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Chapter 5: Viruses – Structure, Classification, and Multiplication

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Viruses: General Structure and Classification

Introduction

Viruses are unique infectious agents that occupy a distinct position in biology due to their acellular nature and dependence on host cells for replication. This section explores their structure, classification, and genome types, providing foundational knowledge for understanding viral biology in anatomy and physiology.

General Structure of Viruses

  • Microscopic Particles: Viruses are much smaller than most cells, ranging from about 20 nm (parvovirus) to 450 nm (poxvirus). Some, like the mimivirus, can be as large as 750 nm.

  • Genome: The viral genome consists of either DNA or RNA, but never both.

  • Obligate Intracellular Parasites: Viruses cannot reproduce independently; they require a host cell's machinery for replication.

  • Host Range: Viruses infect both eukaryotic and prokaryotic cells. Bacteriophages are viruses that specifically infect bacteria.

Classification of Viruses

  • Taxonomic Classification: Virus classification is complex and evolving. The International Committee on Taxonomy of Viruses (ICTV) oversees the official taxonomy, using hierarchical ranks such as order (-virales), family (-viridae), subfamily (-virinae), and genus.

  • Classification Criteria: Viruses are classified based on morphology, nucleic acid type, mode of replication, host organism, and the diseases they cause.

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

    • 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)

Viral Morphology

  • Capsid: The protein shell enclosing the viral genome, composed of subunits called capsomeres, which are themselves made of protomers.

  • Nucleocapsid: The combination of the viral genome and the capsid.

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

  • Spikes: Protruding structures from the capsid or envelope that aid in host cell attachment.

  • Virion: A fully assembled, infectious virus particle.

Classification by Morphology

  • Helical Viruses: Capsids with rod-shaped capsomeres; genetic material can be ssRNA or ssDNA. Examples: tobacco mosaic virus (naked), influenza virus (enveloped).

  • Icosahedral Viruses: Symmetrical, 20-sided structures with 12 corners. Examples: Herpesviridae, Adenoviridae, Papovaviridae, Parvoviridae.

  • Complex Viruses: Include bacteriophages (with head, tail, and base plate) and poxviruses (lacking a regular capsid, with a nucleoid and lateral bodies).

Table: Comparison of Viral Morphologies

Type

Capsid Shape

Genome Type

Envelope

Examples

Helical

Rod-shaped

ssRNA or ssDNA

Naked or Enveloped

Tobacco mosaic virus, Influenza virus

Icosahedral

20-sided, 12 corners

Varies

Naked or Enveloped

Adenovirus, Herpesvirus

Complex

Irregular/Head-tail

Varies

Usually Naked

Bacteriophage, Poxvirus

Viral Genomes

  • Types: Viral genomes can be DNA or RNA, single-stranded (ss) or double-stranded (ds), linear or circular.

  • DNA Viruses: Most use DNA-dependent DNA polymerase for replication. Examples: Adenoviridae, Herpesviridae.

  • RNA Viruses: Use RNA as genetic material and replicate without a DNA intermediate. Examples: Picornaviridae, Orthomyxoviridae.

  • Reverse Transcribing Viruses: Have an RNA genome but replicate via a DNA intermediate (e.g., retroviruses like HIV).

Viral Multiplication and Effects

Viral Multiplication

Viruses do not replicate by cell division. Instead, they hijack the host cell's machinery to produce new viral particles. The process differs slightly between bacteriophages and animal viruses.

Multiplication of Bacteriophages

  1. Adsorption: Phage attaches to specific receptors on the bacterial cell wall, pili, or flagella.

  2. Penetration: Phage injects its nucleic acid into the host cell; the capsid remains outside.

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

  4. Assembly: Viral components are assembled into new phage particles.

  5. Maturation: Final modifications to form mature virions.

  6. Release: Host cell lyses, releasing new phages.

Lytic vs. Lysogenic Cycles

  • Lytic (Virulent) Cycle: Results in host cell lysis and release of new viruses.

  • Lysogenic (Temperate) Cycle: Viral DNA integrates into the host genome as a prophage, replicating with the host cell until triggered to enter the lytic cycle.

Multiplication of Animal Viruses

  1. Adsorption: Virus attaches to host cell receptors.

  2. Penetration: Entry by endocytosis or membrane fusion; nucleocapsid enters the cell.

  3. Uncoating: Viral genome is released from the capsid.

  4. Replication: DNA viruses replicate in the nucleus; RNA viruses replicate in the cytoplasm, often using their own polymerases.

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

  6. Release: Viruses exit by budding (enveloped viruses) or lysis (naked viruses).

Types and Effects of Viral Infections

  • Abortive: Infection occurs without production of new viruses.

  • Lytic/Cytocidal: Host cell is killed; new viruses are released.

  • Persistent: Virus remains in the cell without causing death; can be chronic, latent, or slow infections.

  • Transforming: Viral nucleic acid remains in the cell, potentially causing oncogenic changes.

Host Cell Damage

  • Morphological Effects: Changes in cell shape, detachment, lysis, membrane fusion, inclusion bodies.

  • Physiological Effects: Altered membrane properties, ion movement, and cellular activities.

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

  • Genotoxic Effects: DNA damage, mutations, and potential cancer initiation.

Major Groups of Viruses in Vertebrates

DNA Viruses

  • Adenoviruses: Cause respiratory illnesses, gastroenteritis, conjunctivitis.

  • Hepadnaviruses: Cause hepatitis B, can lead to chronic liver disease.

  • Herpesviruses: Includes herpes simplex, varicella-zoster (chickenpox, shingles).

  • Papillomaviruses/Polyomaviruses: Cause warts, some linked to cancers.

  • Parvoviruses: B19 causes disease in humans; others are zoonotic.

  • Poxviruses: Cause smallpox; largest known viruses.

RNA Viruses

  • Bunyaviridae: Arthropod-borne viruses.

  • Coronaviruses: Cause respiratory and enteric diseases (e.g., SARS).

  • Hepatitis Viruses: Cause hepatitis, cirrhosis, and liver cancer.

  • Orthomyxoviruses: Influenza viruses A, B, and C.

  • Paramyxoviruses: Includes measles, mumps, and respiratory pathogens.

  • Picornaviruses: Includes enteroviruses, rhinoviruses, and hepatitis A.

  • Rhabdoviruses: Includes rabies virus.

  • Reoviruses: Rotavirus causes gastroenteritis in children.

  • Retroviruses: Includes HIV.

  • Togaviruses/Flaviviruses: Includes rubella, yellow fever, dengue, hepatitis C, West Nile virus.

Subviral Agents

  • Viroids: Small, circular RNA molecules causing plant diseases; do not require a helper virus.

  • Virusoids: Similar to viroids but require a helper virus for replication; often associated with plant infections (e.g., hepatitis delta virus in humans requires hepatitis B co-infection).

  • Prions: Infectious proteins lacking nucleic acid; cause transmissible spongiform encephalopathies (e.g., mad cow disease, kuru).

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