Chapter 6

  • Viruses: Infectious agents that can replicate only inside the living cells of organisms, consisting of genetic material encased in a protein coat.

  • Viroids: Short, circular RNA molecules that infect plants and can disrupt regular plant growth processes.

  • Prions: Misfolded proteins that can induce other proteins to misfold, often leading to neurodegenerative diseases.

6.1 Overview of Viruses

Definition of Viruses

  • Viruses: Very small, infectious particles that only replicate inside the living cells of a host organism.

Characteristics of Viruses

  • Obligate Intracellular Parasites: Require a host cell to multiply.

  • Size: Very small, usually classified as ultramicroscopic (most <0.2 μm), requires an electron microscope for visualization.

  • Non-Cellular Nature: Not cellular in nature; referred to as infectious particles not as organisms.

  • Infection Spectrum: Infect every type of cell.

  • Compact Structure: The structure is very compact and economical, not fulfilling independent characteristics of life.

6.2 General Size of Viruses

Size Range of Various Viruses

  • Ultramicroscopic: Most viruses are less than 0.2 μm.

  • Examples of Virus Sizes:

    • Mimivirus: 500 nm

    • Poxvirus: 400 nm

    • Herpes Simplex Virus: 150 nm

    • Rabies Virus: 125 nm

    • Influenza Virus: 100 nm

    • HIV: 110 nm

    • Adenovirus: 95 nm

    • Poliomyelitis Virus: 30 nm

    • Yellow Fever Virus: 22 nm

  • Comparison with Other Biological Entities:

    • Bacterial Cells:

    • Escherichia coli (E. coli): 2 μm long

    • Bacterial dimension examples such as Streptococcus (1 μm wide).

    • Hemoglobin Molecule: 15 nm for reference size.

6.3 General Structure of Viruses

Structural Components of Viruses

  • Simple Structure: Viruses contain only the parts needed to invade and control a host cell.
    Virus Particle Structure:

  1. Central Core: Contains nucleic acid molecules (either DNA or RNA).

  2. External Coating:

    • Capsid: A protein shell made of capsomers that covers and protects the genetic material.

    • Envelope (not found in all viruses): An additional protective covering made from a small amount of the host cell’s membrane.

Types of Viruses Based on Structure

  • Naked Virus: Composed of nucleic acid and a capsid, without an envelope.

  • Enveloped Virus: A virus with an outer lipid membrane (envelope).

  • Spikes: Protruding molecules important for viral attachment to host cells.

6.4 Modes of Viral Replication

Viral Multiplication Phases

  • Viruses undergo 6 general phases during an animal viral life cycle:

    1. Adsorption: Virus attaches to host cell by binding of its spikes to specific cell receptors.

    2. Penetration: The virus is engulfed by the cell membrane.

    3. Uncoating: Release of viral capsid and nucleic acid into the cytoplasm.

    4. Synthesis: Under the control of viral genes, the cell synthesizes components of new viruses.

    5. Assembly: Newly produced viral components are assembled.

    6. Release: Assembled viruses exit the host cell by budding off or through exocytosis.

Step-by-Step Breakdown of Phases

Step 1: Adsorption

  • Adsorption: Coincidentally, the virus collides with a susceptible host cell, causing spikes to bind to receptor sites on the membrane.

  • Host Range: Different viruses have specific cells they can infect, e.g.,

    • Hepatitis B: Targets human liver cells.

    • Rabies: Infects various cells of many mammals.

    • Coronaviruses: Bind to ACE-2 receptors on human cells.

    • Adenoviruses: Have surface molecules that connect with epithelial membranes.

Step 2: Penetration

  • Penetration: The process through which the virus enters the host cell and delivers its nucleic acid.

    • Modes of entry:

    • Fusion (for envelope viruses): The viral envelope fuses directly with the host cell membrane.

    • Endocytosis (for either enveloped or naked viruses): The entire virus is taken in via a vesicle or endosome.

Step 3: Uncoating

  • Uncoating: The viral nucleocapsid or nucleic acid is released into the cytoplasm after the vesicle membrane fuses with the viral envelope.

Step 4: Synthesis

  • Synthesis of Viral Components: The virus utilizes the host cell’s metabolic machinery to produce RNA molecules, capsomers, and spikes under the influence of viral genes.

Step 5: Assembly

  • Assembly: The process where mature virus particles are constructed from available viral components.

    • Initial Formation: Capsid is assembled as an empty shell to house the nucleic acid strands.

Step 6: Release

  • Release Mechanisms:

    1. Budding: Viruses bud off from the host membrane, acquiring a membrane coat in the process.

    2. Exocytosis: Viruses packaged within the ER or Golgi apparatus exit as a group.

Summary of General Phases

  • General Phases in Animal Virus Multiplication Cycle:

    • Adsorption | Penetration | Uncoating | Synthesis | Assembly | Release

6.5 Latent and Persistent Viral Infections

Persistent Infections

  • Definition: Persistent infections occur when the host cell harbors the virus without immediate lysis.

  • Duration: Can last for weeks or the host's lifetime and may experience periodic reactivations.

  • Latent State: The virus remains inactive or nonproductive for extended periods, e.g.,

    • Herpes Simplex Virus: Responsible for cold sores and genital herpes.

    • Herpes Zoster Virus: Associated with chickenpox and shingles.

6.6 Cytopathic Effects

Damage to Host Cells

  • Cytopathic Effects: Indicate cellular damage that alters microscopic appearance. This can include:

    • Disorientation of cells

    • Changes in cell shape or size

    • Intracellular changes

Table 6.3: Cytopathic Changes in Selected Virus-Infected Animal Cells

  • Smallpox Virus: Cells round up; inclusions appear in cytoplasm.

  • Herpesvirus: Cells fuse to form multinucleated syncytia with nuclear inclusions.

  • Adenovirus: Clumping of cells; nuclear inclusions evident.

  • Poliovirus: Causes cell lysis without inclusions.

  • Reovirus: Enlarges cells and causes vacuoles/inclusions in cytoplasm.

  • Influenza Virus: Causes rounding of cells without inclusions.

  • Rabies Virus: No shape change; forms cytoplasmic inclusions (Negri bodies).

  • Measles Virus: Causes formation of syncytia (multinucleate giant cells).

Viral Damage

  • Some viruses can enter host cells and permanently alter genetic material, resulting in cancers termed oncogenic.

  • Transformed Cells: Characterized by increased growth rates, chromosomal alterations, and the capacity for indefinite division.

  • Oncoviruses: Mammalian viruses responsible for initiating tumors, e.g.,

    • Papillomavirus: Linked to cervical cancer.

    • Epstein-Barr Virus: Associated with Burkitt's lymphoma.

6.7 Treating Viruses

Challenges in Viral Treatment

  • Due to their nature, viruses are difficult to treat as antibiotics primarily target bacteria, not viruses.

  • Antivirals: Drugs targeting specific steps in the viral life cycle, e.g.,

    • HIV Drugs: Block the fusion of HIV with host cells or interfere with its release from the host.

  • Vaccines: Stimulate the immune response against viruses; while valuable, they are limited.

6.8 Prions

Definition and Mechanism

  • Prions: PRoteinaceous INfectious particles; noncellular infectious agents distinct from viruses.

  • Infection Mechanism:

    1. Prion proteins infect nerve cells.

    2. Normal proteins change their conformation upon contact and convert into prions.

    3. Prions aggregate to form long chains.

    4. These chains disrupt normal cell function, leading to cellular destruction.

Diseases Caused by Prions

  • Prions cause diseases known as transmissible spongiform encephalopathies (TSEs).

  • Transmission: Spread by direct contact or contaminated food, resulting in a spongiform appearance in nervous tissue.

  • Examples:

    • Bovine Spongiform Encephalopathy (BSE): Known as mad cow disease, exhibits behavioral changes, movement issues, and abnormal posture.

    • Creutzfeldt-Jakob Disease (CJD): Develops from consuming BSE-infected cattle, leading to rapid dementia, memory loss, confusion, and behavioral alterations.