Viruses, Viroids, and Prions

General Characteristics of Viruses

  • Obligatory intracellular parasites
    • Require living host cells to multiply.
  • Contain DNA or RNA.
  • Contain a protein coat.
  • Lack ribosomes.
  • Lack ATP-generating mechanism.
  • Size is on the nanometer order.

Host Range

  • The spectrum of host cells a virus can infect.
  • Most viruses infect only specific types of cells in one host.
    • Determined by specific host attachment sites and cellular factors.
  • Bacteriophages are viruses that infect bacteria.
  • Range from 20 nm to 1000 nm in length.

Viral Structure

  • Virion: Complete, fully developed viral particle.
    • Nucleic acid: DNA or RNA can be single- or double-stranded; linear or circular.
    • Capsid: Protein coat made of capsomeres (subunits).
    • Envelope: Lipid, protein, and carbohydrate coating on some viruses.
    • Spikes: Projections from the outer surface.

General Morphology

  • Helical viruses: hollow, cylindrical capsid.
  • Polyhedral viruses: many-sided.
  • Enveloped viruses.
  • Complex viruses: complicated structures.

Taxonomy of Viruses

  • Genus names end in -virus (e.g., Simplexvirus, Enterovirus).
  • Family names end in -viridae (e.g., Herpesviridae, Poxviridae).
  • Order names end in -virales (e.g., Caudovirales, Nidovirales, Mononegavirales).
  • Viral species: designated by descriptive common names (e.g., Herpes simplex virus, Rabies virus, Rubella virus, Human immunodeficiency virus).
  • Subspecies (if any) are designated by a number (e.g., Herpes simplex virus: HSV-1; HSV-2, Human immunodeficiency virus: HIV-1; HIV-2).

Viral Multiplication

  • For a virus to multiply:
    • It must invade a host cell.
    • It must take over the host’s metabolic machinery.
  • One-step growth curve is used to analyze viral multiplication.

Multiplication of Bacteriophages

  • Lytic cycle
    • Phage causes lysis and death of the host cell.
  • Lysogenic cycle
    • Phage DNA is incorporated into the host DNA.
    • Phage conversion.
    • Specialized transduction.

Lytic Cycle of a T-even Bacteriophage

  1. Attachment: Phage attaches to host cell.
  2. Penetration: Phage penetrates host cell and injects its DNA.
  3. Biosynthesis: Phage DNA directs synthesis of viral components by the host cell.
  4. Maturation: Viral components are assembled into virions.
  5. Release: Host cell lyses, and new virions are released.

Bacteriophage Lambda (λ): The Lysogenic Cycle

  • Lysogeny: phage remains latent.
  • Phage DNA incorporates into host cell DNA
    • Inserted phage DNA is known as a prophage
    • When the host cell replicates its chromosome, it also replicates prophage DNA
    • Results in phage conversion — the host cell exhibits new properties
  • Specialized transduction
    • Specific bacterial genes transferred to another bacterium via a phage
    • Changes genetic properties of the bacteria

Multiplication of Animal Viruses

  • Attachment: viruses attach to the cell membrane.
  • Entry: by receptor-mediated endocytosis or fusion.
  • Uncoating: by viral or host enzymes.
  • Biosynthesis: production of nucleic acid and proteins.
  • Maturation: nucleic acid and capsid proteins assemble.
  • Release: by budding (enveloped viruses) or rupture.

The Biosynthesis of DNA Viruses

  • DNA viruses replicate their DNA in the nucleus of the host using viral enzymes.
  • Synthesize capsid in the cytoplasm using host cell enzymes.

The Biosynthesis of RNA Viruses

  • Virus multiplies in the host cell’s cytoplasm using RNA-dependent RNA polymerase.
    • ssRNA; + (sense) strand (Picornaviridae)
      • Viral RNA serves as mRNA for protein synthesis.
    • ssRNA; − (antisense) strand (Rhabdoviridae)
      • Viral RNA is transcribed to a + strand to serve as mRNA for protein synthesis.
    • dsRNA; double-stranded RNA

Biosynthesis of RNA Viruses That Use DNA

  • Single-stranded RNA, produce DNA
    • Use reverse transcriptase to produce DNA from the viral genome.
      • Viral DNA integrates into the host chromosome as a provirus.
    • Belong to the family of Retroviridae
      • Lentivirus (HIV)
      • Oncoviruses

Viruses and Cancer

  • Several types of cancer are caused by viruses.
    • May develop long after a viral infection.
    • Cancers caused by viruses are not contagious.
  • Sarcoma: cancer of connective tissue.
  • Adenocarcinomas: cancers of glandular epithelial tissue.

The Transformation of Normal Cells into Tumor Cells

  • Oncogenes transform normal cells into cancerous cells.
  • Oncogenic viruses become integrated into the host cell’s DNA and induce tumors
    • Examples: Epstein-Barr virus; Human papillomavirus; Hepatitis B virus

Plant Viruses and Viroids

  • Plant viruses: enter through wounds or via insects.
    • Plant cells are generally protected from disease by an impermeable cell wall.
  • Viroids: short pieces of naked RNA.
    • Cause potato spindle tuber disease.
  • Virusoids: viroids enclosed in a protein coat.
    • Only cause disease when plant cell is coinfected with a virus.

Prions

  • Proteinaceous infectious particles.
  • Inherited and transmissible by ingestion, transplant, and surgical instruments.
    • Spongiform encephalopathies
      • “Mad cow disease”
      • Creutzfeldt-Jakob disease (CJD)
      • Gerstmann-Sträussler-Scheinker syndrome
      • Fatal familial insomnia
      • Sheep scrapie
  • PrPC: normal cellular prion protein, on the cell surface.
  • PrPSc: scrapie protein; accumulates in brain cells, forming plaques.