Cell Communication and the Cell Cycle: Study Notes

Cell Communication

Quorum Sensing and Cell Signaling

Cells communicate with each other to coordinate behavior, respond to environmental changes, and regulate growth and development. In bacteria, quorum sensing allows populations to act collectively once a critical density is reached.

• Quorum sensing: Bacteria release signaling molecules (autoinducers) to coordinate behavior such as biofilm formation and sporulation.

• Paracrine signaling: Local signaling where cells secrete molecules that affect nearby cells.

• Example: Bacterial biofilms and spore dispersal.

Mating in Yeast Cells

Yeast cells use chemical signaling to identify and mate with cells of the opposite type.

• Each mating type secretes a mating factor that binds to receptors on the other type.

• Binding leads to fusion and genetic recombination.

Local and Long-Distance Signaling

• Local regulators: Signaling molecules that affect nearby cells (e.g., neurotransmitters).

• Synaptic signaling: Occurs in the nervous system; neurotransmitters cross synapses to stimulate target cells.

• Hormones: Used for long-distance signaling; secreted by endocrine glands and travel via the circulatory system.

• Plant hormones: Vary in size and function; can act locally or systemically.

Signal Reception and Transduction

Signal Reception

Cells detect signals using receptor proteins that bind specific signaling molecules (ligands).

• Receptors: Proteins on the cell surface or inside the cell that recognize and bind ligands.

• G-protein coupled receptors (GPCRs): Largest family of cell-surface receptors; involved in many physiological processes.

• Steps (GPCRs):

  1. G protein is inactive when bound to GDP.

  2. Ligand binding activates the receptor, which activates the G protein by exchanging GDP for GTP.

  3. Activated G protein interacts with an enzyme, triggering a cellular response.

  4. G protein hydrolyzes GTP to GDP, returning to inactive state.

• Receptor tyrosine kinases (RTKs): Enzyme-linked receptors that phosphorylate tyrosine residues on themselves and other proteins.

• Steps (RTKs):

  1. Ligand binding causes dimerization of two receptor monomers.

  2. Each monomer phosphorylates the other, activating the receptor.

  3. Activated RTKs trigger multiple signal transduction pathways.

• Ion channel receptors: Ligand binding opens or closes ion channels, altering cell membrane potential (common in the nervous system).

Intracellular Receptors

• Located in the cytoplasm or nucleus.

• Small or hydrophobic ligands (e.g., steroid hormones) can cross the plasma membrane and bind these receptors.

• Activated receptors often act as transcription factors, regulating gene expression.

Signal Transduction Pathways

Phosphorylation Cascade

Signal transduction often involves a series of protein kinases that phosphorylate each other in sequence, amplifying the signal.

• Protein kinase: Enzyme that transfers phosphate groups from ATP to proteins.

• Protein phosphatase: Enzyme that removes phosphate groups, turning off the signal.

• Abnormal kinase activity: Can lead to diseases such as cancer.

Second Messengers

Small, non-protein molecules that relay signals inside the cell.

• cAMP (cyclic AMP): Synthesized from ATP by adenylyl cyclase; activates protein kinase A.

• Ca2+ ions: Released from the endoplasmic reticulum; involved in muscle contraction, secretion, and cell division.

• IP3 and DAG: Produced by cleavage of membrane phospholipids; IP3 triggers Ca2+ release.

Example: cAMP Pathway

1. Ligand binds to GPCR.

2. G protein activates adenylyl cyclase.

3. Adenylyl cyclase converts ATP to cAMP.

4. cAMP activates protein kinase A, leading to a cellular response.

Cellular Responses to Signals

• Regulation of gene expression: Turning specific genes on or off.

• Regulation of protein activity: Modifying existing proteins.

• Changes in cell shape or movement: Via cytoskeletal rearrangement.

Specificity and Coordination

• Different cells respond differently to the same signal due to variations in receptor types and intracellular pathways.

• Scaffolding proteins organize signaling components for efficiency.

• Branching pathways and cross-talk allow integration of multiple signals.

Apoptosis (Programmed Cell Death)

• Triggered in cells that are damaged or no longer needed.

• Involves activation of proteases (e.g., caspases) that dismantle the cell.

• Example: In C. elegans, the death signal activates a cascade leading to cell death.

The Cell Cycle

Definitions

• Genome: All of a cell's DNA.

• Chromosomes: Packaged DNA molecules.

• Chromatin: DNA and associated proteins.

• Somatic cells: Body cells (diploid).

• Gametes: Reproductive cells (haploid).

Chromosome Duplication and Distribution

• During S phase, each chromosome is duplicated.

• Sister chromatids are joined at the centromere.

• During mitosis, sister chromatids separate into two daughter cells.

Phases of the Cell Cycle

• Interphase: 90% of the cycle; includes G1, S, and G2 phases.

  • G1 phase: Growth and metabolic activity.

  • S phase: DNA synthesis and chromosome duplication.

  • G2 phase: Preparation for mitosis; centrosomes duplicate.

• Mitotic (M) phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis

• Prophase: Chromatin condenses, spindle forms, nucleoli disappear.

• Prometaphase: Nuclear envelope fragments, spindle microtubules attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

• Cytokinesis: Cytoplasm divides, forming two daughter cells.

Mitotic Spindle

• Composed of microtubules and associated proteins.

• Centrosomes organize spindle formation.

• Kinetochores attach chromosomes to spindle fibers.

Cytokinesis

• In animals: Occurs by cleavage furrow (actin-myosin ring contracts).

• In plants: Vesicles form a cell plate that becomes the new cell wall.

Binary Fission

• Prokaryotic cell division; DNA replicates, cell elongates, and divides into two.

• Evidence suggests mitosis evolved from binary fission.

Cell Cycle Control System

• Regulated by checkpoints (G1, G2, M) that ensure proper division.

• Cyclins and cyclin-dependent kinases (Cdks): Proteins that regulate the cell cycle.

• Progression depends on synthesis and degradation of cyclins.

Table: Comparison of Mitosis and Binary Fission

Feature	Mitosis	Binary Fission
Organisms	Eukaryotes	Prokaryotes
Chromosome Structure	Linear, multiple	Circular, single
Spindle Apparatus	Present	Absent
Process	Multiple phases (prophase, metaphase, etc.)	Simple division after DNA replication
Examples	Animal and plant cells	Bacteria

Apoptosis and Disease

• Defects in apoptosis can lead to diseases such as cancer or degenerative disorders.

• Proper regulation ensures removal of damaged or unnecessary cells.

Key Terms and Definitions

• Genome: Complete set of genetic material in an organism.

• Chromosome: DNA molecule with associated proteins.

• Chromatin: DNA-protein complex forming chromosomes.

• Somatic cell: Any cell other than a gamete.

• Gamete: Reproductive cell (sperm or egg).

• Centromere: Region joining sister chromatids.

• Kinetochores: Protein structures on centromeres for spindle attachment.

• Checkpoints: Control points in the cell cycle.

• Cyclin: Regulatory protein controlling cell cycle progression.

• Cdk: Cyclin-dependent kinase; enzyme that phosphorylates target proteins.

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard biology textbooks.