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General Properties of Viruses

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1. Definition and Size

• They are obligate intracellular parasites - they replicate only inside living host cells and are metabolically inert outside cells
• They are parasites at the genetic level - the viral nucleic acid directs the host cell's biosynthetic machinery to produce viral components
• They contain no ribosomes, no mitochondria, and no metabolic enzymes of their own (unlike bacteria)

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2. Structural Components (Virology Terminology)

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3. Genome (Nucleic Acid)

• Each virus contains only one type of nucleic acid - either DNA or RNA, never both (exception: during certain replication strategies like retroviruses/hepadnaviruses)
• The genome may be:
  • Single-stranded (ss) or double-stranded (ds)
  • Linear or circular
  • Positive-sense, negative-sense, or ambisense (for RNA viruses)
  • Segmented (multiple separate RNA segments, e.g., influenza) or non-segmented
• Genome size ranges from ~1.7 kb (parvoviruses) to ~375 kbp (poxviruses)
• The nucleic acid carries all information necessary to direct the host cell to synthesize virus-specific macromolecules for producing viral progeny

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4. Capsid Symmetry

A. Icosahedral Symmetry

• The capsid is built in the shape of an icosahedron (20 equilateral triangular faces, 12 vertices)
• The number of capsomeres is characteristic for each virus family (e.g., 32 in parvoviruses, 252 in adenoviruses, 162 in herpesviruses)
• Examples: adenovirus, poliovirus, herpesviruses, papillomaviruses

B. Helical Symmetry

• Capsomeres and nucleic acid are wound together in a helical/spiral arrangement
• Results in a rod-shaped or filamentous nucleocapsid
• Examples: influenza virus (negative-strand RNA), rabies virus, paramyxoviruses

C. Complex Symmetry

• Some large viruses (e.g., poxviruses) have complex coats that do not conform to either icosahedral or helical symmetry
• Poxviruses have a brick-shaped morphology with complex internal membranes

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5. Envelope vs. Non-Enveloped (Naked) Viruses

• Attachment to host cell receptors
• Immune evasion
• Membrane fusion (entry)

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6. Viral Replication - Overview

1. Attachment (Adsorption) - Virion binds to specific receptors on the host cell surface via surface proteins or glycoproteins
2. Penetration/Entry - The virion or its nucleic acid enters the cell (by endocytosis, membrane fusion, or direct injection)
3. Uncoating - The capsid is removed, releasing the nucleic acid into the appropriate cellular compartment
4. Biosynthesis - The viral genome directs synthesis of:
   • Viral nucleic acid (genome replication)
   • Viral proteins (structural + non-structural)
5. Assembly - Numerous copies of viral nucleic acid and coat proteins assemble to form new nucleocapsids (the capsid assembles around the genome)
6. Release - New virions are released by:
   • Lysis of the host cell (naked viruses)
   • Budding through host cell membrane (enveloped viruses - acquiring their envelope in the process)

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7. Classification of Viruses

1. Virion morphology - size, shape, type of symmetry, presence/absence of peplomers, membranes
2. Genome properties - type of nucleic acid (DNA/RNA), size, strandedness (ss/ds), linear/circular, sense (+/-/ambisense), number of segments, nucleotide sequence, %GC content
3. Genome organization and replication - gene order, open reading frames, strategy of replication, cellular sites of replication and assembly
4. Viral protein properties - number, size, amino acid sequence, modifications (glycosylation, phosphorylation, myristoylation), functional activities (transcriptase, reverse transcriptase, neuraminidase)
5. Antigenic properties - serologic relationships detected by neutralization, hemagglutination inhibition, complement fixation
6. Physicochemical properties - molecular mass, buoyant density, pH stability, thermal stability, susceptibility to physical and chemical agents
7. Biologic properties - natural host range, mode of transmission, vector relationships, pathogenicity, tissue tropisms

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8. Summary Table: Major Animal Virus Families (Infecting Humans)

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9. Special Properties of DNA vs. RNA Viruses

DNA Viruses (Medical Microbiology 9e, Box 36.7)

• DNA is not transient or labile
• Many establish persistent/latent infections (e.g., herpesviruses)
• DNA genomes reside in the nucleus (except poxviruses, which replicate in cytoplasm)
• Viral DNA resembles host DNA for transcription and replication
• Gene transcription is temporally regulated: early genes encode DNA-binding proteins and enzymes; late genes encode structural proteins
• DNA polymerases require a primer to replicate the genome

RNA Viruses

• RNA is more labile and mutation-prone
• Positive-sense RNA (+ssRNA) can act directly as mRNA and be immediately translated
• Negative-sense RNA (-ssRNA) must carry an RNA-dependent RNA polymerase (transcriptase) in the virion
• RNA viruses have a higher mutation rate (no proofreading) leading to quasispecies and antigenic drift/shift
• Most RNA viruses replicate in the cytoplasm; exceptions include influenza (nuclear) and retroviruses

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10. Evolutionary Origin of Viruses

1. Cellular origin: Viruses may be derived from DNA or RNA components of host cells that became able to replicate autonomously - essentially "escaped genes" that evolved independently. Some viral sequences are related to portions of cellular genes.
2. Degenerate parasite theory: Viruses may be degenerate forms of intracellular parasites. Poxviruses, being so large and complex, might represent evolutionary products of some cellular ancestor.

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11. Host Range and Effects on Host

• The host range of a given virus may be broad or extremely limited (determined by receptor specificity and intracellular factors)
• Viruses infect organisms from mycoplasmas and bacteria (bacteriophages) to algae, all higher plants and animals
• Effects of viral infection on cells may range from:
  • No visible effect
  • Cytopathic effects (CPE) - rounding, lysis, inclusion bodies, multinucleated giant cells
  • Transformation/immortalization (oncogenic viruses)
  • Cell death (necrosis or apoptosis)

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