📚Study Guide: Cell Communication and Cell Cycle
Unit 4: Cell Communication and Cell Cycle
Cells do not exist in isolation; they constantly communicate with each other to coordinate growth, defense, metabolism, and reproduction. This unit explores the mechanisms of cell signaling, from direct contact to long-distance hormonal signaling, and examines how signals are transduced into cellular responses through phosphorylation cascades and second messengers like cAMP. Understanding signal transduction pathways is essential because disruptions in these pathways can lead to cancer and other diseases. The second half of this unit covers the cell cycle, including mitosis and meiosis, and the regulatory mechanisms that ensure accurate cell division. Checkpoints at G1, S, and G2/M phases prevent damaged DNA from being replicated or passed to daughter cells. Cyclins and cyclin-dependent kinases (CDKs) regulate progression through these checkpoints. Cancer arises when these regulatory mechanisms fail, leading to uncontrolled cell division. On the AP exam, students are frequently asked to predict the effect of mutations in signaling proteins or to compare and contrast mitosis and meiosis, making this unit both conceptually dense and exam-critical.
Key Concepts
- Types of Cell Signaling: Autocrine (same cell), paracrine (local nearby cells), endocrine (long-distance via hormones), synaptic (neurotransmitters across synapses), and direct contact (gap junctions, cell surface molecules).
- Signal Transduction Pathway: Reception (ligand binds receptor) -> Transduction (signal relay through proteins, often kinases) -> Response (cellular activity changes). Amplification occurs at each step.
- Second Messengers: Small, non-protein molecules that relay signals inside the cell. cAMP activates protein kinase A (PKA). Calcium ions (Ca2+) and IP3/DAG are also critical second messengers.
- Cell Cycle Phases: Interphase (G1, S, G2) and M phase (mitosis: prophase, metaphase, anaphase, telophase; cytokinesis). DNA replication occurs in S phase; cell growth in G1 and G2.
- Checkpoints and Regulation: G1 checkpoint assesses cell size and DNA damage. S checkpoint ensures DNA replication is complete. G2/M checkpoint verifies DNA replication and damage repair. MPF (maturation-promoting factor, composed of cyclin and CDK) triggers entry into mitosis.
- Mitosis vs. Meiosis: Mitosis produces two genetically identical diploid daughter cells for growth and repair. Meiosis produces four genetically unique haploid gametes for sexual reproduction. Meiosis includes crossing over (prophase I) and independent assortment (metaphase I).
Vocabulary
- Ligand: A signaling molecule that binds to a specific receptor protein to initiate a cellular response.
- Receptor Tyrosine Kinase (RTK): A membrane receptor that phosphorylates tyrosine residues on itself and other proteins, often initiating growth factor signaling pathways.
- Protein Kinase: An enzyme that transfers phosphate groups from ATP to target proteins, activating or deactivating them.
- Apoptosis: Programmed cell death; a controlled process essential for development and maintenance, distinguishable from necrosis (uncontrolled cell death).
- Oncogene: A mutated gene that has the potential to cause cancer, often encoding a hyperactive version of a protein involved in cell growth signaling.
- Tumor Suppressor Gene: A gene that normally inhibits cell division or promotes DNA repair; loss-of-function mutations can lead to cancer (e.g., p53, Rb).
Processes and Diagrams to Know
- G-Protein Coupled Receptor (GPCR) Pathway: Ligand binds GPCR -> G-protein exchanges GDP for GTP -> activates adenylyl cyclase -> produces cAMP -> activates PKA -> phosphorylates target proteins.
- Cell Cycle Diagram: Be able to draw and label G1, S, G2, M phases, and indicate where checkpoints occur.
- Mitosis Stages: Prophase (chromosomes condense, spindle forms), Metaphase (chromosomes align at equator), Anaphase (sister chromatids separate), Telophase (nuclear envelopes reform).
Experimental Designs
- Signal Transduction Investigation: Treating cells with varying concentrations of a hormone and measuring the production of a second messenger (cAMP) to demonstrate dose-response relationships.
- Cell Cycle Analysis: Using flow cytometry or DNA staining to determine the proportion of cells in G1, S, and G2/M phases under different conditions.
Common Mistakes
- Confusing Kinases and Phosphatases: Kinases add phosphate groups (activate usually); phosphatases remove phosphate groups (deactivate usually). Remember "kinases keep adding."
- Thinking All Receptors Are on the Cell Surface: Steroid hormones (lipid-soluble) pass through the membrane and bind intracellular receptors that act as transcription factors.
- Confusing Mitosis and Meiosis I: In meiosis I, homologous chromosomes separate; in mitosis, sister chromatids separate. Meiosis II is similar to mitosis but with haploid cells.
- Forgetting Crossing Over: Genetic recombination occurs during prophase I of meiosis, not during mitosis or meiosis II.
AP Exam Strategies
- Draw the Pathway: For signal transduction FRQs, sketch the receptor, G-protein/kinase cascade, and final response. Label each step clearly.
- Use "If...Then...Because" Reasoning: If a kinase is mutated and cannot be activated, then the pathway stops because phosphorylation cannot occur.
- Compare Explicitly: When comparing mitosis and meiosis, use a table or structured paragraph with direct comparisons (e.g., "In mitosis, sister chromatids separate, whereas in meiosis I, homologous chromosomes separate").
- Explain Cancer at the Molecular Level: Don't just say "cells divide too much." Explain that a mutation in a proto-oncogene creates an oncogene that constantly signals cell division, or that a mutation in a tumor suppressor removes the 'brakes' on division.
Real-World Applications
- Cancer Therapy: Targeted therapies like imatinib (Gleevec) block specific tyrosine kinases in cancer cells, demonstrating the clinical importance of understanding signal transduction.
- Birth Defects: Errors in meiosis (nondisjunction) cause aneuploidies such as Down syndrome (trisomy 21), Turner syndrome (monosomy X), and Klinefelter syndrome (XXY).
- Diabetes: Type 2 diabetes involves insulin resistance, where target cells fail to respond properly to insulin signaling, illustrating the consequences of disrupted cell communication.