Viruses: General Structure and Classification

General Structure of Viruses

Viruses are microscopic, acellular particles that infect living cells. They are obligate intracellular parasites, meaning they require a host cell to reproduce. Viruses can infect both prokaryotic (bacteria) and eukaryotic (plants, animals, fungi) cells.

• Genome: Composed of either DNA or RNA, but never both.

• Virion: The fully assembled, infectious virus particle.

• Capsid: Protein shell surrounding the viral genome, made up of subunits called capsomeres.

• Nucleocapsid: Complex of viral nucleic acid and capsid proteins.

• Envelope: Some viruses have a lipid membrane (envelope) derived from the host cell, often containing viral glycoproteins.

• Bacteriophage (phage): Viruses that infect bacteria.

Example: Influenza virus is an enveloped virus with a segmented RNA genome.

Methods of Virus Classification

Virus classification is complex and evolving. Two main systems are used:

• Taxonomic Classification: Based on morphology, nucleic acid type, mode of replication, host organism, and disease caused. The International Committee on Taxonomy of Viruses (ICTV) uses hierarchical ranks: Order (-virales), Family (-viridae), Subfamily (-virinae), Genus (-virus), Species (-virus).

• Baltimore Classification: Groups viruses into seven categories based on the relationship between their genome and messenger RNA (mRNA).

Baltimore Groups:

• Group I: dsDNA viruses

• Group II: ssDNA viruses

• Group III: dsRNA viruses

• Group IV: (+) ssRNA viruses

• Group V: (−) ssRNA viruses

• Group VI: ssRNA-RT viruses (reverse transcribing)

• Group VII: dsDNA-RT viruses (reverse transcribing)

Example: HIV is a Group VI virus (ssRNA-RT).

Morphology of Viruses

Viruses vary greatly in size and structure:

• Size: Range from ~20 nm (parvovirus) to 450 nm (poxvirus); mimivirus is exceptionally large (750 nm).

• Capsid: Made of protein subunits (protomers form capsomeres).

• Nucleoproteins: Proteins associated with viral nucleic acids.

• Spikes: Long projections from the nucleocapsid, often involved in host cell attachment.

Classification by Morphology

• Helical Viruses: Rod-shaped capsomeres; genetic material can be ssRNA or ssDNA; may be naked (e.g., Tobacco mosaic virus) or enveloped (e.g., Influenza virus).

• Icosahedral Viruses: 3D geometric structure with 12 corners, 20 triangular faces, and 30 edges; includes Herpesviridae, Adenoviridae, Papovaviridae, Parvoviridae.

• Enveloped Viruses: Surrounded by a lipid envelope acquired from the host cell membrane; envelope may contain viral glycoproteins.

• Complex Viruses: Bacteriophages have a head (capsid), tail, sheath, base plate, and fibers; poxviruses lack a regular capsid and have a nucleoid surrounded by a membrane and lateral bodies.

Types of Viral Genomes

• DNA Viruses: dsDNA or ssDNA; most use DNA-dependent DNA polymerase for replication.

• RNA Viruses: dsRNA or ssRNA; replicate using RNA-dependent RNA polymerase.

• Reverse Transcribing Viruses: ssRNA or dsDNA genomes; replicate via a DNA intermediate (e.g., retroviruses).

• Genome Structure: Can be linear or circular; generally, a species has only one type, with rare exceptions (e.g., cytomegalovirus).

Viral Multiplication and Effects in Host Cells

Viral Multiplication Overview

Viruses do not replicate by cell division. Instead, they use host cell machinery to produce new viral particles. Outside the host, viruses are inert.

Multiplication of Bacteriophages

Bacteriophages are highly specific for their bacterial hosts and are used in phage typing. The multiplication cycle includes:

1. Adsorption: Phage attaches to bacterial cell via tail fibers or analogous structures; receptors are bacterial surface proteins.

2. Penetration: Phage injects nucleic acid into bacterium; contractile sheath pushes inner tube through cell wall.

3. Replication: Host metabolism shifts to express viral genes, producing viral components.

4. Assembly: Viral components are assembled into mature phages.

5. Maturation: Final modifications to phage structure.

6. Release: New phages exit the cell, often by lysis.

Lytic (Virulent) Phages: Destroy host cell by lysis after multiplication.

Lysogenic (Temperate) Phages: Phage DNA integrates into host genome as a prophage, can persist or eventually cause lysis.

Multiplication of Animal Viruses

Similar to bacteriophage multiplication, but with additional steps:

1. Adsorption: Virus attaches to host cell via specific receptors.

2. Penetration: Virus enters cytoplasm by endocytosis or fusion.

3. Uncoating: Viral nucleic acid is released from capsid/envelope.

4. Replication: DNA viruses replicate in nucleus; RNA viruses replicate in cytoplasm, often using their own polymerase; retroviruses use reverse transcription.

5. Assembly: Viral components are assembled into mature virions.

6. Release: Virions exit cell by budding (exocytosis) or lysis.

Types of Viral Infections

• Abortive: Infection without viral production.

• Lytic or Cytocidal: Kills host cell.

• Persistent: Includes chronic (productive, not lytic), latent (limited synthesis, no viral production), slow (prolonged incubation), and transforming (possible oncogenic changes).

Example: Herpes simplex virus can cause latent infections (cold sores).

Host Cell Damage by Viruses

• Morphological Effects (Cytopathic): Altered cell shape, detachment, lysis, membrane fusion, inclusion bodies, cell death.

• Physiological Effects: Addition of viral proteins to plasma membrane, altered ion movement, changes in cellular activities.

• Biochemical Effects: Inhibition or alteration of host macromolecule synthesis.

• Genotoxic Effects: Damage to host DNA, mutations, potential cancer initiation.

Major Groups of Viruses Infecting Vertebrates

DNA Viruses

• Adenoviruses: Stable, cause respiratory illness, gastroenteritis, conjunctivitis, cystitis, rashes; used in gene therapy.

• Hepadnaviruses: Cause hepatitis B; acute, chronic, symptomatic, or asymptomatic disease.

• Herpesviruses: Herpes simplex 1 and 2, varicella-zoster.

• Papillomaviruses and Polyomaviruses: Cause warts, possibly cervical carcinomas.

• Parvoviruses: Only B19 causes human disease; others affect animals.

• Poxviruses: Largest viruses; cause smallpox.

RNA Viruses

• Bunyaviridae: Arthropod-borne.

• Coronaviruses: Cause respiratory and enteric diseases; includes SARS.

• Hepatitis Viruses: Cause hepatitis, cirrhosis, hepatocellular carcinoma.

• Orthomyxoviruses: Influenza A, B, C; type A most virulent.

• Paramyxoviruses: Morbillivirus, Paramyxovirus, Pneumovirus, Nipah, Hendra.

• Picornaviruses: Enterovirus, Rhinovirus, Hepatovirus, etc.; smallest RNA viruses.

• Rhabdoviruses: Infect many organisms; rabies virus.

• Reoviruses: Rotavirus causes gastroenteritis in children.

• Retroviruses: HIV, Oncovirinae, Lentivirinae, Spumavirinae.

• Togaviruses: Alphavirus, Rubivirus (rubella).

• Flaviviruses: Yellow fever, encephalitis, dengue, hepatitis C, West Nile virus.

Subviral Agents: Viroids, Virusoids, and Prions

Viroids and Virusoids

• Viroids: Small, circular RNA molecules; cause plant diseases; do not require helper viruses.

• Virusoids: Similar to viroids but require a helper virus for replication; generally infect plants.

• Hepatitis Delta Virus: Human virusoid; requires simultaneous infection with hepatitis B.

Prions

• Prions: Proteinaceous infectious particles; abnormally folded proteins that become infectious.

• Lack nucleic acid.

• Diseases: Cause transmissible spongiform encephalopathies (TSEs), such as mad cow disease and kuru.

Table: Baltimore Classification of Viruses

Additional info: The Baltimore classification is widely used to understand viral replication strategies and genome types.

Table: Types of Viral Infections