Introduction to Virology

Introduction to Virology

• Virology is the study of viruses.
• Asmaa Omar is a Professor of Clinical Pathology at Assiut University.

Viruses: General Properties

• Viruses are the smallest infectious agents (20-300 nm in diameter).
• Bacteria are larger (300-1000 nm).
• Red Blood Cells (RBCs) are also larger (7500 nm).
• Viruses contain either DNA or RNA.
• Viruses lack the cellular machinery for protein synthesis.
• They cannot reproduce independently; they need a host cell.
• Viruses are obligate intracellular parasites.

Basic Virus Structure

• Viruses consist of a genome (DNA or RNA) surrounded by a protein coat (capsid).
• Naked viruses have only a capsid.
• Enveloped viruses have an additional lipid membrane layer.
• The capsid's building blocks are capsomeres.
• Glycoproteins are found on the surface of enveloped viruses.
• The nucleocapsid is the complex of nucleic acid and proteins.
• A complete infective virus particle is a virion.

Viruses: Structure

• The viral genome is either DNA or RNA and can be single-stranded (ss) or double-stranded (ds).
• It can be circular or linear.
• The capsid is the protein coat surrounding the genome.
• It provides structure, attachment to host cell, and facilitates viral nucleic acid transfer.
• Capsomere proteins make up the capsid; they may be one or several per capsomere.

Envelope

• Some viruses have an envelope, a lipid layer around the nucleocapsid.
• The envelope's composition varies—often contains host cell and viral material.
• Glycoproteins on the envelope mediate interactions with host cells.
• Most human helical viruses are enveloped; icosahedral can be both or neither.
• The nucleocapsid is the complex containing the genetic material and proteins.
• A virion is the complete infective viral particle

Significance of Envelope

• Enveloped viruses are more unstable to heat, drying, detergents, and alcohols compared to naked viruses.
• Viruses transmitted by the fecal-oral route (like HAV, poliovirus, rotavirus) are non-enveloped.
• Enveloped viruses often spread by direct contact (like HIV, HBV, HCV, rabies virus), blood, or sexual contact.

Viruses: Symmetry (Shapes)

• Icosahedral symmetry (cubic): 60 capsomeres arranged on a 20-sided figure.
• Helical symmetry: Capsomere proteins arrange in a spiral around the nucleic acid.
• Complex symmetry: Does not fit into the above categories.

Differences Between Different Groups of Microorganisms

• A chart comparing bacteria, mycoplasma, rickettsia, chlamydia, and viruses based on growth, division, genetic material, ribosomes, and antibiotic sensitivity.

Atypical Virus-like Agents

• Defective Viruses: Contain viral nucleic acid and proteins but can't replicate without a "helper" virus.
• Pseudovirions: Contain host cell DNA inside a viral capsid; they can infect cells but not replicate.
• Viroids: Short circular RNA molecules that cause plant diseases.
• Prions: Infectious proteins causing certain slow diseases (e.g., Creutzfeldt-Jacob Disease in humans).

Major Viruses Causing Human Diseases

• A list of virus families, related human pathogens, and the diseases they cause.

Virus Infectious Cycle

• The viral life cycle within a host cell has 6 stages.

1. Attachment/Adsorption: Reactive sites on the virus surface bind to specific receptors on host cells; this limits the infection to specific host species or cell types.
2. Penetration: Enveloped viruses fuse with the host cell membrane, releasing their nucleocapsid into the cytoplasm. Other viruses enter through endocytosis.
3. Uncoating: The viral capsid breaks down, releasing viral nucleic acid.
4. Biosynthesis/Genomic Activation: Viral nucleic acid directs the host cell's machinery to produce viral proteins and nucleic acids.
5. Assembly: Newly synthesized viral components assemble into new virions.
6. Release: New virions leave the host cell. Release may occur through budding (enveloped viruses) or lysis.

Viral Replication Enzymes

• DNA and RNA polymerases.
• Reverse transcriptase.
• Neuraminidase (involved in release of some viruses).

Properties of Naked Capsid Viruses

• Environmentally stable (resistance to heat, acid, proteases, detergents, drying).
• Released from cells by lysis.
• Spread easily in droplets, dust, and on surfaces (fomites).
• Can survive harsh conditions (e.g., in the gut).
• Antibody may be sufficient for protection against these viruses

Properties of Enveloped Viruses

• Environmentally labile (disrupted by heat, acid, detergents, drying).
• Modifies cell membranes during replication.
• Released by budding.
• Spread in large droplets, secretions, and by blood and organ transplants and transfusions.
• Need antibody and cell-mediated immunity for protection.
• Cause hypersensitivity and inflammation (immunopathogenesis)