The Cell Cycle

Overview of the Cell Cycle

The cell cycle is a series of events that cells go through as they grow and divide. It is essential for growth, development, and tissue repair in multicellular organisms.

• Phases of the Cell Cycle:

  • G1 phase (First Gap): Cell grows and carries out normal functions.

  • S phase (Synthesis): DNA is replicated.

  • G2 phase (Second Gap): Cell prepares for division.

  • M phase (Mitosis): Division of the nucleus and cytoplasm (cytokinesis).

• Interphase: Consists of G1, S, and G2 phases; the cell spends most of its time here.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Example: In early embryonic development, cells rapidly cycle through these phases to increase cell number.

Regulation of the Cell Cycle

Variation in Cell Cycle Length

Cell cycle length can vary greatly among different cell types and in response to environmental conditions.

• Rapidly dividing cells: Have a short G1 phase (e.g., embryonic cells).

• Non-dividing cells: Some cells exit the cycle and enter a resting state called G0 (e.g., neurons, muscle cells).

• Environmental response: Cell cycle can adjust to external signals and conditions.

Conclusion: The cell cycle is tightly regulated to ensure proper cell division and function.

Discovery of Cell-Cycle Regulatory Molecules

Key discoveries in the 1970s and 2001 Nobel Prize-winning research identified molecules that regulate the cell cycle, particularly the transition into mitosis.

• Maturation-Promoting Factor (MPF): A complex that induces mitosis in eukaryotic cells.

• Key researchers: Leland H. Hartwell, Tim Hunt, and Sir Paul Nurse.

What is MPF?

MPF (Maturation-Promoting Factor) is a dimer composed of two distinct proteins:

• Cyclin: A regulatory protein whose concentration fluctuates throughout the cell cycle.

• Protein kinase (Cdk): An enzyme that transfers phosphate groups from ATP to specific substrates, regulating their activity.

Definition: A kinase is an enzyme that catalyzes the transfer of phosphate groups, often regulating protein function.

Regulation of MPF Activity

The activity of MPF is controlled by the concentration of cyclin and the phosphorylation state of the Cdk subunit.

• Activation: Cdk has two phosphorylation sites. Both are phosphorylated after cyclin binds, but MPF is inactive until one phosphate is removed in late G2 phase, activating the kinase.

• Function: Active MPF phosphorylates proteins to initiate mitosis (e.g., chromosome condensation, spindle formation).

• Deactivation: During anaphase, cyclin is tagged for destruction, leading to MPF inactivation. This is an example of negative feedback regulation.

Equation:

Practice: Cyclin, Cdk, and MPF Activity by Cell Stage

Cell-Cycle Checkpoints

Definition and Importance

Checkpoints are critical control points where regulatory molecules determine whether the cell proceeds to the next stage or arrests.

• Purpose: Ensure that each phase is completed correctly before moving on.

• Key checkpoint: MPF activation is crucial for the G2 checkpoint, allowing entry into mitosis.

Major Cell-Cycle Checkpoints

Role of p53 in DNA Damage Response

p53 is a protein that acts as a tumor suppressor by regulating the cell cycle in response to DNA damage.

• Activation: Stabilized when DNA damage is detected.

• Function: Activates proteins to pause the cell cycle and repair damage; can initiate apoptosis if damage is irreparable.

• Significance: Acts as a "brake" on the cell cycle; mutations in p53 are common in cancer.

Growth Factors and the G1 Checkpoint

Regulation by External Signals

The G1 checkpoint is the first and most important checkpoint, determining whether a cell will divide or enter a non-dividing state.

• Growth factors: External signals that promote cell cycle progression by activating specific cyclins and Cdks (distinct from MPF).

• G1 Cyclin and Cdk: Regulate passage through the G1 checkpoint.

Cancer and Cell Cycle Regulation

What is Cancer?

Cancer is a group of diseases characterized by uncontrolled cell division, invasion of nearby tissues, and the ability to spread (metastasize) to other parts of the body.

• Prevalence: About 40% of Americans will develop cancer.

• Types: Over 200 types, including prostate, breast, lung, colon, melanoma, and more.

• Origin: All cancers arise from cells in which cell-cycle checkpoints have failed.

Properties of Cancer Cells

• Benign tumors: Noncancerous, noninvasive.

• Malignant tumors: Cancerous, invasive, can metastasize via blood or lymph.

Genetic Defects in Cancer

Cancer cells often have two types of defects related to cell cycle control, especially at the G1 checkpoint:

• Oncogenes (the "gas"): Mutations activate proteins that promote cell division when they should not be active.

• Tumor suppressor genes (the "brakes"): Mutations inactivate proteins that normally stop the cell cycle (e.g., p53).

• Somatic mutations: Most cancer-causing mutations arise in somatic (non-germline) cells.

Common Cancer Mutations at the G1 Checkpoint

• Overproduction of G1 cyclin: Permanently activates Cdk, leading to continuous cell cycle progression.

• Defective Rb protein: Fails to bind E2F, allowing uncontrolled entry into S phase.

• Defective p53: DNA damage is not repaired, leading to rapid mutation accumulation (found in about half of all cancers).

Summary Table: Key Proteins in Cell Cycle Regulation

Additional info: For further study, see recommended videos on cancer biology and mutation causes (links provided in original material).