Virology: Virus Structure and Classification

• Acellular particles contain either DNA or RNA and are obligate intracellular parasites, requiring a host to reproduce.
• Viruses do not replicate through cell division.
• Viruses infect both eukaryotes and prokaryotes.
• Bacteriophages, also known as phages, specifically infect bacteria.

Classification of Viruses

• Viruses are classified based on morphology, capsid shape, nucleic acid type, and viral infection characteristics.

Morphology

• Virus sizes vary, from very small (parvovirus) to large (poxvirus).
• Most viruses are too small to be seen with a light microscope.
• Viruses consist of genetic material inside a viral coat called a capsid.
• Capsids are made of proteins called capsomeres.
• Nucleocapsid refers to the combination of viral capsid proteins with viral nucleic acid.
• Spikes are projections extending from the nucleocapsid.

Capsid Shape

• Helical viruses have rod-shaped capsomeres, and can be naked (e.g., tobacco mosaic virus) or enveloped (e.g., flu virus).
• Icosahedral viruses are three-dimensional with many corners, triangular faces, and edges (e.g., Herpesvirus, adenovirus, papovavirus, parvovirus).
• Complex viruses have capsids that are neither purely helical nor completely icosahedral (e.g., bacteriophages).
  • Bacteriophages possess a head (capsid) containing nucleic acid, a tail (hollow tube surrounded by a sheath), and a base plate with attached fibers.

Nucleic Acid Type

• Viral nucleic acid can be DNA or RNA, and may be linear or a closed loop.
• Reverse transcribing viruses are an exception:
  • Cytomegalovirus (a type of Herpesvirus) is a DNA virus that replicates through an RNA intermediate.
  • Retroviruses (like HIV) are RNA viruses that replicate through a DNA intermediate.

Multiplication of Bacteriophages

• Each phage targets a specific bacterium (host range).
• Stages of multiplication:
  • Adsorption: The phage attaches to a susceptible host cell through irreversible binding.
  • Penetration: The phage injects its nucleic acid into the bacterium, leaving the phage outside.
  • Replication: The bacterial metabolism is used to produce viral components.
  • Assembly: Viral components accumulate within the cell.
  • Maturation: Components are assembled into mature virus particles.
  • Release: New phages are released from the cell.

Multiplication of Animal Viruses

• The multiplication process is similar to bacteriophages:
  • Adsorption
  • Penetration.
  • Uncoating: the viral nucleic acid is released.
  • Replication
  • Assembly
  • Release.

Viral Infections

• Abortive infections: no viral production.
• Lytic or Cytocidal infections: the host cell dies.
• Persistent infections:
  • Chronic: not lytic but productive.
  • Latent: the virus remains in an asymptomatic host for a long period (e.g., cold sores, shingles).
  • Slow infections: have a long incubation period followed by disease.
  • Transforming infections: viral nucleic acid remains indefinitely without virus production, potentially leading to oncogenic changes.

Host Cell Damage

• Morphological Effects (Cytopathic): Altered shape, lysis, membrane fusion, apoptosis.
• Physiological/Biochemical Effects: Changes in ion movement or host cell's chemical molecules.
• Genotoxic/Mutation Effects: Damage to host cell DNA; potential initiation of cancer.

Oncogenic Viruses

• Genotoxic effects can initiate cancer

DNA Viruses

• Adenovirus is a DNA virus:
  • Causes mostly respiratory illnesses, but can also cause gastroenteritis, conjunctivitis, cystitis, and rashes.
• Hepadnavirus is a DNA virus:
  • Causes Hepatitis B.
• Poxvirus is the largest DNA virus:
  • Causes Smallpox (Variola virus): It was eradicated from the world in 1981.
• Parvovirus is the smallest DNA virus:
  • B19 causes 5th disease (erythema infectiosum).
• Herpesvirus is a DNA virus:
  • Herpes Simplex 1 (HHV-1): causes cold sores or fever blisters (vesicles on lips) and herpes whitlow (vesicles on fingers); it remains latent in the trigeminal nerve.
  • Herpes Simplex 2 (HHV-2): causes genital herpes & encephalitis, and is carcinogenic; it remains latent in the sacral nerve.
  • Varicella-Zoster Virus (HHV 3, VZV): causes varicella (chickenpox) and shingles (herpes zoster), which is a reactivation of latent HHV-3 and more common above 50 years. Remains latent in dorsal nerve root.
  • Epstein-Barr Virus (HHV-4, EBV): causes infectious mononucleosis (Mono, Glandular Fever, Kissing disease), more common in young adults; avoid contact sports; carcinogenic.
  • Cytomegalovirus (CMV, HHV 5): can be asymptomatic or cause mild infection, but severe infection in immunocompromised patients; most common virus transmitted during pregnancy; transmitted sexually, by body fluids, or by transplanted tissues; carcinogenic.
  • Human herpes virus 6 (HHV 6): causes 6th disease.
  • HHV 7: carcinogenic.
  • HHV 8: carcinogenic.
• Papillomavirus & Polyomavirus:
  • Includes more than 90 viruses
  • Causes warts, some are sexual and carcinogenic.