Cell Cycle

Cellular Messaging

Cell Communication & Signaling - Essential for both unicellular and multicellular organisms to interact with their environment and each other.

Types of Signals - Light, touch, and primarily chemicals.

Purpose of Chemical Signals

  • Enable cells to respond to their environment.

  • Allow communication without physical contact.

  • Can act over short or long distances.

Importance of Studying Cell Communication

  • Helps in modifying pathways, developing drugs, controlling diseases, improving agriculture, and managing fruit ripening.

Consequences of Failed Communication

  • Abnormal development, diseases, cancer, or cell death.

Chemical Signals

Signal Molecules (Ligands) - Chemical messengers that enable cell communication.

Types of Ligands:

  • Growth Factors - Proteins that promote cell growth and division.

  • Neurotransmitters - Chemicals that transmit signals between nerve cells.

  • Hormones - Lipids or proteins that regulate physiological processes.

  • Peptides - Small proteins that act as signaling molecules.

Lipid-Soluble Ligands - Can cross the cell membrane and bind to intracellular receptors.

Lipid-Insoluble Ligands - Cannot cross the membrane and must bind to membrane receptors.

Types of Cell Communication

Local Signaling - Communication between adjacent cells.

  1. Direct Contact:

    • Cell Junctions - Allow molecules to pass directly between cells.

      • Gap Junctions (Animals)

      • Plasmodesmata (Plants)

    • Cell-Cell Recognition - Interaction between membrane-bound molecules (important in immune response and embryonic development).

  2. Paracrine Signaling - Cells release molecules that act on nearby target cells.

    • Example: Growth factors.

  3. Autocrine Signaling - Cells release signals that affect themselves.

    • Example: Cancer cells promoting their own growth.

  4. Synaptic Signaling - Nerve cells release neurotransmitters across a synapse.

  5. Endocrine (Hormone) Signaling - Hormones travel via the bloodstream to target distant cells.

Evolution of Cell Signaling

Yeast Mating Types - Yeast cells release mating factors (α and β) that bind only to receptors of the opposite type.

Long-Distance Communication

Endocrine System - Glands release hormones into the blood to regulate distant target cells.

Glands - Organs that secrete chemical substances (hormones).

Cell Signaling Stages

1. Reception - A ligand binds to a receptor protein inside or outside the cell.

2. Transduction - The receptor undergoes a shape change, triggering a signaling pathway.

3. Response - The cell produces a specific response (e.g., adrenaline increasing heart rate).

Signal Reception

Target Cells - Cells with receptors for a specific ligand.

Intracellular Receptors - Found inside the cell; bind small or nonpolar ligands (e.g., steroid hormones like testosterone).

Membrane Receptors - Located on the cell membrane; bind large or polar ligands.

Types of Membrane Receptors:

  1. G-Protein-Coupled Receptors (GPCRs)

    • Involved in vision, smell, growth, and embryonic development.

    • Activate G-proteins, which switch from GDP to GTP and trigger a signaling pathway.

    • Over 60% of modern drugs target GPCRs.

  2. Receptor Tyrosine Kinases (RTKs)

    • Activate multiple signal transduction pathways.

    • Help regulate cell growth and division.

  3. Ligand-Gated Ion Channels

    • Open/close in response to ligand binding, allowing ions (Na+, Ca2+) to pass.

    • Important in nervous and muscular system function.

Signal Transduction

Phosphorylation Cascade - A chain reaction where protein kinases activate other kinases.

Dephosphorylation - Phosphatases remove phosphate groups, deactivating proteins.

Signal Transduction Pathways - Act as molecular switches to regulate cell activity.

Secondary Messengers - Small molecules that help pass signals inside the cell.

  • cAMP (Cyclic AMP) - Made from ATP, activates protein kinases.

  • Calcium Ions (Ca2+) - Trigger cellular responses like muscle contraction and cell division.

Response (Final Step of Cell Signaling)

Cellular Responses:

  • Cytoplasmic or nuclear activity regulation.

  • Gene expression changes (transcription activation/inhibition).

Signal Amplification - A single signal molecule triggers multiple responses.

The Cell Cycle

Definition - The life cycle of a cell, from formation to division.

Phases:

  1. Interphase (90% of the cycle) - Cell growth and DNA replication.

    • G1 Phase - Growth, organelle duplication.

    • S Phase - DNA replication.

    • G2 Phase - Further growth, preparation for division.

  2. Mitotic (M) Phase - Division of the nucleus (mitosis) and cytoplasm (cytokinesis).

Mitosis (PMAT)

  1. Prophase - Chromosomes condense, spindle fibers form, nuclear membrane breaks down.

  2. Prometaphase - Chromosomes attach to spindle fibers.

  3. Metaphase - Chromosomes align at the metaphase plate.

  4. Anaphase - Sister chromatids separate and move to opposite poles.

  5. Telophase - Nuclear envelope reforms, chromosomes de-condense.

Cytokinesis:

  • Animal Cells - Cleavage furrow forms.

  • Plant Cells - Cell plate forms, creating a new cell wall.

Binary Fission (Prokaryotic Cell Division)

Process:

  1. Chromosome replication begins at the origin of replication.

  2. Daughter chromosomes move apart as the cell elongates.

  3. Plasma membrane pinches inward, dividing the cell.

Evolutionary Link - Mitosis likely evolved from binary fission.

Cell Cycle Regulation

Cell Cycle Control System - Regulates cell division using checkpoints.

Checkpoints:

  • G1 Checkpoint - Determines if the cell proceeds to S phase or enters G0 (non-dividing state).

  • G2 Checkpoint - Ensures proper DNA replication before mitosis.

  • M Checkpoint - Ensures chromosomes are properly aligned before anaphase.

G0 Phase - A non-dividing state for specialized cells (e.g., neurons and muscle cells).

Growth Factors - Proteins that signal cells to divide.