Module 25

Cell Cycle Regulation

  • The cell cycle is regulated to ensure cells only divide when they are ready, at the appropriate time and location.

    • Key insights into cell cycle regulation came from studies of early embryos.

    • The cell cycle is most dynamic during embryonic development where fertilized eggs rapidly divide to form an organism’s tissues and organs.

    • Cells in some embryos, like those of sea urchins, can divide every 30 minutes.

Phases of the Cell Cycle

  • During rapid cell divisions, M phase and S phase alternate, generally skipping G1 and G2 phases.

    • M phase refers to the mitotic phase; S phase is the DNA synthesis phase.

Discovery of Cyclins

  • Tim Hunt and colleagues (1980s) studied embryonic cells during rapid divisions.

    • They extracted proteins and observed that proteins had varying abundances across different phases of the cell cycle.

    • Hunt named these proteins "cyclins" due to their cyclical patterns of presence and absence.

Functions of Cyclins and CDKs

  • Different types of cyclins exist in eukaryotic cells; their concentrations change dynamically during the cell cycle.

    • Graphs (e.g., FIGURE 25.2) depict the cyclical rise and fall of cyclins, indicating synthesis during interphase and degradation before the next cycle.

  • Cyclins bind to kinases known as cyclin-dependent kinases (CDKs), which are present in the cell but only active when bound to a cyclin.

    • The activation of cyclins-CDKs promotes cell cycle progression.

  • CDK activity cycles; they transition between active and inactive states upon the association and dissociation with cyclins.

Cyclin-CDK Mechanism

  • Active cyclin-CDK complexes transfer phosphate groups to target proteins, modifying their functions and shapes.

    • After activation, cyclins degrade, leading to a short-lived signal since the inactive CDK cannot initiate actions without its cyclin partner.

  • Both cyclins and CDKs are evolutionarily conserved, highlighting their essential roles across various organisms including yeast, mice, and humans.

Cell Cycle Checkpoints

  • The cell cycle includes several checkpoints that monitor progress and readiness before a cell advances to the next phase.

Major Checkpoints

  1. DNA Damage Checkpoint (G1 to S phase)

    • Monitors for DNA damage prior to the initiation of DNA synthesis (S phase).

    • Environmental factors like UV radiation can cause DNA damage (e.g., double-stranded breaks).

    • If damage is detected, cell cycle progression is halted until repairs are made.

  2. DNA Replication Checkpoint (G2 to M phase)

    • Ensures that all parental DNA has been accurately copied before moving into mitosis.

    • Uncopied or damaged DNA prevents transition to mitosis.

  3. Spindle Assembly Checkpoint (During Metaphase)

    • Verifies that all chromosomes are properly attached to the mitotic spindle before sister chromatids separate.

    • This checkpoint ensures that cell division is completed correctly, preventing the separation of unaligned chromosomes.

Consequences of Checkpoint Failure

  • Checkpoints not only prevent the division of damaged cells but can also pause the cycle until issues are resolved.

Cell Death Mechanisms

  • When damage is irreparable, programmed cell death (apoptosis) or necrosis can occur.

    • Necrosis: Results from acute damage, causing cell contents to leak, possibly harming nearby cells.

    • Apoptosis: A regulated and orderly process for removing unneeded cells (e.g., during development) while maintaining tissue integrity.

    • During apoptosis, specific enzymes are deployed to disassemble cellular components, resulting in cell shrinkage and DNA fragmentation, which immune cells later clear.

Role of p53 Protein

  • Upon DNA damage, a kinase is activated, which phosphorylates the p53 protein.

    • Active p53 binds to DNA and activates genes that block cyclin-CDK formation necessary for transitioning from G1 to S phase.

    • This process gives the cell time to repair its damaged DNA.

    • The p53 protein is a critical component in cellular defense against DNA damage and is often referred to as the “guardian of the genome.”

  • Mutations in p53 can disable its function, allowing for continued division in the presence of DNA damage, leading to cancer.

Cancer Development

  • Cancer arises when checkpoint systems or programmed cell death (apoptosis) fail, leading to unchecked cell division.

    • Carcinogens, such as radiation and certain chemicals, often damage DNA, generating breaks that stimulate checkpoint responses.

  • The p53 protein's dysfunction can allow for a sequential buildup of mutations within cells, rapidly accelerating the potential for cancer progression through distinct stages:

    • Initial Mutation: Damages p53, removing its ability to arrest the cycle at the G1 checkpoint.

    • Second Mutation: May enhance cellular proliferation, leading to tumor formation.

    • Third Mutation: Further accelerates growth.

    • Metastasis: Finally, additional mutations can enable invasive growth and spread of cancer cells throughout the body.

  • Early detection through cancer screenings is crucial to manage and treat cancers effectively before metastasis occurs.

Conclusion

  • Understanding the intricate mechanisms of the cell cycle, including regulating checkpoints, the role of proteins like p53, and the consequences of their failure is vital for appreciating how cancer develops and progresses.