Characterizing and Classifying Viruses, Viroids, and Prions - CH 13

Characterizing and Classifying Viruses, Viroids, and Prions

General Information about Viruses

Viruses are unique infectious agents that require a host cell to replicate. They are considered obligate intracellular parasites because they cannot reproduce or carry out metabolic processes outside a living cell.

• Intracellular parasites: Viruses must infect a host cell to replicate their genetic material and produce new virus particles.

Types of Viruses (Based on Host Cell)

Viruses are classified according to the type of host cell they infect.

• Animal viruses: Infect animal cells, including humans.

• Bacteriophages (phages): Infect bacterial cells.

Anatomy of Viruses

Viruses have a simple structure, typically consisting of the following components:

• Receptors: Surface molecules that allow the virus to attach to specific host cells.

• Envelope: A lipid membrane derived from the host cell, present in some viruses (enveloped viruses).

• Capsid: A protein shell that encases and protects the viral genetic material.

• Genetic material: Can be either DNA or RNA, single-stranded or double-stranded.

Replication Scheme of a Bacteriophage – Lytic Cycle

Bacteriophages (viruses that infect bacteria) can replicate via the lytic cycle, which results in the destruction of the host cell.

1. Attachment: The phage attaches to the bacterial cell using specific receptors.

2. Entry: The phage injects its genetic material into the host cell, often using lysozyme to drill a hole through the bacterial cell wall.

3. Synthesis: The host cell's machinery is hijacked to replicate the phage genome and synthesize capsid proteins.

4. Assembly: New bacteriophage particles are assembled inside the host cell.

5. Release: The host cell bursts (lyses), releasing new phage particles.

Note: In the lysogenic cycle, the bacteriophage integrates its genome into the host's DNA and remains dormant after entry, without immediately destroying the host cell.

Replication Scheme of an Animal Virus – Lytic Cycle

Animal viruses can also replicate via a lytic cycle, with some differences in entry and release mechanisms compared to bacteriophages.

1. Attachment: The virus binds to specific receptors on the animal cell surface.

2. Entry: Entry occurs by endocytosis (for non-enveloped viruses) or by fusion with the cell membrane (for enveloped viruses).

3. Uncoating: The viral capsid is removed, releasing the genetic material into the host cell.

4. Synthesis: The host cell's machinery is used to replicate the viral genome and synthesize viral proteins.

5. Assembly: New virus particles are assembled within the host cell.

6. Release: Non-enveloped viruses are released by cell lysis, while enveloped viruses are released by budding from the cell membrane.

Note: In the lysogenic (latent) cycle, the animal virus becomes dormant after uncoating, integrating into the host genome or remaining in the cell without causing immediate lysis.

Viruses and Cancer

Some viruses can contribute to the development of cancer by integrating their genetic material into the host cell's genome, potentially disrupting normal cell regulation and leading to uncontrolled cell growth.

• Oncogenic viruses: Viruses that can induce cancer formation (e.g., Human papillomavirus, Epstein-Barr virus).

Culturing Viruses

Viruses require living cells for replication, so they are cultured in systems that provide suitable host cells.

• Bacteria: Used for culturing bacteriophages.

• Plants: Used for plant viruses.

• Tissue culture: Used for animal viruses, including human viruses.

Prions

Prions are infectious proteins that lack nucleic acids. They can cause neurodegenerative diseases by inducing abnormal folding of normal cellular proteins.

• Infectious proteins: Prions are composed solely of protein, with no genetic material.

• Diseases: Prion diseases include Mad Cow Disease (Bovine Spongiform Encephalopathy), Creutzfeldt-Jakob disease, and others.