Untitled Quiz

Cell Signaling Overview

• Cell signaling is the transfer of information from outside a cell to obtain an internal response.
• Cells communicate using chemical signals, enabling communication over short or long distances.
• Four elements essential for cell communication:
  • Signaling cell (initiates signal).
  • Signaling molecule (carries signal).
  • Receptor molecule (captures signal).
  • Responding cell (receives signal).

Cell Signaling in Multicellular Organisms

• Multicellular organisms require the same four elements for cell communication.
• Examples include cells of the adrenal glands releasing epinephrine into the bloodstream, affecting target organs like the heart.
• E.g. adrenaline binds to receptors on heart muscle, increasing heart rate. This is part of the fight-or-flight response.

Cell Signaling in Bacteria

• Bacteria communicate to regulate their behavior, particularly related to DNA uptake.
• At low density, peptide levels are low, and genes for DNA uptake are turned off.
• High density stimulates elevated peptide levels, triggering DNA uptake.

Cell Signaling Steps

• Receptor activation: A signaling molecule (ligand) binds to a receptor protein.
• Signal transduction: The signal is transmitted into the cell.
• Cellular response: The cell responds (e.g., activates an enzyme or changes gene expression).
• Signal termination: The response is stopped to allow for new signals.

Types of Signals

• Endocrine signaling: Long-distance communication via bloodstream (hormones).
• Paracrine signaling: Short-distance communication between neighboring cells.
• Autocrine signaling: A cell secretes a signaling molecule that acts on the same cell.
• Contact-dependent signaling: Direct contact between cells, involving transmembrane proteins.

Cellular Receptors

• Intracellular receptors: Nonpolar signals can pass through the membrane & bind intracellular receptors in the cytosol or nucleus.
• Cell-surface receptors: Polar signaling molecules require surface receptors (e.g., G protein-coupled receptors, receptor kinases, ion channels).

G Protein-Coupled Receptors (GPCRs)

• GPCRs are common in eukaryotic cells.
• GPCR activation involves ligand binding, GDP replacement with GTP, and activation of a target protein. This causes a cellular response.
• E.g., epinephrine binding to a GPCR in heart muscle activates adenylyl cyclase, leading to cAMP production, increasing heart rate.

Signal Amplification

• A small signal from a ligand binding to a receptor can produce a large response.
• This is achieved by multiple steps in a signal transduction pathway.

Signal Termination

• Ligands detach from receptors, inactivating the receptors.
• G proteins convert GTP to GDP, becoming inactive.
• These processes stop cAMP formation and enzyme activation.

Receptor Kinases

• Receptor kinases need a ligand binding to activate.
• They dimerize and phosphorylate themselves, activating signal transduction pathways.
• Involved in various cellular processes like limb development, insulin signaling, and wound healing. The Ras protein is often activated in this pathway.

Ligand-Gated Ion Channels

• Channels allow specific ions to pass across the membrane in response to binding a ligand.
• This can cause rapid, short-term signal changes in cells.
• Crucial in muscle contractions, digestion, and brain function.

Cell Communities-Cell Structure and Function

• Cells in multicellular organisms work together as communities.
• Interactions are aided by the cytoskeleton and extracellular matrix (ECM).

The Cytoskeleton

• Provides internal support and facilitates movement within cells.
• Components include:
  • Microtubules: Largest, hollow tubes of tubulin dimers. Maintain cell shape, guide movement of organelles, form cilia & flagella.
  • Microfilaments: Smallest, double helix of actin monomers. Reinforce cell membrane, participate in cytokinesis and muscle contraction.
  • Intermediate filaments: Intermediate size, strong, cable-like polymers, provide mechanical strength.

Cell Junctions

• Adherens junctions: Belt-like cadherin complexes attached to actin filaments for cell-cell adhesion.
• Desmosomes: Button-like cadherin complexes linked to intermediate filaments, providing structural support.
• Hemidesmosomes: Half-desmosomes, integrin-based cell-ECM attachment.
• Tight junctions: Seal extracellular spaces between cells, regulating movement of substances.
• Gap junctions (connexons): Small channels allowing communication through cytoplasm and the passage of small molecules.
• Plasmodesmata: Plant cell counterparts to gap junctions, enabling extensive cell-cell communication.

Extracellular Matrix (ECM)

• Provides support and protection to cells and tissues.
• Animal ECM: Composed of proteins (e.g. collagen, elastin) and polysaccharides embedded in a gel. A more flexible type of support than a cell wall in plants. Collagen is the most abundant animal protein and often forms a strong triple helical structure.
• Plant ECM: The cell wall, made of middle lamella (carbohydrates binding the cells together), primary walls (cellulose, pectin), and secondary walls (rigid, cellulose, lignin).

DNA Replication and Cell Division

• Binary fission: Bacterial cell division with a single origin of replication proceeds bidirectionally.
• Eukaryotic cell cycle: Includes Interphase (G1, S, G2) and M phase (Mitosis and Cytokinesis).
• Mitosis: A process of nuclear division (karyokinesis). Produces two identical daughter cells. Has four stages: Prophase, pro-metaphase, metaphase, anaphase and telophase.
• Cytokinesis: A process of cytoplasmic division. Different in animal and plant cells. Animal cells use actin filaments to pinch and divide the cell; plant cells form a cell plate.
• Meiosis: Germ cell division, producing haploid gametes. Two divisions (meiosis I and meiosis II) yield four daughter nuclei. Meiosis I separates homologous chromosomes, and Meiosis II separates sister chromatids.
• Telomeres and telomerase: Telomeres protect ends of linear chromosomes; telomerase is an enzyme that maintains their length in specific cell types like germ and stem cells. If not maintained, DNA shortening could occur.
• Genome organization: Eukaryotic DNA is packaged with histones to form chromatin, which condense into chromosomes.
• Checkpoints: Control points in the cell cycle that ensure DNA is replicated correctly and that chromosomes are properly attached to the mitotic spindle.
• Apoptosis: Programmed cell death; key to development and tissue homeostasis. Imbalances in proteins like p53 can lead to uncontrolled cell division in cancer.
• Cancer: Cancer develops from uncontrolled cell division, caused by mutations in proto-oncogenes which promote cell division and in tumor suppressor genes that inhibit division.