop the cell cycle.
4. p27 binds to cyclin and CdK blocking entry into S phase. Breast
cancer prognosis is determined by p27 levels. Reduced levels of p27
predict a poor outcome.
•a G1 cyclin (cyclin D)
•S-phase cyclins (cyclins E & A)
•mitotic cyclins (cyclins B & A)
Their levels in the cell rise and fall with the stages
of the cell cycle.
Cyclin-dependent kinases (Cdks)
• G1 Cdk (Cdk4)
• S-phase Cdk (Cdk2)
• M-phase Cdk (Cdk1) Their
levels in the cell remain fairly stable, but
each must bind the appropriate cyclin in
order to be activated. They add phosphate
groups to a variety of protein substrates
that control processes in the cell cycle.
The anaphase-promoting complex (APC)
•The APC is also called the cyclosome, and the complex is often
designated as the APC/C.
•triggers the events leading to destruction of the cohesins
thus allowing the sister chromatids to separate
•degrades the mitotic cyclin B.
Steps in the cycle
•A rising level of G1-cyclins bind to their Cdks and signal the cell to
prepare the chromosomes for replication.
•A rising level of S-phase promoting factor (SPF) — which includes
cyclin A bound to Cdk2 — enters the nucleus and prepares the cell to
duplicate its DNA (and its centrosomes).
•As DNA replication continues, cyclin E is destroyed, and the level of
mitotic cyclins begins to rise (in G2).
One example of a well-known transcription factor activation is the Rb/E2F protein
regulation of the G1 to S phase transition in mammals cells
M-phase promoting factor (the complex of mitotic cyclins
with the M-phase Cdk) initiates
• assembly of the mitotic spindle
• breakdown of the nuclear envelope
• condensation of the chromosomes
These events take the cell to metaphase of mitosis.
At this point, the M-phase promoting factor activates the
anaphase-promoting complex (APC/C)
•allows the sister chromatids at the metaphase plate to separate and
move to the poles (= anaphase), completing mitosis;
•destroys cyclin B. It does this by attaching it to the protein
ubiquitin which targets it for destruction by proteasomes.
• turns on synthesis of G1 cyclin for the next turn of the cycle;
•degrades geminin, a protein that has kept the freshly-synthesized
DNA in S phase from being re-replicated before mitosis.
This is only one mechanism by which the cell ensures that every
portion of its genome is copied once — and only once —during S
Checkpoints: Quality Control of the Cell Cycle
The cell has several systems for interrupting the cell cycle if something goes wrong.
A check on completion of S phase. The cell seems to monitor the presence of the
Okazaki fragments on the lagging strand during DNA replication. The cell is not
permitted to proceed in the cell cycle until these have disappeared.
DNA damage checkpoints. These sense DNA damage
•before the cell enters S phase (a G1 checkpoint);
•during S phase, and
•after DNA replication (a G2 checkpoint).
•detect any failure of spindle fibers to attach to kinetochores and
arrest the cell in metaphase (M checkpoint);
•detect improper alignment of the spindle itself and block cytokinesis
•trigger apoptosis if the damage is irreparable
•The p53 protein senses DNA damage and can halt progression of
the cell cycle in G1.
•Both copies of the p53 gene must be mutated for this to fail so
mutations in p53 are recessive, and p53 qualifies as a tumor
•.The p53 protein is also a key player in apoptosis, forcing "bad"
cells to commit suicide.
•Mutant p53 develops into a cancer.
•More than half of all human cancers harbor p53 mutations and
have no functioning p53 protein.
The p53 gene is found in chromosome 17.
In the cell, p53 protein binds DNA and stimulates p21 that interacts
When p21 is complexed with cdk2 the cell cannot pass through to the
next stage of cell division.
Mutant p53 can no longer bind DNA in an effective way, and the p21
protein is not made available to act as the 'stop signal' for cell division.
Thus cells divide uncontrollably, and form tumors.
A genetically-engineered adenovirus, called ONYX-015, can
only replicate in human cells lacking p53.
Thus it infects, replicates, and ultimately kills many types of
cancer cells in vitro.
Clinical trials are now proceeding to see if injections of ONYX015 can shrink a variety of types of cancers in human patients.
p53 seems to evaluate the extent of damage to DNA, at least for
damage by radiation.
•At low levels of radiation, producing damage that can be repaired,
p53 triggers arrest of the cell cycle until the damage is repaired.
•At high levels of radiation, producing hopelessly damaged DNA,
p53 triggers apoptosis. Possible mechanism:
•Serious damage, e.g., double-stranded breaks (DSBs), causes a
linker histone (H1) to be released from the chromatin.
H1 leaves the nucleus and enters the cytosol where it triggers the
release of cytochrome c from mitochondria leading to apoptosis
ATM (ataxia telangiectasia mutated) gets its name from a human
disease of that name, whose patients — among other things —
are at increased risk of cancer.
The ATM protein is involved in
•detecting DNA damage, especially double-strand breaks
•interrupting (with the aid of p53) the cell cycle when damage is
•maintaining normal telomere length
MAD (mitotic arrest deficient) genes (there are two) encode proteins that
bind to each kinetochore until a spindle fiber (one microtubule will do)
attaches to it. If there is any failure to attach, MAD remains and blocks
entry into anaphase.
Mutations in MAD produce a defective protein and failure of the
checkpoint. The cell finishes mitosis but produces daughter cells with too
many or too few chromosomes (aneuploidy).
Aneuploidy is one of the hallmarks of cancer cells suggesting that
failure of the spindle checkpoint is a major step in the conversion of a
normal cell into a cancerous one.
Infection with the human T cell leukemia virus-1 (HTLV-1)
leads to a cancer (ATL = adult T cell leukemia) in about 5% of
HTLV-1 encodes a protein, called Tax, that binds to MAD
protein causing failure of the spindle checkpoint.
The leukemic cells in these patients show many chromosome
abnormalities including aneuploidy
Many times a cell will leave the cell cycle, temporarily or
It exits the cycle at G1 and enters a stage designated G0 (G zero).
A G0 cell is often called "quiescent", but that is probably more a
reflection of the interests of the scientists studying the cell cycle
than the cell itself.
Many G0 cells are anything but quiescent. They are busy carrying
out their functions in the organism. e.g., secretion, attacking
Often G0 cells are terminally differentiated: they will never
reenter the cell cycle but instead will carry out their function in
the organism until they die.
For other cells, G0 can be followed by reentry into the cell cycle.
Most of the lymphocytes in human blood are in G0. However, with
proper stimulation, such as encountering the appropriate antigen, they
can be stimulated to reenter the cell cycle (at G1) and proceed on to new
rounds of alternating S phases and mitosis.
G0 represents not simply the absence of signals for mitosis but an active
repression of the genes needed for mitosis.
Cancer cells cannot enter G0 and are destined to repeat the cell cycle
The retinoblastoma gene was isolated in 1986.
It was the
first tumor suppressor gene that was isolated based on knowledge of its
chromosomal location: chrom 13 band q14. Germ line mutations in the
Rb gene, predispose to a pediatric malignancy of the eye:
Loss of Rb predisposes to a variety of other tumors later in life, with
osteosarcoma being the most prominent secondary tumor.
Loss of Rb function: lung cancer, lymphoma & breast
(pRb) that is expressed in
almost every cell of the human body and contributes to growth
regulation in these cells. Reintroduction of a functional Rb gene in
retinoblastoma tumor cells results in growth arrest, indicating that
the function of the gene is to restrict proliferation.
The Rb gene encodes a 110 kDa phosphoprotein
ATM acts as a tumor suppressor. ATM activation, via IR damage to DNA,
stimulates DNA repair and blocks cell cycle progression. p53 can cause growth
arrest of the cell at a checkpoint to allow for DNA damage repair or can cause
the cell to undergo apoptosis. p53 is mutated in over 50% of cancers.
•The p53 tumor suppressor protein can bind to specific DNA elements and in
free from can activate transcription of genes that harbor p53 response elements in their
•The activity of p53 is counteracted by a protein called Mdm2.
•Mdm2 can bind to the transactivation domain of p53 and thereby
prevent transactivation by p53. In addition, Mdm2 acts as a “ubiquitin ligase”
that can target p53 for degradation by the proteasome. Thus, p53 in complex with
Mdm2 is both inactive and unstable.
•Binding of Mdm2 to p53 can be disrupted by a protein named p19ARF
(or ARF for short). ARF can bind Mdm2 which renders it unable to
interact with p53.
•Expression of ARF therefore activates p53 by releasing Mdm2.
Proliferation vs. Apoptosis Controls
•These are interrelated, and may induce apoptosis in cells that fail to successfully complete some
phase of cell cycle.
•cell cycle negative controls: inhibition of CDK-cyclin
•cell cycle positive controls: activation of CDK-cyclin
•MITOGENS are polypeptide ligands
•Some of these are growth factors that activate receptor tyrosine kinases (RTK
proteins). This initiates a signal cascade that affects the configuration of many different
transcription factors, affecting the gene activity in the cell
•apoptosis positive controls: leakage of cytochrome c from defective mitochondria acts
as a trigger for apoptosis
•apoptosis negative controls: proteins such as Bcl-2 & Bcl-x block the release of
cytochrome c from mitochondria, possibly stabilizing the mitochondrial membrane and
preventing its rupture).
•based on cell-cell communication
節食減少代謝 使NAD, Sir2增加
What is (and is not) mitosis?
Mitosis is nuclear division plus cytokinesis, and produces two
identical daughter cells during prophase, prometaphase,
metaphase, anaphase, and telophase.
Interphase is often included in discussions of mitosis, but
interphase is technically not part of mitosis, but rather
encompasses stages G1, S, and G2 of the cell cycle.
The cell is engaged in metabolic activity and performing its prepare for mitosis.
Chromosomes are not clearly discerned in the nucleus, although a dark spot
called the nucleolus may be visible. The cell may contain a pair of centrioles
both of which are organizational sites for microtubules.
Chromatin in the nucleus begins to condense and becomes
visible in the light microscope as chromosomes. The
nucleolus disappears. Centrioles begin moving to opposite
ends of the cell and fibers extend from the centromeres.
Some fibers cross the cell to form the mitotic spindle.
The nuclear membrane dissolves, marking the beginning of
prometaphase. Proteins attach to the centromeres creating the
Microtubules attach at the kinetochores and the chromosomes
Spindle fibers align the chromosomes along the middle of
the cell nucleus. This line is referred to as the metaphase
plate. This organization helps to ensure that in the next phase,
when the chromosomes are separated, each new nucleus will
receive one copy of each chromosome.
The paired chromosomes separate at the kinetochores and move to
opposite sides of the cell. Motion results from a combination of
kinetochore movement along the spindle microtubules and through
the physical interaction of polar microtubules.
Chromatids arrive at opposite poles of cell, and new membranes
form around the daughter nuclei. The chromosomes disperse and
are no longer visible under the light microscope. The spindle fibers
disperse, and cytokinesis or the partitioning of the cell may also
begin during this stage.
In animal cells, cytokinesis results when a fiber ring composed of a
protein called actin around the center of the cell contracts pinching
the cell into two daughter cells, each with one nucleus. In plant cells,
the rigid wall requires that a cell plate be synthesized between the
two daughter cells.