Viruses

Introduction to Microbiology and Viruses


What do you already know?

  • Key Questions:

    • Differences between viruses and bacteria?

    • Are all bacteria harmful?

    • Should antibiotics be taken for a cold?

    • Best practices to avoid getting sick?


Microbiology

  • Definition: The study of microscopic organisms including:

    • Viruses

    • Bacteria (Archaea and Eubacteria)

    • Protists (e.g., paramecium)

    • Some Fungi (e.g., yeast)


Viruses

  • Characteristics:

    • Non-cellular particles

    • Composed of genetic material (either DNA or RNA) and protein

    • Ability to invade all types of life forms: animals, plants, and microorganisms (e.g., eubacteria, archaea)

  • Examples of viruses: HIV, Herpes, Influenza


Structure of Viruses

  • Lack cellular structures like:

    • Cell membrane

    • Membrane-bound organelles (nucleus, mitochondria, Golgi apparatus)

  • Inactivity outside a host cell

    • Viruses are entirely inert until they invade a host cell


Components of Viruses

  1. Capsid

    • Protein coat providing protection for genetic material

  2. Genetic Material (DNA/RNA)

    • Instructions for replication within the host cell

  3. Enzymes

    • Facilitate viral entry and takeover of the host cell

  4. Specific Envelope Proteins

    • Allow virus attachment to specific host cells, e.g., bacteriophages have tail fibers for this purpose


Viral Life Cycle

Lytic Cycle

  • Definition: Involves takeover of the host cell to produce new viruses.

Steps:

  1. Attachment: Virus connects to host cell surface.

  2. Entry: Virus injects genetic material, using an enzyme to weaken entry point.

  3. Replication & Assembly: Viral genetic information commandeers host machinery to create virus components.

  4. Lysis: Host cell bursts, releasing new viruses and continuing the cycle.

  5. Virulence: Indicates the infectious capacity of a virus based on its ability to complete the lytic cycle.


Lysogenic Cycle

  • Definition: Host cell remains unaltered, viral DNA integrates into host DNA (forming a Prophage).

Steps:

  1. Integration: Viral genetic material merges into the host DNA, generally non-destructive.

  2. Cell Multiplication: Host cell divides normally, copying viral DNA alongside.

  3. Latency: Prophage remains inactive while the host cell continues to function.

  4. Activation: Triggers (like stress) may lead prophage to exit and switch to lytic cycle.


Viruses: Living or Non-Living?

  • Characteristics of Life:

    • Growth: NO for viruses, YES for cells.

    • Homeostasis: NO for viruses, YES for cells

    • Metabolism: NO for viruses, YES for cells.

    • Genetic Material: BOTH have DNA/RNA

    • Reproduction: Only with a host for viruses, Cell division for cells

  • Conclusion: Viruses are considered NON-LIVING.


Viruses and Diseases

  • Examples of viral diseases include:

    • Herpes simplex

    • Chicken pox

    • HIV

    • Hepatitis

  • Viral Specificity: Certain viruses infect specific hosts only.


The Body’s Defense Against Viruses

Primary Defense

  • Prevents virus entry:

    • Skin (barrier unless breached)

    • Mucous membranes in respiratory tract (trap pathogens)

    • Hydrochloric acid in stomach (destroys viruses)

Secondary Defense

  • Immune response:

    • White blood cells (WBCs) produced (lymphocytes, phagocytes)

    • Phagocytes engulf and destroy intruders.

Tertiary Defense (Antibody-Mediated Immunity)

  • Produced by lymphocytes: Antibodies target specific pathogens.

  • Interferons: Proteins from infected cells signaling nearby cells to prepare an immune response.


Oncogenic Viruses

  • Definition: Viruses that can cause cancer.

  • Mechanism: Insert DNA into host cell and can disrupt cell cycle regulation.

  • Tumor Types:

    • Benign: Dormant, non-invasive cells.

    • Malignant: Active, invasive cancer cells.


Gene Therapy

  • Innovative method potentially curing genetic diseases by delivering healthy genes into patient cells using viruses.

Process:

  1. Alter virus to eliminate harmful effects.

  2. Insert healthy gene into viral vector.

  3. Virus introduces new gene into patient's cells, promoting production of necessary proteins.

  4. Example: Cystic Fibrosis, leading to blockage in lungs, intestines due to thick mucus.


Diagram of Gene Therapy Process

  • Steps include:

    • Virus binding to cell and releasing new gene into nucleus for gene expression

    • Ensuring altered cells can produce desired proteins and are injected back into the patient's body.

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