DNA Part III:
The Cell Cycle
“The Life of a Cell”
Cells & Cell Reproduction
Cells must reproduce else they die. The "life of a cell" is termed the
cell cycle. The cell cycle has distinct phases, which are called G1, S,
G2, and M.
Cells that have temporarily or reversibly stopped dividing are said to
have entered a state of quiescence called G0 phase.
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The G1 Phase of the Cell Cycle
• During this time
organelles are
reproducing, protein
synthesis is occurring for
growth and
differentiation.
• Because, transcription
is occurring, the DNA is
uncoiled. .
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The G1 Phase of the Cell Cycle
The average time for G1 is around 10
hours if the cell cycle lasts 24 hours.
Most cells that differentiate will do
so during this phase.
Cells arrested in G1 may no longer
have the capability of reproducing
and are said to be in G0.
Nerve and muscle cells are usually
arrested in G0.
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S Phase of the Cell Cycle
The S or synthesis phase is the
second phase of the cell cycle.
• DNA uncoils
• DNA replication occurs
• Additional organelle
replication occurs
• This phase ensures that each
emerging daughter cell will
have the same genetic content
as the mother cell.
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G2 Phase of the Cell Cycle
The G2 or Gap 2 phase
occupies the time from the end
of S until the onset of mitosis.
•During this time, the cell
prepares for mitosis by
making and organizing
necessary proteins such as the
tubulin.
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The Amount of DNA Varies During the Cell Cycle
This graph represents the amount of DNA found in the
cell during the cell cycle.
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Internal Controls of the Cell Cycle
• The control of the cell cycle
is dependent on an
accumulation of “signal
molecules”.
• Quite often these signal
molecules must be
phosphorylated in order to
be functional.
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Cyclins vs. Kinases
• Cyclins are a family of proteins that
control the progression of cells through the
cell cycle by activating cyclin-dependent
kinase (Cdk) enzymes.
• A kinase is a type of enzyme that
transfers phosphate groups from highenergy donor molecules, such as ATP, to
specific substrates, a process referred to
as phosphorylation.
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Cyclins vs. Kinases
• Certain cyclins are made at certain times during the cell
cycle, and their concentration will rise and fall. Cyclins are
also destroyed after they are no longer needed by the cell.
• CDKs are not destroyed as they are only activated or
deactivated.
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Kinases Phosphorylate Cell Signal Molecules
•Certain kinases may
have two forms (active
and inactive).
•Kinases are enzymes
(proteins) that
phosphorylate certain
molecules or other
enzymes.
•Most cell cycle signals
are phosphorylated by
kinases.
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Cyclins Activate Kinases
Most cell cycle kinases are activated by molecules called
cyclins. A kinase that requires a cyclin for activation is called
a cyclin-dependent kinase or CDK.
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Cyclins Activate Kinases
Once the CDK phosphorylates certain signals, the
cyclin is destroyed.
In the cell, the concentration
of cyclins will rise and fall
depending on the phase of
the cell cycle.
When the cyclin is
destroyed the CDK returns
to an inactive form.
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Cyclins Activate Kinases
Above is an example of how the M cyclin concentration
affects MPF or M/CDK activity.
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Cyclic Nature of Cyclins in the Cell Cycle
This graph displays the cyclic nature of various cyclins in
a given cell cycle
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Different Types of Cyclins
Cyclin/CDK
Complex
Cyclin
Function of Cyclin/CDK Complex
G1-CDK
Cyclin D
Drives the transition G1  S transition
G1/S-CDK
Cyclin E
Cyclins bind to CDK at the end of G1 and
commits the cell to DNA replication.
S-CDK
Cyclin A
Cyclins bind the CDK during S and are necessary
for the initiation of DNA replication
M-CDK
Cyclin B
Cyclins promote the events of Mitosis
These are some known cyclin/CDK complexes and their
role in the cell cycle.
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Cyclins/CDKs Control the Cell Cycle
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External Factors Also Influence the Cell Cycle
Cell reproduction
and the healing of
a wound.
I. Hemostasis
II. Inflammation
III. Proliferation
IV. Remodeling
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