Viruses, Viroids, and Prions: Structure, Replication, and Pathogenicity

Distinctive Features of Viruses

General Characteristics

• Viruses are obligate intracellular parasites, meaning they can only reproduce inside living host cells.

• They require a living host cell to replicate.

• Viruses contain either DNA or RNA as their genetic material, but never both.

• They are surrounded by a protein coat called a capsid.

• Viruses lack cellular structures such as ribosomes and do not possess ATP-generating mechanisms.

• They use the host cell’s biosynthetic machinery to multiply.

Host Range

Definition and Determinants

• Host range refers to the spectrum of host cells a virus can infect.

• Determined by:

  • Specific host attachment sites (receptors) on the cell surface

  • Cellular factors necessary for viral replication

Bacteriophages

Viruses That Infect Bacteria

• Bacteriophages (or phages) are viruses that infect bacteria.

• Attachment sites may include:

  • Cell wall

  • Pili

  • Flagella

Animal Viruses

Attachment and Entry

• Usually attach to receptors on the plasma membrane of animal cells.

Viral Structure

Virion

• A complete, fully developed viral particle is called a virion.

Nucleic Acid

• Viral genomes may be:

  • Single-stranded or double-stranded

  • Linear or circular

  • Segmented (in some viruses)

Capsid

• The capsid is the protein coat that surrounds the viral nucleic acid.

• Composed of subunits called capsomeres.

• Functions:

  • Protects viral genetic material

  • Aids in attachment to host cells

Envelope

• Some viruses possess an envelope outside the capsid.

• The envelope is composed of:

  • Lipids

  • Proteins

  • Carbohydrates

• Envelopes are often derived from the host cell’s plasma membrane.

Spikes

• Spikes are projections from the envelope made of glycoproteins and protein.

• Function: Aid in attachment to host cells and may be involved in immune recognition.

Viral Morphology

Types of Viruses

• Helical Viruses: Have rod-shaped, helical capsids. Examples: Tobacco mosaic virus, Ebola virus.

• Polyhedral Viruses: Usually icosahedral (20-sided). Examples: Adenovirus, Poliovirus.

• Enveloped Viruses: Most are roughly spherical in shape.

• Complex Viruses: Have complicated structures, such as bacteriophages (infect bacteria).

Growing Viruses in the Laboratory

Methods

• Animal viruses may be grown in:

  • Living animals

  • Embryonated eggs

• Growth in eggs may cause:

  • Embryo death

  • Visible pocks or lesions

• Embryonated eggs are used to produce some vaccines.

Multiplication of Bacteriophages

Two Viral Reproductive Cycles

• Lytic cycle

• Lysogenic cycle

The Lytic Cycle

1. Attachment: Phage attaches to host cell using tail fibers.

2. Penetration: Phage lysozyme opens the bacterial cell wall; viral DNA enters the host cell.

3. Biosynthesis: Production of viral nucleic acids and proteins.

4. Maturation: New viral particles are assembled.

5. Release: Host cell undergoes lysis, releasing new phages.

The Lysogenic Cycle

• During lysogeny, viral DNA integrates into the host chromosome.

• The integrated viral DNA is called a prophage.

• Lysogenic bacteria are immune to reinfection by the same phage.

Multiplication of Animal Viruses

1. Attachment: Virus attaches to plasma membrane.

2. Entry: Occurs by receptor-mediated endocytosis or membrane fusion.

3. Uncoating: Separation of viral nucleic acid from the capsid.

4. Biosynthesis: Production of viral nucleic acids and proteins.

5. Maturation: Assembly of viral particles.

6. Release: Enveloped viruses released by budding; nonenveloped viruses released by cell lysis.

Viruses and Cancer

Oncogenic Viruses

• Viruses that cause cancer are called oncogenic viruses.

• Virus-associated cancers are not contagious.

Transformation of Cells

• Proto-oncogenes: Genes involved in normal cell growth and division.

• Oncogenes: Mutated proto-oncogenes that promote uncontrolled cell growth (cancer).

• Transformation: Process where normal cells become cancerous.

DNA Oncogenic Viruses

• Epstein-Barr Virus (EBV): Associated with Burkitt’s lymphoma.

• Human Papillomavirus (HPV): Associated with cervical and anal cancers.

• Hepatitis B Virus (HBV): Associated with liver cancer.

RNA Oncogenic Viruses

• Retroviruses use reverse transcriptase to convert RNA into DNA.

• Examples: HIV-1, HTLV-2, Feline leukemia virus.

Oncolytic Viruses

• Viruses used to infect and destroy cancer cells and stimulate immune responses.

Latent and Persistent Viral Infections

Latent Viral Infections

• Latent viruses remain inactive for long periods.

• All herpesviruses can establish latency.

• Reactivation may occur when immunity decreases.

• Examples:

  • HHV-1 (Herpes simplex virus 1) → cold sores

  • Varicellovirus (Varicella-zoster virus) → shingles

Persistent Viral Infections

• Develop gradually over long periods and are often fatal.

Prions

Nature and Transmission

• Prions are infectious protein particles that contain no nucleic acid.

• Transmission can occur by ingestion, transplants, or contaminated surgical instruments.

Prion Diseases

• Examples:

  • "Mad cow disease" (Bovine spongiform encephalopathy)

  • Creutzfeldt-Jakob disease

  • Fatal familial insomnia

  • Sheep scrapie