Viral structure and classification

Viral Structure - The Basic Blueprint

• Core Components: All viruses possess a genome shielded by a protein capsid, forming the nucleocapsid.
  • Genome: The genetic material, either DNA or RNA, which can be single-stranded (ss) or double-stranded (ds).
  • Capsid: A protective protein coat composed of subunits called capsomeres. Common shapes include icosahedral (polyhedral) and helical (spiral).
• Envelope (Optional):
  • A lipid bilayer acquired from the host cell membrane, studded with viral glycoproteins (spikes) essential for attachment.
  • Viruses lacking this layer are termed naked or nonenveloped.

⭐ Enveloped viruses are generally susceptible to heat, detergents, and alcohol because their lipid envelope is easily disrupted. Naked viruses are more environmentally stable.

📌 Naked DNA/RNA Viruses: Remember "PAPP Smears and a CRiSPy HePe" (Papilloma, Adeno, Polyoma, Parvo - Calici, Reo, Picorna - Hepe).

Viral Symmetry & Envelope - Shape and Shield

• Symmetry (Capsid Shape): Determines viral morphology.

  • Icosahedral: Polyhedron with 20 triangular faces. Very strong & stable. E.g., Adenovirus, HSV.
  • Helical: Spiral, rod-like structure. E.g., Influenza, Rabies.
  • Complex: No simple symmetry. E.g., Poxvirus (brick-shaped).

• Envelope (Lipid Bilayer):

  • Acquired from host cell membrane via budding.
  • Contains viral glycoproteins (spikes) for attachment.

⭐ Enveloped viruses are generally sensitive to heat, acid, and detergents. Loss of envelope results in loss of infectivity.

Viral Genome - Genetic Codebreakers

• Core Material: DNA or RNA, defining the virus family.
• Strandedness: Single-stranded (ss) or double-stranded (ds).
• Polarity (for ssRNA):
  • Positive-sense (+ssRNA): Acts directly as mRNA.
  • Negative-sense (-ssRNA): Must be transcribed to +ssRNA by viral RNA polymerase.
• Topology: Linear (most), circular (e.g., Papillomavirus), or segmented (e.g., Influenza).

⭐ High-Yield: Positive-sense RNA viruses (except Retroviruses) are infectious by themselves as their genome can be directly translated by host ribosomes, functioning like host mRNA.

Virus Classification - The Major Leagues

Viruses are obligate intracellular parasites classified by their nucleic acid (DNA/RNA), strandedness (double/single), and presence of a lipid envelope.

• DNA Viruses

  • Generally dsDNA, icosahedral, and replicate in the nucleus using host polymerase.
  • 📌 Mnemonic: HHAPPPy (Hepadna, Herpes, Adeno, Pox, Parvo, Papilloma, Polyoma).
  • Enveloped: Herpesviridae, Hepadnaviridae, Poxviridae.
  • Naked: Papillomaviridae, Adenoviridae, Polyomaviridae (dsDNA); Parvoviridae (the only ssDNA).

• RNA Viruses

  • Generally ssRNA and replicate in the cytoplasm. They must encode their own RNA-dependent polymerases.
  • +ssRNA (infectious): Genome acts as mRNA. Examples: Picornaviridae, Caliciviridae, Flaviviridae, Retroviridae.
  • -ssRNA (non-infectious): Must carry their own RNA-dependent RNA polymerase. All are enveloped.
  • dsRNA: Reoviridae (naked).

⭐ Poxvirus is the major DNA virus exception: it's complex (not icosahedral) and replicates fully in the cytoplasm using its own enzymes.

High‑Yield Points - ⚡ Biggest Takeaways

• Viruses are obligate intracellular parasites; they cannot make their own ATP or proteins.
• Viral genomes are DNA or RNA, either single-stranded (ss) or double-stranded (ds), and linear or circular.
• The capsid protects the nucleic acid; enveloped viruses have an outer lipid membrane derived from the host cell.
• Naked viruses are generally more resistant to environmental stress than enveloped viruses.
• All DNA viruses are dsDNA except for Parvoviridae. All RNA viruses are ssRNA except for Reoviridae.
• Positive-sense RNA is infectious like mRNA, while negative-sense RNA must be transcribed first.