1. Aguda & Friedman Chapter 6 The Eukaryotic Cell-Cycle Engine

2. Physiology Eukaryotic Cell Cycle –Chromosome Cycle DNA is replicated and split between daughter cells –Growth Cycle Cellular mass doubles before being split in two

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. Goldbeter Model (1991) Parameters –Cyclin (C) –Active protease (X) –Active MPF (M) –Rate constants (v i ) Dynamics –Delay in activation of X gives cyclin time to grow before it get degraded

10. The DEs

11. Results

12. 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

13. 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

14. Results

15. T&N Model of Eukaryotic Cell Cycle

16. 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

17. 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

18. 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