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General Properties of Viruses

  • Viruses Defined

    • Smallest infectious agents, 20-300 nm in diameter.

    • Contain only one type of nucleic acid (either RNA or DNA).

    • Encased in a protein shell (capsid) possibly surrounded by a lipid membrane (envelope).

    • The entire infectious unit is termed a virion.

  • Nature of Viruses

    • Parasites at the genetic level, they replicate only within living cells.

    • Viruses are inert in the extracellular environment.

    • Viral nucleic acid carries information necessary for synthesizing virus-specific macromolecules essential for viral propagation.

    • Pathogenic effects on host cells vary: little effect, cell damage, or cell death.

    • Diversity in viruses based on structure, genome organization, and transmission strategies.

  • Virus Infection and Replication

    • Viral nucleic acids replicate and produce numerous copies along with coat proteins during the cycle.

    • Coat proteins assemble into capsid, protecting the viral genome.

    • Attachment and penetration into new cells are facilitated by the virion structure.

  • Viral Properties

    • Capsid: Protein shell enclosing the genome.

    • Capsomeres: Morphologic units visible in electron microscopy, representing clusters of polypeptides.

    • Defective virus: Lack functional replication aspects.

    • Envelope: Acquired during viral maturation via budding, contains lipid and virus-encoded glycoproteins (peplomers).

    • Nucleocapsid: Protein-nucleic acid complex of the packaged viral genome.

    • Structural units/Protomer: Basic protein building blocks of the capsid.

    • Subunit: Single folded viral polypeptide chain.

    • Virion: Complete virus particle.

Evolutionary Origin of Viruses

  • Unknown origins with two possible theories:

    1. Viruses may derive from genetic components of host cells that evolved to replicate independently.

    2. Viruses are degenerate forms of intracellular parasites.

  • Complexity exists among DNA, RNA viruses, and those using both for replication.

Classification of Viruses

  • Basis of Classification

    • Virion morphology: Size, shape, symmetry, presence of peplomers.

    • Virus genome properties: Nucleic acid type, size, strandedness (single/double), sense (positive/negative), specific features (repetitive elements, 5'-terminal caps).

    • Genome organization & replication: Gene order, open reading frames, replication strategy, cell locations involved in viral life cycle.

    • Virus protein properties: Amino acid sequence, modifications, functional activities.

    • Antigenic properties: Host responses to antisera.

    • Physicochemical properties: Molecular mass, density, stability against agents, susceptibility.

    • Biologic properties: Host range, transmission mode, pathology.

  • Virus Taxonomy System

    • Organized based on morphology, genome structure, replication strategies.

    • Virus families named with the suffix -viridae, genera with -virus.

    • Over 4400 virus species categorized into 122 families and 735 genera.

Major Families of Animal Viruses

  • Summary of various families and their properties (Table 29-1).

  • Overview of specific families:

    • DNA Viruses: Parvoviridae, Herpesviridae, Poxviridae.

    • RNA Viruses: Picornaviridae, Paramyxoviridae, Retroviridae, Orthomyxoviridae.

Principles of Virus Structure

  • Viruses possess various symmetrical structures determined by intended functions:

    • Cubic Symmetry

      • Icosahedral pattern is most efficient; 20 faces, 12 vertices.

    • Helical Symmetry

      • Protein subunits bind to nucleic acid in a helical structure; only synthesized with their RNA genomes.

    • Complex Structures

      • More intricate architecture seen in some viruses, like poxviruses.

Chemical Composition of Viruses

  • Viral Protein

    • Protects the nucleic acid, facilitates cell attachment, provides structural symmetry. Enzymatic activity may also be present for infection initiation.

  • Viral Nucleic Acid

    • Contains either DNA or RNA; types vary (single/double stranded, segmented/nonsegmented).

  • Viral Lipid Envelopes

    • Acquired during maturation; sensitive to solvents, indicating their protective role.

  • Viral Glycoproteins

    • Provide attachment to host cells, act as antigens.

Laboratory Safety and Virus Handling

  • Precautionary measures to prevent infections from working with viruses.

  • Best practices include using protective equipment, proper waste disposal, and excellent hygiene.

Replication Overview

  • Viruses multiply only in living cells using host cell's machinery.

  • Steps in viral replication cycle include attachment/penetration, uncoating, gene expression, synthesis of viral components, assembly, and release.

  • Various outcomes include productive infections, abortive infections, and latent infections, which may not kill host cells.

Transmission of Viruses

  • Modes of transmission include direct (person-to-person) and indirect routes (fomites, fecal-oral).

  • Specific patterns surrounding transmission cycles highlight the diverse ecology of viruses and their adaptation strategies.

Emerging Viral Diseases

  • Changes in society and environment lead to the emergence of new viral diseases.

  • Factors influencing emergence include environmental changes, human behavior, travel, and microbial adaptation.

Summary and Review Questions

  • Summary discusses properties, replication, classification, and ecological roles of viruses. Review questions test comprehension of essential concepts discussed.