Viral Morphology and Physiology

Viral Morphology and Physiology

• Viruses are small, obligate intracellular parasites, containing nucleic acid (DNA or RNA).
• They range from 20 to 300 nanometers in diameter.
• Viruses replicate only within living cells; outside cells they are inert.
• A virion is a fully assembled, infectious viral particle.

Virion Structure

• Virions have two basic components: nucleic acid and a capsid.
• The nucleic acid may be single- or double-stranded RNA or DNA.
• The viral genome codes for structural and regulatory proteins involved in replication.
• The capsid is a protein shell that encloses and stabilizes the nucleic acid, facilitating entry into host cells.
• Viral symmetry patterns determine the capsid shape (e.g., helical, icosahedral, complex).

Viral Characteristics

• Viruses are obligate intracellular parasites, requiring living cells for replication, and are cultured using viable cells, not artificial media.
• Viruses contain a single type of nucleic acid (DNA or RNA, but never both).
• They lack cellular machinery necessary for protein and nucleic acid synthesis.
• Viral replication relies on host cell metabolic machinery.

Viral Classification

• Viruses are classified based on their genome (type of nucleic acid, strand type, genome organization, replication site).
• RNA viruses include Picornaviridae, Togaviridae, Retroviridae, Orthomyxoviridae, Coronaviridae.
• DNA viruses include Parvoviridae, Papovaviridae, Adenoviridae, Herpesviridae, Poxviridae.
• Replication sites for viruses may be in the nucleus or cytoplasm.

Viral Cultivation

• Methods include:
  • Embryonated chicken eggs (in inoculation sites like the chorio-allantoic membrane, amniotic cavity).
  • Tissue cultures (primary cell cultures, diploid cell strains, continuous cell lines).
  • Animals (mice, guinea pigs, rabbits).

Viral Replication Cycle

• Adsorption: Viruses attach to host cell receptors.
• Penetration and Uncoating: Enveloped viruses fuse with the host membrane; naked viruses enter via phagocytosis or direct penetration. The nucleic acid is released.
• Replication and Biosynthesis: Viral components are synthesized using host machinery.
• Viral Assembly: Nucleic acids and proteins are assembled to form new virions.
• Viral Release: Occurs through cell lysis or budding.

Viral Infections

• Viral replication disrupts host cell biochemistry and structure.
• Cytopathic effects include cell lysis, apoptosis, and syncytia formation.
• Inclusion bodies (like Negri bodies in rabies) sometimes form.

Viral Virulence and Host Response

• Virulence Factors: Viral genes affect replication, host defense response, tissue tropism, and transmission.
• Host Immune Response: Includes a disseminated antibody response (IgG, IgA, IgM), and cell-mediated immunity crucial for recovery and controlling viral spread. IgE also amplifies immune responses.

Explore the fascinating world of viruses and their unique characteristics. This quiz covers the structures of virions, including nucleic acids and capsids, and how they function as obligate intracellular parasites. Test your understanding of viral replication and symmetry patterns.