Transcript Chapter 6 The Eukaryotic Cell

Aguda & Friedman
Chapter 6
The Eukaryotic Cell-Cycle Engine
Physiology
• Eukaryotic Cell Cycle
– Chromosome Cycle
• DNA is replicated and split between daughter cells
– Growth Cycle
• Cellular mass doubles before being split in two
Cell Cycle
• S phase
– DNA is replicated
• M phase
– Mitosis
– Chromosomes
condense and
separate
• G1 & G2
– Gap phases
– No Gap phases in
embryonic cell cycles
Cell Cycle (cont)
• Interphase
– G1, S & G2
• Mitosis
– Prophase
– Chromosomes migrate to poles
– Metaphase
• Each chromosome duplicate pair
migrates to equator
– Anaphase
• Sister chromatids are separated
– Telophase
• New nuclear membranes formed
• Cytokinesis
– Cells split
Revving up the Cell Engine
• Cyclin-dependent kinases (CDKs)
– Enzymes that drive the cell cycle
• Cyclins
– Proteins that activate CDKs through binding
– Oscillations of levels of this protein during cell cycle
causes variation of CDKs
• Cyclin bonding is followed by phosphorylation
– CAK: puts and activating phosphate on CDK
– Wee1: puts an inhibitory phosphate on CDK
• This phosphate is removed by Cdc25 Phosphatase
Slowing Down the Cell Engine
• Other important enzymes are those that target
cyclins for degradation
• Anaphase promoting complex (APC)
– Target mitotic cyclins for destruction
– Inactivates mitotic CDKs
– Causes exit from mitosis
• Mutual antagonism between mitotic CDKs and
APC
– CDK activity is low interphase due to high APC
– During mitosis APC activity decreases and CDK
activity increases
Embryonic Cell Cycles
• Stem cells derived
from an early-stage
embryo
• Alternating S & M
cycles (no gap
phases)
• Maturation-promoting
factor (MPF)
– Suggested that
oscillations drive
embryonic cell cycles
Experimental Observations
Incorporated in Models
• Cyclin synthesis is necessary and
sufficient to enter mitosis
• MPF enhances its own activation
• Cyclin degradation is necessary to exit
mitosis
Goldbeter Model (1991)
• Parameters
–
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–
Cyclin (C)
Active protease (X)
Active MPF (M)
Rate constants (vi)
• Dynamics
– Delay in activation of X
gives cyclin time to
grow before it get
degraded
The DEs
Results
Novak & Tyson Model (1993)
• Modeling mitotic control in frog embryos
• Contains same cyclin/CDK structure as
Goldbeter model
– Cascading enzyme reactions
• Incorporates positive feed-back loops that
account for MPF self-amplification
– Not included in Goldbeter model
– Leads to potential of bistability
Tyson & Novak Model (2001)
• Cell-cycle model of yeast
• Steady states based on mutual antagonism
between Anaphase-promoting complex
(APC) and CDK
• Two-states
– G1: high APC, low CDK
– S-G2-M: low APC and high CDK
• CDK needed to start, APC needed to finish
Results
T&N Model of Eukaryotic Cell Cycle
The Whole Shebang
• Novak and Tyson propose
that their cell cycle network
characterize all eukaryotic
cells
• Model validated through
gene knockout experiments
• Csikasz-Nagy (2006)
integrate this model for
various organisms
–
–
–
–
Budding yeast
Fission yeast
Xenopus (frog) embryo
Mammalian cells
Concept Summary
• The cell cycle is split into 4 main phases
– Synthesis, Mitosis, and two Gap phases
• Experimental evidence supports the
existence of a “cell-engine” in the form of
an autonomous CDK oscillator
– This mechanism drives the cell cycle
• The cycle is generated through
interactions of CDK with cyclin enzyme
and cyclin degradation enzymes
Model Summary
• Goldbeter model
– Focuses on negative feedback between cyclin and a
cyclin protease
• Novak-Tyson (1993) model
– Incorporates CDK positive feedback
– Predicts bistable behavior
• Novak-Tyson (2001) model
– Mutual antagonism between APC and CDK
– Generates two steady states: G1 & S-G2-M
– Coupling between positive and negative feedback
loops generate oscillations