Lecture Notes: Chapter 13 - Viruses and Prions (Part Two)

Key Focus

• Animal Viruses
• Prions

Structure of Animal Viruses

• Capsid: All animal viruses have a capsid composed of capsomeres.
• Nucleic Acid: Can be DNA or RNA.
• Envelope: Some have an envelope; not required for all.

Viral Life Cycle Stages

1. Attachment: Determined by outer covering (capsid or envelope).
2. Penetration: Type of penetration depends on envelope presence.
3. Biosynthesis: Determined by whether it's a DNA or RNA virus.
4. Maturation: Depends on genome type.
5. Release: Process varies based on structure.

Naked vs. Enveloped Viruses

• Naked Viruses:

  • Outer covering: Capsid with protein fibers for attachment.
  • Penetration: Receptor-mediated endocytosis.
  • Release: By lysis (cell rupture).

• Enveloped Viruses:

  • Outer covering: Phospholipid bilayer envelope with viral spikes.
  • Penetration: Fusion with host cell membrane.
  • Release: By budding (cell remains intact).

DNA Viruses

• Examples: Herpes virus (e.g., cold sore, chickenpox), Hepatovirus (hepatitis), Papova virus (warts), Pox viruses (e.g., smallpox).
• Biosynthesis:
  • Genome replication in nucleus.
  • Capsomere synthesis in cytoplasm.
• Maturation: Occurs in the nucleus.

RNA Viruses

• Examples: Coronavirus (e.g., COVID-19), Filoviruses (Ebola), Flaviviruses (e.g., yellow fever), Paramyxoviruses (measles, mumps), Orthomyxoviruses (influenza).
• Biosynthesis and Maturation: Both occur in the cytoplasm.

Retroviruses

• Structure: Enveloped RNA virus with reverse transcriptase.
• Examples: HIV, MMTV (mouse breast cancer model).
• Life Cycle:
  • Reverse transcription: Viral RNA to DNA.
  • DNA splices into host chromosome (provirus).
  • Triggered activation leads to biosynthesis.

Viral Infections

• Acute: Short duration, high virus production (e.g., flu, cold, Ebola).
• Chronic: Long duration, constant low-level virus (e.g., hepatitis).
• Latent: Virus production only during outbreaks (e.g., herpes).

Cytoplasmic Effects

• Inclusion Bodies: Dark staining areas in infected cells.
• Giant Cell Formation: Fused cells with multiple nuclei.

Viruses and Cancer

• HPV and Cervical Cancer: DNA virus linked to cancer.
• Mechanism: Viral DNA integrates into host DNA, potentially causing mutations.

Prions

• Nature: Infectious proteins causing brain diseases.
• Diseases: Mad cow disease, CWD in deer.
• Types in Humans:
  • CJD: Spontaneous, causes neurological symptoms.
  • Kuru: Acquired by ingestion, historically in Papua New Guinea.
  • FFI: Genetic, inherited prion disease.
• Impact: Fatal, untreatable, causes spongy form encephalopathies.

Study Tips

• Use tables to compare life cycles: focus on stages and virus types (naked vs. enveloped, DNA vs. RNA).
• Understand attachments, penetration, biosynthesis, maturation, and release processes for each virus type.