Gen Viro Chapter 1: General Virology – Introduction & Viral Components

Virology

The scientific study of viruses, viral diseases, and virus-like agents, covering infection mechanisms, pathogenesis, and treatment or prevention.

Virus

A tiny, acellular infectious agent composed of genetic material inside a protein coat that cannot reproduce on its own and requires a living host cell to replicate.

Viral Size Range

20 to 300 nanometers (nm), making viruses small and highly filterable through standard bacterial filters.

Viral Filterability

The ability of viruses to pass through ultra-fine filters that retain bacteria due to their small size (20–300 nm).

Obligate Intracellular Parasite

An organism or agent that cannot replicate independently and depends entirely on the host cell's metabolic machinery and living tissues.

Acellular Infectious Agent

An infectious entity that lacks cellular organization, possessing no cytoplasm, organelles, or autonomous metabolism.

Mobile Gene Element

A genetic element capable of transferring between cells and incorporating into or modifying host genomes.

Viral Extracellular State

Metabolically inert and dormant when outside a living host cell.

Viral Host Spectrum

Viruses can infect all known forms of life, including animals, plants, bacteria, and archaea.

Viral Genome Types

Consists of either DNA or RNA (never both), occurring in single-stranded (ss) or double-stranded (ds) configurations.

Helical Virus Morphology

Cylindrical capsid architecture where protein subunits wind around a central spiral nucleic acid (e.g., Tobacco Mosaic Virus).

Polyhedral (Icosahedral) Morphology

Regular geometric capsid structure consisting of 20 triangular faces and 12 corners (e.g., Adenovirus).

Spherical Virus Morphology

A roughly rounded outer shape typical of enveloped viruses (e.g., Influenza virus, Coronaviruses).

Complex Virus Morphology

A specialized, multi-part viral structure featuring combined polyhedral heads and helical tail assemblies (e.g., Bacteriophages).

Bacteriophage

A naked (non-enveloped), complex virus that specifically infects bacterial cells and carries DNA as its genome.

Bacteriophage Structural Components

Composed of an icosahedral head (capsid), internal DNA genome, collar/neck, helical tail sheath, base plate, and tail fibres.

Influenza Virus Structure

An enveloped, spherical virus carrying an RNA genome, internal capsid, and surface glycoprotein spikes.

External Viral Coating

The outermost protective layer of a virus that interacts directly with the environment and binds host cell receptors.

Capsid

The outer protein shell of a virus that encloses the genetic material and serves as the outermost layer in naked (non-enveloped) viruses.

Capsomere

Individual structural protein subunits that self-assemble to construct the viral capsid.

Naked Virus

A virus consisting solely of a nucleocapsid with no outer lipid membrane (e.g., Norovirus, Human Papillomavirus / HPV).

Viral Envelope

An outer lipid bilayer membrane found in enveloped viruses (e.g., Influenza, HIV), stolen directly from host cell membranes during viral exit.

Peplomers

Glycoprotein spike projections embedded in the viral envelope that function as ligands to recognize and bind host cell receptors.

Viral Core

The central interior region of a virion containing the nucleic acid genome.

Nucleic Acid (Core Component)

The genetic material (either DNA or RNA, ss or ds) encoding instructions to synthesize new viral components.

Nucleocapsid

The structural complex formed by the viral nucleic acid genome tightly bound inside the protein capsid.

Virion

A single, complete, and fully functional virus particle existing outside a host cell capable of establishing infection.

Viroid

A subviral plant pathogen simpler, smaller, and more primitive than a virus, completely lacking a protein capsid and consisting solely of naked RNA.

Viroid Genetic Structure

A short, naked single-stranded circular RNA molecule that does not encode proteins.

Viroid Replication Characteristics

Produces proteins without depending on host translation machinery and replicates autonomously in plant cells.

Potato Spindle Tuber Disease

A plant disease caused by a viroid resulting in elongated, spindle-shaped potato tubers and stunted growth.

Chrysanthemum Stunt Disease

A plant disease caused by a viroid resulting in reduced plant height and diminished flower size in chrysanthemums.

Adenoviridae (Etymology)

Derived from "Adeno" (gland); named after the human adenoid tissue from which the virus was first isolated.

Astroviridae (Etymology)

Derived from "Astron" (star); describes the distinct star-shaped virion surface morphology under electron microscopy.

Arenaviridae (Etymology)

Derived from "Arena" (sand); describes the sand-like appearance of the virion interior caused by packaged host ribosomes.

Bunyaviridae (Etymology)

Named after Bunyamwera, Uganda (Africa), the geographical locality where the type strain was first isolated.

Caliciviridae / Calicivirus (Etymology)

Derived from "Calix" (cup or goblet); refers to the cup-shaped surface depressions on the viral capsid.

Coronaviridae (Etymology)

Derived from "Corona" (crown); refers to the crown-like halo of club-shaped peplomers protruding from the viral envelope.

Filoviridae (Etymology)

Derived from Latin "filum" (thread or filament); describes the long, filamentous, thread-like virion morphology (e.g., Ebola).

Herpesviridae (Etymology)

Derived from Greek "Herpes" (creeping); describes the creeping, spreading nature of recurring skin lesions.

Orthomyxoviridae (Etymology)

Derived from "Ortho" (true) + "myxo" (mucus); viruses possessing a strong biochemical affinity for mucoproteins and mucus secretions.

Paramyxoviridae (Etymology)

Derived from "Para" (closely resembling / alongside) + "myxo" (mucus); a family closely resembling Orthomyxoviruses.

Parvoviridae (Etymology)

Derived from Latin "Parvus" (small); a family of extremely small, non-enveloped single-stranded DNA viruses.

Poxviridae (Etymology)

Derived from "Pock" (pustule); refers to the characteristic pustular, pus-filled skin eruptions and pox lesions produced.

Rhabdoviridae (Etymology)

Derived from Greek "Rhabdo" (rod); describes the bullet- or rod-shaped virion morphology (e.g., Rabies virus).

Togaviridae (Etymology)

Derived from Latin "Toga" (cloak); refers to the tight-fitting lipid bilayer envelope that cloaks the virion.