Virus Structure and Classification

Viral Basics - Tiny Terrors Unveiled

• Definition: Acellular, obligate intracellular parasites; hijack host machinery for replication.
• Size: Range 20 nm (Parvovirus) to 300 nm (Poxvirus).
• Core Components:
  • Nucleic Acid (Genome):
    • DNA or RNA (never both).
    • ss/ds; linear/circular/segmented.
    • Haploid (most); Retroviruses are diploid (two +ssRNA copies).
  • Capsid: Protein shell (capsomeres).
    • Functions: Protects genome, aids entry, antigenicity.
  • Envelope (if present): Host-derived lipid bilayer with viral glycoproteins (spikes).
    • Spikes: Attachment, fusion, major antigens.
• Key Viral Enzymes (virally coded, not all possess):
  • e.g., RNA-dependent RNA polymerase (RdRp), Reverse Transcriptase (RT), Integrase, Protease.

virus structures, highlighting nucleic acid, capsid, envelope, and glycoprotein spikes)

⭐ Retroviruses (e.g., HIV) are diploid (+ssRNA) and carry reverse transcriptase for DNA synthesis from RNA.

Viral Architecture - Shape Shifters

Viruses exhibit diverse capsid symmetries, crucial for their structure and function. The capsid protects the viral genome.

• Three main types of symmetry:
  • Icosahedral
  • Helical
  • Complex

Viral Taxonomy - Naming Nightmares

• ICTV Classification: Hierarchical system: Order (-virales), Family (-viridae), Subfamily (-virinae), Genus (-virus), Species. Note: Family names always end in -viridae.
• Baltimore Classification: 7 groups based on mRNA synthesis pathway:
  • Group I: dsDNA (e.g., Herpesvirus)
  • Group II: ssDNA (e.g., Parvovirus)
  • Group III: dsRNA (e.g., Reovirus)
  • Group IV: +ssRNA (e.g., Poliovirus)
  • Group V: -ssRNA (e.g., Influenza virus)
  • Group VI: ssRNA-RT (e.g., HIV)
  • Group VII: dsDNA-RT (e.g., Hepatitis B virus) 📌 Mnemonic: Dapper Students Devour Positive News, Reading Diligently (dsDNA, ssDNA, dsRNA, +ssRNA, -ssRNA, RNA-RT, DNA-RT).

• Other Criteria for Classification: Nucleic acid type (DNA/RNA), strandedness (single/double), envelope (presence/absence), capsid symmetry (e.g., icosahedral, helical), host range.

⭐ Poxviruses (dsDNA) are unique: DNA viruses that replicate entirely in the cytoplasm of the host cell.

Oddballs Out - Rogue Agents

Atypical infectious agents, distinct from viruses, lacking some core viral components.

• Prions (Proteinaceous Infectious Particles)

  • No nucleic acid; solely abnormal PrPSc protein.
  • Mechanism: Converts normal PrPc to pathogenic PrPSc.
  • Diseases: CJD, Kuru, BSE (mad cow), Scrapie.
  • Highly resistant to standard sterilization.

    ⭐ Prions resist standard autoclaving (121°C, 15 min). Require 134°C for 18+ min or specific chemical treatments (e.g., NaOH, NaOCl).

• Viroids

  • Smallest infectious agents: circular ssRNA.
  • No protein coat (capsid).
  • Primarily plant pathogens (e.g., Potato Spindle Tuber Viroid - PSTVd).

• Virusoids (Satellites)

  • Small ssRNA entities; depend on a helper virus for replication & encapsidation.
  • Example: Hepatitis D Virus (HDV) requires Hepatitis B Virus (HBV).

Table: Comparison of Rogue Agents

High‑Yield Points - ⚡ Biggest Takeaways

• Viruses: obligate intracellular parasites with DNA or RNA genomes.
• Structure: Nucleic acid + capsid (protein coat) = nucleocapsid; some have lipid envelope.
• Capsid symmetry: Icosahedral, helical, or complex.
• Baltimore classification: Based on mRNA synthesis pathway; 7 groups.
• Naked viruses are generally more resistant to disinfectants than enveloped viruses.
• All DNA viruses are dsDNA except Parvoviridae (ssDNA).
• Most RNA viruses are ssRNA; Reoviridae (e.g., Rotavirus) is dsRNA.