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The Cell cycle
• Overview:
The Cell cycle
• Cell division in bacteria
– Importance of the cell cycle.
– Not considered mitosis.
– Progress through the cell cycle is controlled.
– Does not undergo the stages we will refer to as
“the cell cycle” of eukaryotic cells.
• Consequences of improper control for unicellular
organisms.
• Consequences of improper control for multicellular
organisms.
• Rest of the chapter we will concern
ourselves only with eukaryotic cells.
– The precision of cell replication.
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The Cell cycle
The Cell cycle
• Mitosis and cytokinesis
• Stages of the cell cycle. (Fig. 18.2)
– Mitosis is the duplication of the duplication of
the nucleus
– Cytokinesis is the division of the cytosol into
multiple components
– Usually mitosis and cytokinesis happen at
about the same time.
– There are numerous exceptions to this rule
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M-phase
• The definition of M phase
– Mitosis (or meiosis).
– (Usually) cytokinesis.
• Where a cell becomes two.
• Most dramatic of the cell
cycle stages.
M-phase
• This stage includes prophase,
pro-metaphase, metaphase,
anaphase and telophase.
• These stages are really about
distributing the chromosomes
to daughter cells.
• Usually short in duration.
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M-phase
• Other organelles also are distributed during
division.
– Many organelles are present in high copy number
- randomly end up in each daughter cell. Ex:
ribosomes, ER, etc.
Interphase
• The term suggests an.
interlude between mitoses
• Why this is a misnomer.
– This is the stage where a cell
really does its work and growth.
– Some organelles are duplicated before division
(ex: some chloroplasts, mitochondria)
– Some organelles reassemble in the daughter cells
(ex. Some Golgi)
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– Contains several biochemically
distinct stages within it.
– Differentiated cells (eg nerve
and muscle spend all their time
in interphase.
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Interphase
• The S-phase
Interphase
• The G1-phase
– The name refers to DNA
synthesis (RNA synthesis and
protein synthesis occur
throughout interphase)
– The period after M but before
S.
– Like all parts of interphase,
proteins and RNA are also
synthesized.
– Starts with 1 C DNA and ends
up at 2 C.
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– Cell is 1c.
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Interphase
• The G2-phase
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The Cell cycle
• Not all cell types have all these phases.
– The period after S but before
M.
– Most differentiated cells remain in G1 (actually
as we shall see G0)
– Like all parts of interphase,
proteins and RNA are also
synthesized.
– Some cell types, for example some fungi, lack
G1, other cell types (for example other fungi)
lack G2.
– Cell is 2C
– In some specialized cases (for example
embryonic cleavage) G1 and G2 are markedly
shortened or eliminated.
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Control of the cell cycle
Control of the Cell cycle
• Review: the importance of controlling the
cell cycle.
• The three main
checkpoints:
– Late G1 (=start
=restriction point)
• There is a central
control system to
trigger entry/exit
of the various
stages
– Late G2
– Late M
Fig. 18-3
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Fig. 18-3
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Control of the Cell cycle
• Checkpoints are also places where input
from multiple sources, internal and external
can influence the controller and thus the
progress through the cell cycle.
• Consider the input a cell needs before
entering M-phase.
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Mitosis ?
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Does the organism
Need me to divide?
Does the organism
Need me to divide?
Mitosis ?
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Do I have enough
Cytosol?
Mitosis ?
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Does the organism
Need me to divide?
Do I have enough
Cytosol?
The nature of the controller
• Hint #1: something in M-phase cells can
cause other cells to enter M.
– The system: the Xenopus embryo/ oocyte
Have I finished
DNA replication?
Mitosis ?
Division in the Xenopus embryo
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The nature of the controller
The nature of the controller
• Hint #1: something in M-phase cells can
cause other cells to enter M.
• MPF activity was thus cyclic.
– What is an oocyte?
– The experiment:
Fig. 18-9
Fig. 18-10
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The nature of the controller
• Hint #2: When MPF was finally purified, it
was found to be a protein kinase (=CdK).
– Regulated the activity of a host of proteins
• At least one causes chromosome condensation.
• At least one causes lamin (IF) disassembly and thus
nuclear envelope breakdown.
The nature of the controller
• Hint #3: Almost all proteins are present
throughout the cell cycle at relatively
constant concentrations. (Bummer!)
• At least one causes changes in the microtubule
cytoskeleton which in turn promotes development of a
spindle.
– BUT: the kinase (the protein itself) was present
throughout the cell cycle, and not restricted to
M-phase.
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The nature of the controller
The nature of the controller
• Hint #4: Work in clam eggs showed a
unusual protein whose abundance oscillated
with the cell cycle. They named this protein
cyclin.
• Cyclin is rapidly destroyed by the ubiquitin
dependent proteolytic pathway as the cell
passes the checkpoint.
• BUT: cyclin did not have any enzyme
activity. How could it cause a cellular
change?
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• Hint #5: Still others were working with
yeast. They isolated a whole host of cdc
(cell division control) mutants that had
defects in particular portions of the cell
cycle.
• By identifying and sequencing these genes
they were able to show that some of the
yeasts were defective in a cyclin proteins
while others were defective in the MPF
kinase!
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The nature of the controller
The nature of the controller
• In fact, the yeast cell can be cured if the
appropriate human gene is inserted into the
mutant.
• So where are we?
– A universal eukaryotic controller.
– One that requires both cyclin and MPF kinase
(=Cdk).
– It looks like cyclin turns on the kinase to give
functional MPF!
• This indicates that the factors that control
progress through the cell cycle are universal
in eukaryotes.
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The nature of the controller
Fig. 18-6
The nature of the controller
• The idea is that rising cyclin concentrations
can turn on the kinase.
• It turns out that cyclin binding was only one
a series of events required to activate Cdk
• After the cell progresses past the
checkpoint the cyclin is rapidly destroyed.
Fig. 18.11
Fig. 18-7
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The nature of the controller
• Positive feedback of the activating
phosphatase further sharpens the response.
– Right before
triggering, there
are generally lots
of molecules ready
to go.
The nature of the controller
• In yeast one Cdk can complex with each of
the cyclins in turn. Therefore combinations
are important in specificity.
• In other cell types there may be more than
one type of Cdk as well.
– The first active Cdk
dephosphorylates
additional Cdk
molecules.
Fig. 18-12
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• The complete model. (Fig 18-13)
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How does active CdK cause cellular
effects?
• Each active Cdk regulates a separate set of
proteins
• For mitotic Cdk
• At least one causes chromosome condensation.
• At least one causes lamin (IF) disassembly and thus
nuclear envelope breakdown.
• At least one causes changes in the microtubule
cytoskeleton which in turn promotes development of
a spindle.
Fig. 18-13
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How does active CdK cause cellular
effects?
• Each active Cdk regulates a separate set of
proteins
• For S-phase Cdk.
– The challenge for replication: 1 round and only 1 round.
– Remember that DNA replication starts at “origins of
replication”.
– There is a complex present at these places that is
inactive in G1
– It becomes activated at S and starts replication.
– How?
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How does active CdK cause cellular
effects?
• Cdc6 made in early G1
• ORC now “ready”
• S-Cdk now phosphorylates at the ORC replication starts.
• Also S- Cdk phosphorylates cdc6, as a signal
for destruction.
• Prevents re-replication.
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Fig. 18-14
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The nature of the controller
Do I have enough
Cytosol?
Does the organism
Need me to divide?
The nature of the controller
Have I finished
DNA replication?
Do I have enough
Cytosol?
Does the organism
Need me to divide?
Have I finished
DNA replication?
Mitosis ?
• Remember the above questions?
Mitosis ?
• How are the answers to these questions
communicated to the Cdk machinery?
• How are the answers to these questions
communicated to the Cdk machinery?
– Not completely known.
– Perhaps the phosphatase and the two kinases
that regulate Cdk are themselves regulated by
other cellular pathways using mechanisms that
should be familiar to you. As far as I know
these remain hypothetical.
– Cyclin is completely destroyed during the
previous stage. Takes time and protein
synthesis to complete enough for another round.
This might answer the question about enough
cytoplasm, for example.
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The nature of the controller
Do I have enough
Cytosol?
Does the organism
Need me to divide?
Have I finished
DNA replication?
Mitosis ?
• How are the answers to these questions
communicated to the Cdk machinery?
– DNA damage halts cell cycle by inhibiting Cdk
through p53 and p21 phosphoproteins. (Fig. 1813)
Fig. 18.15
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The nature of the controller
Do I have enough
Cytosol?
Does the organism
Need me to divide?
The nature of the controller
Have I finished
DNA replication?
Does the organism
Need me to divide?
Do I have enough
Cytosol?
Have I finished
DNA replication?
Mitosis ?
• How are the answers to these questions
communicated to the Cdk machinery?
Mitosis ?
• Cells in multicellular organisms require mitogens
factors to progress through the cell cycle.
Fig. 18-23
– External signaling molecules
– Older terminology called “growth factors”.
– Probably should be broken up into:
• Mitogens - stimulate cell division.
• Growth factors - stimulate increase in cell mass
• Survival factors- necessary to go on living.
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The cell has multiple checkpoints
• There are multiple checkpoints in the cell cycle.
• We have already mentioned the restriction point,
and the DNA damage checkpoint.
The Cell cycle
• Some cells effectively withdraw from the
cell cycle.
– The general importance of the G1 checkpoint
(=start).
• Another interesting checkpoint:
– Chromosomal attachment checkpoint
– Cells held at this point can refuse to play the
cell cycle game and enter G0
– Unattached chromosomes create a signal that prevents
start of anaphase.
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– Often true for specialized and differentiated
cells.
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The Cell cycle
• A summary Fig 18-14
• Design an experiment
to determine if a
particular cell is in G1
or G0
Fig. 18.16
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