Cell Cycle Regulation and Checkpoints

Week 9 – Learning Objectives

• Timing of Major Checkpoints

  • Identify the three critical checkpoints in the cell cycle: G1, G2, and Metaphase.
  • Explain what each checkpoint monitors:
  • G1 Checkpoint: Checks for DNA damage and cell size before entry to S-phase.
  • G2 Checkpoint: Ensures DNA is fully replicated and checks for damage before entering M-phase.
  • Metaphase Checkpoint: Verifies that all chromosomes are properly attached to the spindle apparatus before proceeding to anaphase.

• CDKs vs. Cyclins

  • Cyclins: Regulatory proteins whose levels fluctuate during the cell cycle.
  • Types include Cyclin D, E, A, B.
  • Cyclin-Dependent Kinases (CDKs): Enzymes that, when activated by cyclins, phosphorylate target proteins to progress through the cell cycle.

• CDK:cyclin Activation

  • Review images representing activation at various cell cycle stages.

• Mitogen: A signaling molecule that stimulates a cell to begin cell division, particularly important for cells in G0 to re-enter the cell cycle into G1.

• Proto-Oncogenes, Oncogenes, and Tumor Suppressor Genes

  • Proto-Oncogenes: Normal genes that promote cell growth and division; mutations can lead to oncogene formation (cancer-causing).
  • Oncogenes: Mutated forms of proto-oncogenes that drive uncontrolled cell proliferation.
  • Tumor Suppressor Genes: Genes that inhibit cell cycle progression; mutations can result in loss of function, leading to cancer. Example: p53.

• Role of p53

  • Known as the "Guardian of the Genome"; functions to regulate the cell cycle and promote apoptosis in response to stress.
  • Activates transcription of CDK inhibitors like p21, which halts cycle progression.

• Loss-of-Function vs. Gain-of-Function Mutations

  • Loss-of-Function Mutations: Result in reduced or abolished function of proteins, often leading to unregulated cell division if tumor suppressor genes are affected.
  • Gain-of-Function Mutations: Result in proteins that are hyperactive or have novel functions, often contributing to cancer when occurring in oncogenes.

• Regulatory Balance in the Cell Cycle

  • The balance between CDK activity and kinase inhibitors ("p" proteins like p21) determines cell cycle progression versus inhibition. Higher kinase activity promotes progression, while higher inhibitor activity halts it.

• Metaphase Checkpoint Mechanisms

  • Key proteins include cohesin (holds sister chromatids together), separase (enzyme for chromatids separation), APC (Anaphase-Promoting Complex), Mad/Bub (regulators).
  • Disruption in these proteins can result in improper chromatid separation and aneuploidy.

• Consequences of Metaphase Checkpoint Disruption

  • Analyze literature on effects of checkpoint failures, which may result in chromosomal instability, a common precursor to cancer.

Week 9 Key Terms

• CDKs vs. Cyclins: Regulatory proteins for the cell cycle
• Checkpoints: G1, G2, Metaphase checkpoints
• Mitogen: Stimulates cell cycle entry from G0
• Proto-Oncogenes vs. Oncogenes: Genes promoting and causing cancer, respectively
• Loss-of-Function vs. Gain-of-Function Mutations: Impact on gene expression and function
• Tumor Suppressor Genes: Include p53, inhibit uncontrolled division
• Kinase Inhibitors: "p" proteins that regulate cell cycle transitions
• Cohesin, Separase, APC, Mad/Bub: Key proteins in the metaphase checkpoint

Recap Events of the Cell Cycle

• Phases of Interphase:

  • G1: Critical checkpoint for DNA damage and growth signals.
  • S: DNA synthesis, ensuring accurate replication.
  • G2: Checkpoint verifying complete DNA replication and damage.

• Mitosis Stages: Prophase, prometaphase, metaphase, anaphase, telophase

  • Metaphase checkpoint plays a crucial role in ensuring proper chromatid separation.

Important Regulatory Molecules

• CDKs: Phosphorylate target proteins, ensuring progression through the cell cycle.
• Cyclins (D, E, A, B): Regulators activating corresponding CDKs at specific cell cycle phases.
• Inhibitors: "p" proteins that prevent premature progression through the cell cycle.

p53 and Its Role in Cell Cycle Regulation

• Activation: p53 is phosphorylated during cellular stress, acting as a tumor suppressor.
• Functionality: Halts the cell cycle if DNA damage is detected, preventing propagation of damaged cells by upregulating p21, which inhibits CDKs.

The Importance of the Metaphase Checkpoint

• APC: Promotes transition to anaphase by activating separase, regulating sister chromatid separation.

• Signal Mechanisms: The metaphase checkpoint uses a signaling cascade to detect unaligned chromosomes signaling the APC for action.

• Chemotherapy Impact: Drugs that disrupt microtubule function lead to checkpoint signals being activated, with potential adaptations affecting cell survival and tumor treatment efficacy.