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Lecture 20

BIOL 3113 Lecture Notes - Lecture 20: Growth Factor, Mitosis, Cytokinesis

Course Code
BIOL 3113
Barbara S

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20 Cell Cycle Control System
The cell cycle - orderly sequence of events by which a cell duplicates its contents and
divides into two
How cell coordinates various steps of its reproductive cycle?
Cell-cycle control system - network of regulatory proteins that governs progression of a
eucaryotic cell through the cell cycle
Cell-cycle control system - responds to various signals from inside (monitoring of the
progression through cell cycle) and outside the cell (monitoring of the conditions in cell
The cell cycle control system consists of a series of biochemical checkpoints at which the
cycle arrests until certain conditions are met and/or certain external signals are received
Eucaryotic cell cycle - 4 phases
*M phase (mitosis + cytokinesis)
*S phase
*G1 phase
*G2 phase
S + G1 + G2 = Interphase
Duration of the cell cycle varies greatly from one cell type to another (from minutes to
The cell cycle control system - Series of biochemical checkpoints
Most important: G1 and G2
G1 checkpoint
Principal checkpoint
Sometimes referred to as "Start"
Most interphase cells are arrested at this point
Passage through this point triggers entry into the S phase
*Conditions and signals:
Cell size: Is the cell big enough?
Environment: Are environmental conditions favorable for division?
DNA: Is the DNA damaged?
Growth factors: Are appropriate external signals present?

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G2 checkpoint
Controls entry into mitosis
Passage through this point triggers entry into the M phase
Conditions and signals:
Cell size: Is the cell big enough?
DNA: Is all of the DNA replicated?
External Signals - Growth Factors
Growth Factors are external signals that act at the checkpoints to affect the cell cycle in
one of two ways:
Promote cell proliferation
Inhibit cell proliferation
Mechanism of Cell Cycle Control
The cell cycle is controlled by cycles of activation (by phosphorylation) and inactivation
(by dephosphorylation) of key control proteins:
*The enzymatic reactions are mediated by a specific set of protein kinases and protein
*The activities of these enzymes rise and fall cyclically, in synchrony with the cell cycle
*Enzyme activity controlled by cyclins
control proteins
not enzymes
concentration rises and falls in a cyclical manner
responsible for cyclical activity of enzymes
Enzymes controlled by cyclins are called cyclin-dependent protein kinases (Cdks), e.g.,
relatively constant concentration
activity varies depending on the concentration of the appropriate cyclin e.g., M-cyclin -
helps drive cells into M phase
Concentration of cyclins rises gradually during the cell cycle and their rapid elimination
initiates the exit from one phase and progression to the next
Example: M-cyclin (acts in G2 to trigger entry into M phase)
synthesis starts immediately after cell division and continues at the steady level during
interphase - accumulation - the highest level just before mitosis
sudden fall in M-cyclin concentration toward the end of mitosis due to rapid M-cyclin
ubiquitination and destruction in proteasomes
destruction of M-cyclin inactivates M-Cdk
Activation of M-cyclin-Cdk complex
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