Download cell cycle - Chair of Computational Biology

V7: cell cycle
The cell cycle, or cell-division cycle, is the
series of events that takes place in a cell
leading to its division and duplication
(replication).
In cells without a nucleus (prokaryotes),
the cell cycle occurs via a process termed
binary fission.
In cells with a nucleus
(eukaryotes), the cell cycle can
be divided in 2 brief periods:
interphase—during which the
cell grows, accumulating
nutrients needed for mitosis and
duplicating its DNA—and
the mitosis (M) phase, during
which the cell splits itself into two
distinct cells, often called
"daughter cells".
Each turn of the cell cycle divides the
chromosomes in a cell nucleus.
www.wikipedia.org
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Phases
The cell cycle consists of 4 distinct phases:
- G1 phase,
- S phase (synthesis),
- G2 phase (collectively known as interphase)
- and M phase (mitosis).
Activation of each phase is dependent on the
proper progression and completion of the
previous one.
Cells that have temporarily or reversibly stopped
dividing are said to have entered a state of
quiescence called G0 phase.
Schematic of the cell cycle.
Outer ring:
I = Interphase, M = Mitosis;
Inner ring:
M = Mitosis, G1 = Gap 1, G2 =
Gap 2, S = Synthesis.
www.wikipedia.org
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Activity during 4 phases
M phase itself is composed of 2 tightly coupled processes:
- mitosis, in which the cell's chromosomes are divided between the two daughter
cells, and
- cytokinesis, in which the cell's cytoplasm divides in half forming distinct cells.
www.wikipedia.org
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Regulation of the eukaryotic cell cycle
Regulation of the cell cycle involves
processes crucial to the survival of a
cell, including the detection and repair
of genetic damage as well as the
prevention of uncontrolled cell
division.
The molecular events that control the
cell cycle are ordered and directional;
that is, each process occurs in a
sequential fashion.
It is impossible to "reverse" the cycle.
Leland Hartwell
Tim Hunt
Paul Nurse
Noble Price in Physiology/Medicine 2001
„for their discoveries of key regulators of
the cell cycle“
Two key classes of regulatory molecules,
cyclins and cyclin-dependent kinases
(CDKs), determine a cell's progress
through the cell cycle.
www.wikipedia.org
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protein kinase A
Susan S. Taylor
UC San Diego
Masterson et al. Nat Chem Biol. 6, 825 (2010)
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Cellular Programs
Taylor et al. Phil Trans R.Soc. B (1993) 5
Cyclin – cdk2 complex crystal structure
Cyclin A – cdk 2
complex
red: PSTAIRE motif
yellow: activation loop
Nikola Pavletich
Memorial Sloan-Kettering
Cancer Center
Cyclin A – cdk2 phosphorylated
at Thr160
www.wikipedia.org
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Crystal structure
p27 (Kip1) is shown bound to the
CyclinA-Cdk2 complex, provoking
profound changes in the kinase
active site and rendering it inactive.
p27(Kip1)-CyclinA-Cdk2 Complex
p27 also interacts with the secondary
substrate recognition site on the
cyclin.
www.wikipedia.org
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Targets of Cdk1 (also known as Cdc28)
Sofar, 75 targets of Cdk1
are known.
Cdk1 is involved in
positive and negative
feedback loops that
regulate transcriptional
programs to control
cell cycle progression;
Clb, Cln: cyclins
WS 2010 – lecture 7
Cellular Programs
Enserink and Kolodner Cell
Division 2010 5:11
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Cdk1-phosphorylation sites
Cdk1 substrates frequently contain multiple phosphorylation sites that are clustered in regions
of intrinsic disorder, and their exact position in the protein is often poorly conserved in
evolution, indicating that precise positioning of phosphorylation is not required for regulation of
the substrate.
Cdk1 interacts with nine different cyclins throughout the cell cycle.
Expression of human cyclins
through the cell cycle.
Enserink and Kolodner
Cell Division 2010 5:11
WS 2010 – lecture 7
www.wikipedia.org
Cellular Programs
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Cdk1 modulates the activity of several DNA damage
checkpoint proteins
Enserink and Kolodner Cell
Division 2010 5:11
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Cd1-controlled targets and processes
Abstract
The cyclin dependent kinase Cdk1 controls the cell cycle, which is best understood in the
model organism S. cerevisiae. Research performed during the past decade has significantly
improved our understanding of the molecular machinery of the cell cycle. Approximately 75
targets of Cdk1 have been identified that control critical cell cycle events, such as DNA
replication and segregation, transcriptional programs and cell morphogenesis.
....
Conclusions
In conclusion, the identification of Cdk1 targets during the past decade has greatly improved
our understanding of the molecular mechanism of the cell cycle. Nonetheless, much work still
needs to be done because many targets remain to be identified, the exact phosphorylation
sites of many known Cdk1 targets have not been mapped and the consequences of these
phosphorylations at the molecular often remain elusive.
Enserink and Kolodner
Cell Division 2010 5:11
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Cell cycle checkpoints
Cell cycle checkpoints are control mechanisms that ensure the fidelity of cell
division in eukaryotic cells.
These checkpoints verify whether the processes at each phase of the cell cycle
have been accurately completed before progression into the next phase.
An important function of many checkpoints is to assess DNA damage, which is
detected by sensor mechanisms.
When damage is found, the checkpoint uses a signal mechanism either to stall the
cell cycle until repairs are made or, if repairs cannot be made, to target the cell for
destruction via apoptosis (effector mechanism).
All the checkpoints that assess DNA damage appear to utilize the same sensorsignal-effector mechanism.
www.wikipedia.org
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G1 checkpoint
The first checkpoint is located at the end of the cell cycle's G1 phase, just before
entry into S phase, making the key decision of whether the cell should divide, delay
division, or enter a resting stage.
Many cells stop at this stage and enter a resting state called G0.
Liver cells, for example, enter mitosis only around once or twice a year.
The G1 checkpoint is where eukaryotes typically arrest the cell cycle if
environmental conditions make cell division impossible or if the cell passes into G0
for an extended period.
In animal cells, the G1 phase checkpoint is called the restriction point, and in yeast
cells it is called the Start point.
www.wikipedia.org
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G2 checkpoint
The second checkpoint is located at the end of G2 phase, triggering the start of the
M phase (mitosis). In order for this checkpoint to be passed, the cell has to check a
number of factors to ensure the cell is ready for mitosis.
If this checkpoint is passed, the cell initiates many molecular processes that signal
the beginning of mitosis. The CDKs associated with this checkpoint are activated by
phosphorylation of the CDK by the action of a "Maturation promoting factor" (or
Mitosis Promoting Factor, MPF).
The molecular nature of this checkpoint involves the activating phosphatase Cdc25
which under favourable conditions removes the inhibitory phosphates present within
the MPF complex.
However, DNA is frequently damaged prior to mitosis, and, to prevent transmission
of this damage to daughter cells, the cell cycle is arrested via inactivation of the
Cdc25 phosphatase.
www.wikipedia.org
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Metaphase checkpoint
The mitotic spindle checkpoint occurs at the point in metaphase where all the
chromosomes have/should have aligned at the mitotic plate and be under bipolar
tension.
The tension created by this bipolar attachment is what is sensed, which initiates the
anaphase entry. This sensing mechanism allows the degradation of cyclin B, which
harbours a D-box (destruction box).
Degradation of cyclin B ensures that it no longer inhibits the anaphase-promoting
complex, which in turn is now free to break down securin. The latter is a protein
whose function is to inhibit separase, the protein composite responsible for the
separation of sister chromatids.
Once this inhibitory protein is degraded via ubiquitination and subsequent
proteolysis, separase then causes sister chromatid separation. After the cell has
split into its two daughter cells, the cell enters G1.
www.wikipedia.org
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The classical model of cell-cycle control
Nature Reviews Molecular Cell Biology 9, 910-916 (2008)
Cyclin-dependent kinases (cDKs) trigger the transition from G1 to S phase and
from G2 to M phase by phosphorylating distinct sets of substrates.
The metaphase-to-anaphase transition requires the ubiquitylation and
proteasome-mediated degradation of mitotic B-type cyclins and various other
proteins, and is triggered by the anaphase-promoting complex/cyclosome
(APc/c) e3 ubiquitin ligase
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The classical model of cell-cycle control
cDK1 and cDK2 both show promiscuity in
their choice of cyclin partners and can bind
cyclins A, B, D and E,
whereas cDK4 and cDK6 only partner Dtype cyclins.
Thick lines represent the preferred pairing
for each kinase
Nature Reviews Molecular Cell Biology 9, 910-916 (2008)
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The classical model of cell-cycle control
According to the classical model of cell-cycle
control,
D-type cyclins and cDK4 or cDK6 regulate
events in early G1 phase (not shown),
cyclin e–cDK2 triggers S phase,
cyclin A–cDK2 and cyclin A–cDK1 regulate the
completion of S phase,
and cDK1–cyclin B is responsible for mitosis.
But see Paper 7 ....
Nature Reviews Molecular Cell Biology 9, 910-916 (2008)
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Tim Hunt about these new experiments ...
According to the classical model of cell-cycle control ...
...
The first serious blow to this oderly scheme was the discovery that mice that lack
CDC2, although infertile, are viable and healthy.
...
Deletion of other CDKs and cyclins in mice led to a ruther revision of the
„specialized CDK“ hypothesis for the mammalian cell cycle.
...
Santamaria et al. Recently published the ultimate step in this line of work.
...
Nature Reviews Molecular Cell Biology 9, 910-916 (2008)
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Tim Hunt about these new experiments ...
Why were the earlier experiments so misleading?
Quite simply, they were not sufficiently rigorous.
Antibody injection and antisense experiments are inherently difficult to control
and interpret correctly, and they cannot substitute for ablations that are achieved
using gene-targeting.
Although dominant-negative mutants give clear results, they can be problematic if
several kinases share the same activating partners.
Nature Reviews Molecular Cell Biology 9, 910-916 (2008)
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