Virology: Introduction and Virus Structure

Viruses: Introduction and Characteristics

• Virology is the study of viruses, complex structures of nucleic acids and proteins capable of replicating in animal, plant, and bacterial cells.
• Viruses are parasites that utilize host cell functions for replication, causing changes in the host cell, including altered antigenicity and metabolic processes.
• Viral activity can lead to cell death.
• Viral replication results in the manifestations of viral disease within the host.
• Viruses infect all living organisms.
• Humans regularly ingest and inhale billions of virus particles.
• Viruses are components of human genetic material.

Virus Structure

• Viruses are small, typically 0.01-0.3 µm in size, and remain infectious after passing through bacterial filters.
• Viruses are fully dependent on living cells for replication—eukaryotic or prokaryotic cells—obligatory intracellular parasites.
• Some viruses may possess complex enzymes (RNA or DNA polymerase) but cannot replicate their genome independently.
• Viruses are acellular; they replicate by utilizing the cellular machinery of other organisms.

History of Virus Discovery

• The term “virus” originated in the early 1790s, pre-germ theory, and referred to poisons.

• Significant milestones (dates are approximate):

  • 1798: Jenner's work on smallpox
  • 1890: Iwanowski's discovery of viruses.
  • 1911: Beijerinck's research.

• 1930s-1970s: Continued advancement of virology and development of techniques.

• 1984: Identification of the AIDS virus.

• 1995: Development of multiple drug treatments for HIV/AIDS (protease inhibitors and reverse transcriptase inhibitors)

Virus Size

• Virus sizes vary.
• Example virus sizes are given in nanometers (e.g., poliovirus - 30 nm, Ebola virus - 970 nm, bacteriophages - 24-225 nm).
• Virus sizes are compared to bacteria and human cells.

Virus Structure: Composition

• Capsid: The protein coat surrounding the viral genetic material.
  • Capsid subunits are called capsomers; capsomer arrangement defines the virus's symmetry.
• Viral envelope (not all viruses): Some viruses gain a lipoprotein coat ("budding").
• Function of the capsid:
  • Protection of Nucleic Acid.
  • Transport of Nucleic Acid From Cell to Cell.
  • Provides Specificity for Attachment.

Virus Morphology

• Helical viruses: Resemble rods (e.g., rabies, Ebola).
• Polyhedral viruses: Many-sided (e.g., poliovirus).
• Complex viruses: More complex structure (e.g., bacteriophages).

Unconventional Agents

• Viroids: Small, single-stranded circular RNA molecules that are not packaged and appear to cause plant diseases only.
• Prions: Proteinaceous particles that affect the structure of other proteins, causing various neurological diseases (e.g., Creutzfeldt-Jakob Disease).

Viral Replication

• Viral replication has six stages:
  • Attachment: Viruses bind to host cell receptors.
  • Penetration: Entry into the host cell by endocytosis or fusion.
  • Uncoating: Viral capsid is degraded.
  • Replication: Viral genetic material and proteins are produced.
  • Assembly: Viral components assemble into new viruses.
  • Release: Viruses exit the host cell (lysis or budding).

Viral Classification

• Classification based on nucleic acid type (DNA or RNA):
• RNA-containing viruses are classified into various groups (Picornaviruses, REOviruses, Bunyaviruses, retroviruses, and more)
• DNA-containing viruses are categorized (Parvoviruses, HepaDNAviruses, Papovaviruses, Adenoviruses, Herpesviruses, and Poxviruses).
• Classification also based on disease symptoms: generalized disease such as smallpox, or specific organ, as in nervous system, eye, lungs and more.
• Additional classification based on the geometry of the viral capsid or morphological characteristics.

Virus Genomes

• Viruses can have either DNA or RNA genomes but not both.
• The nucleic acid can be single-stranded or double-stranded.
• Different types of viral genomes correlate with different replication mechanisms.