AP Biology Unit 4: Cell Communication and Cell Cycle

Cell Communication

• Cell communication involves three steps: reception, transduction, and response.
• Reception: A signaling molecule (ligand) binds to a receptor protein, causing a conformational shape change.
  • Receptor types include:
    • G protein-coupled receptors (GPCRs): Bind GTP to activate a G protein, initiating a transduction pathway.
    • Ligand-gated ion channels: Ligand binding causes a conformational shape change, opening/closing the ion channel, allowing ion flow.
    • Tyrosine receptor kinases (RTKs): Two receptors dimerize, kinases add phosphates to tyrosine residues, initiating multiple transduction pathways.
  • Ligands can be:
    • Steroid hormones (e.g., testosterone, estrogen): Nonpolar, bind to intracellular receptors.
    • Protein hormones (e.g., insulin): Polar, bind to extracellular membrane receptors. Protein hormones are released via exocytosis.

Transduction

• Transduction is the intermediate step between reception and response.
• It's a signal transduction pathway often involving a cascade (like dominoes falling).
• The ligand itself does not directly participate in transduction.
• Transduction methods:
  • Phosphorylation cascade: Enzymes (kinases) add phosphate groups to relay molecules in a chain reaction.
  • Secondary messengers (e.g., calcium, cAMP): Small molecules that relay signals within the cytoplasm.
    • cAMP is produced from ATP by adenyl cyclase.

Response

• Response is the final effect of the signal transduction pathway.
• Examples include:
  • Cell growth
  • Secretion of molecules
  • Modification of gene expression (turning genes on/off)
  • Altering cell function/phenotype
  • Apoptosis (programmed cell death)
• Receptors can be enzymes (e.g., RTKs) or just proteins.
• cAMP structure differs from ATP (one less phosphate, cyclic structure).

Cell Cycle

• The cell cycle is the life cycle of a cell from its origin to division.
• It comprises interphase (G1, S, G2) and the mitotic phase (mitosis and cytokinesis).
• Interphase:
  • G1 (Gap 1): Cell growth, organelle duplication, protein synthesis, signal transduction.
  • S (Synthesis): Replication of DNA and centrosomes.
  • G2 (Gap 2): Cell growth, protein/RNA synthesis, reorganization of cellular components.
• Mitotic phase:
  • Mitosis (prophase, metaphase, anaphase, telophase): Nuclear division.
  • Cytokinesis: Cytoplasmic division, separating the two cells.

Cell Cycle Regulation

• Checkpoints ensure proper cell cycle progression:
  • G1 checkpoint: Determines if cell should divide. Checks for growth factors, resources, damage.
  • G2 checkpoint: Verifies DNA replication completeness and accuracy. Activates p53 if needed, stops cell cycle for repair.
  • M checkpoint: Ensures sister chromatids are attached to microtubules before separation.
• Genes regulating cell cycle:
  • Proto-oncogenes: Normal genes promoting cell division. If mutated, become oncogenes. Oncogenes can cause uncontrolled cell division.
  • Tumor suppressor genes: Normally halt cell cycle if damage detected (e.g., p53). If mutated, cell growth can occur with errors.
  • Cyclin-dependent kinases (Cdks): Enzymes that regulate the cell cycle by phosphorylation.

Clinical Examples

• Cell communication pathways featured in previous exam questions demonstrate analysis of experimental results.
• Understanding of feedback mechanisms (negative and positive) is crucial for answering questions on homeostasis.
• Specific questions regarding signal transduction pathways demonstrate analysis of component effects (e.g., inhibitors).