The Eukaryotic Cell Cycle and Regulation

Nobel Prize Winners in Cell Cycle Research

  • Leland Hartwell (Budding Yeast)

    • Awarded for his discoveries concerning the cell cycle control in budding yeast, which laid the groundwork for understanding eukaryotic cell cycle regulation.

  • Tim Hunt

    • Discovered cyclins, which are crucial for regulating the cell cycle.

  • Paul Nurse (Fission Yeast) 

    • Identified key regulators of the cell cycle, such as cyclin-dependent kinases (CDKs), which are essential for cellular division and the cycle progression.

Budding and Fission yeast

  • Budding: identifying control of G1\rightarrowS transition

    • closed mitosis

  • Fission: identifying control of G2\rightarrowM transition

    • closed mitosis

Frog Experiment:

This experiment demonstrated how the cell cycle transitions from G2 to M phase are tightly regulated through the action of cyclins and cyclin-dependent kinases (CDKs), indicating their critical role in controlling cell division.

Cell Fusion Experiments: Gave insight into cell cycle regulation

  1. Maturating Promotion Factor

    1. Caused by diffusible M-phase C/CDK Complexes

Overview of the Eukaryotic Cell Cycle

  • Highly regulated series of events.

  • Major phases: G1, S, G2, and M (Mitosis).

Key Phases of the Cell Cycle

  • G1 Phase(INTERPHASE): Controls entry into S phase; pass START/Restriction Point signifies commitment.

    • The Cohesin Complex 

  • S Phase: Chromosomal replication occurs under tight regulation.

  • G2 Phase: Prepares the cell for mitosis; important checkpoints.

  • M Phase (Mitosis): Divided into stages:

    • Prophase,

    • Prometaphase,

    • Metaphase

    • Anaphase,

    • Telophase,

    • Cytokinesis (post-mitosis)

Regulation of the Cell Cycle:

  • Regulated by:

    • Protein synthesis/degradation

    • poly-Ub/proteasomes

    • Protein phosphorylation/dephosphorylation

    • Cyclins/cyclin dependent kinases (C/CDKs)

  • KEY PLAYERS: Cyclins and Cyclin-dependent kinases (CDKs):

    • CDKs require Cyclin to be active.

    • Cyclins: regulate synthesis/degradation

    • Activate CDKs→ protein phosphorylation

    • Different cyclins are active at different cell cycle stages.

  • Oscillating activity to control cell cycle transitions. (THINK: Checkpoints)

    • Generated by positive feedback (CDK/cyclins promote their activation)

    • Followed by negative feedback (CDK/cyclins promote their inactivation)

Regulation of Cyclins: they are regulated in multiple ways

  • Regulated transcription

  • Regulated activation/inhibition (kinases and phosphatases)

  • Cyclin degradation (poly-ubiquitination, proteasomes)

Checkpoints: Without them, mutations would rapidly accumulate!

  • Critical for DNA integrity and proper cell division:

    • G1 checkpoint: Checks for damaged DNA before replication.

    • G2 checkpoint: Checks if DNA was replicated correctly.

    • M checkpoint: Ensures all chromosomes are attached to spindle.

CDKs and Cyclins: regulated phosphorylation

  • CDKs: Phosphorylate target proteins to promote cell cycle progression.

    • Cyclin-DEPENDENT-kinases (regulated by the cyclins)

  • Cyclins: Regulate CDK activity; levels fluctuate with cell cycle phases.

    • regulatory subunits for a family of kinases

    • Levels of cyclins rise and fall depending on the stage in the cell cycle

  • The four classes of C/CDK-complexes:

Key Mechanisms of Cyclin regulation: protein degradation

  • Two main ubiquitin-protein ligases in the cell cycle:

    • SCF: Controls the G1/S transition

    • Is always active, but targets need to be phosphorylated

    • Destroys G1 Cyclins

    • Destroys S-phase cyclin inhibitor

    • APC/C: Anaphase-Promoting Complex or Cyclostome ( controls mitosis)

    • Needs to be phosphorylated to be active

      • Specificity Subunits:

      • Cdc20: destroys anaphase inhibitor protein

      • Cdh1: destroys S-phase and M-phase Cyclins

Destruction of Cyclins by poly-ubiquitination

  • E1

  • E2

  • E3

(Universal CKIs: Inhibit all CDKs)

Conclusion

  • Proper regulation is crucial for cell cycle progression; errors can lead to cancer and other diseases.

The most important topics from this lecture encompass the groundbreaking discoveries in cell cycle regulation, the detailed phases of the eukaryotic cell cycle, and the critical molecular mechanisms that control it. Key areas include:

  • Nobel Prize Winners and their Discoveries: Understanding the contributions of Leland Hartwell (budding yeast, G1-S control), Tim Hunt (cyclins), and Paul Nurse (fission yeast, CDKs, G2-M control).

  • Cell Cycle Phases: A thorough understanding of G1, S, G2 (interphase), and M (mitosis: prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis) and their respective functions.

  • Regulation by Cyclins and CDKs: The central role of Cyclin-Dependent Kinases (CDKs) and their regulatory subunits, Cyclins, including how their oscillating activity (synthesis, degradation, phosphorylation) drives cell cycle progression.

  • Cell Cycle Checkpoints: The significance of G1, G2, and M checkpoints in ensuring DNA integrity and proper chromosome segregation, preventing mutations.

  • Mechanisms of Cyclin Regulation: How cyclins are regulated at multiple levels, including transcription, activation/inhibition by kinases and phosphatases, and crucial degradation pathways involving poly-ubiquitination via ubiquitin-protein ligases like SCF (G1/S transition) and APC/C (anaphase inhibitor, S-phase and M-phase cyclins).

These topics collectively explain how the cell meticulously controls its division, and why errors can lead to serious conditions like cancer.