Meaning and Definition of a Virus

Meaning and Definition of a Virus

• Viruses are derived from the Latin word for "poison"
• Dimitri Ivanosky discovered it in 1892
• Beijerinck coined the term "virus" in 1897
• Wendell Stanley found viruses consist of nucleic acids, protein, and lipids in 1935
• Bawden (1943) defined a virus as an obligatory parasitic organism smaller than 200mµ
• Loria (1953) described viruses as sub-microscopic entities reproducing inside living cells
• Luria and Darnell defined viruses as entities with a nucleic acid genome (DNA or RNA) that uses the host cell's machinery to synthesize
• Konin et al. (2006) defined a virus as a small infectious agent replicating only inside living cells
• Virion is another term used for virus
• Virology, a branch of Microbiology, is the study of viruses
• Millions of virus types exist but only about 5,000 have been described in detail

Origin of Viruses

• The evolutionary origin of viruses is unclear
• Some evolved from plasmids while others may have evolved from bacteria
• Viruses contain genetic material, reproduce, and evolve through natural selection
• Viruses lack key characteristics generally needed to be considered a life-form
• Viruses are seen as organisms at the edge of life, possessing some but not all characteristics of living things

Morphology of a Virus (Virion)

• Virions consist mainly of a nucleoid/genome/genetic material and a capsid/protein coat
• Some viruses have an external envelope

The Genetic Material

• The genetic material is nucleic acid inside the capsid
• Nucleic acid contains pentose, phosphorus, and nitrogenous bases like pyrimidine (RNA) or purine (DNA)
• A virus contains DNA or RNA, but not both
• Plant viruses typically have RNA, while animal and bacterial viruses usually have DNA
• Nucleic acid constitutes 1% to 50% of the virion's content
• RNA virus nucleic acid tends to be smaller than DNA virus nucleic acid
• DNA or RNA can be single or double-stranded, circular, or linear
• Single-stranded nucleic acid resembles half a ladder, while double-stranded resembles a complete ladder
• Single-stranded RNA can carry a positive or negative charge, depending on whether it is complimentary to the viral mRNA
• Virions may contain linear nucleic acid like Tobacco Mosaic Virus (TMV)
• Virions may contain circular nucleic acid like bacteriophages, such as Coliphage X
• Virions may contain double-stranded RNA, such as Reovirus
• Virions may contain double-stranded DNA, such as Herpes virus
• Virions may contain single-stranded DNA, such as Coliphage and Influenza virus
• Virions may contain single-stranded RNA+ molecule, such as Tobacco Mosaic Virus and Picornavirus
• Virions may contain single-stranded RNA- molecule, such as Rhabdovirus
• Nucleic acid determines the genetic properties of the virus
• The infectious part of the virus contains codes for protein and virus synthesis

The Capsid

• The capsid is a protein coat around the nucleic acid
• Capsids comprise smaller units called capsomeres, made of one or more protein units (protomeres)
• Some capsids consist of one type of capsomere, while others consist of multiple
• Capsid shapes can be spherical, helical, polyhedral, or binal/complex
• The capsid protects nucleic acid from the external environment, facilitates entry into host cells, aids in nucleic acid/protein synthesis, and provides morphological distinction
• Capsids are antigenic

The Viral Envelope

• Some viruses have a 10-18 mm thick envelope over the capsid
• It's a modified form of the host cell's membranes, containing proteins, lipids, and carbohydrates from the host
• Viruses with envelopes are called enveloped viruses and rely on the envelope for infectivity
• The envelope provides rigidity and shape to enveloped viruses
• Viruses lacking envelopes are called naked viruses
• Some enveloped viruses possess spikes for host cell attachment

Other Components of a Virion

• Other components include enzymes and carbohydrates

Enzymes

• The first discovered viral enzyme was neuraminidase, found in Influenza and Paramyxo viruses
• DNA-dependent RNA polymerase was later found in Vaccinia virus
• These enzymes, called transcriptases, synthesize messenger RNA (mRNA) from the viral nucleic acid
• Reverse transcriptase, found in RNA Tumor virus, synthesizes DNA from an ssRNA template
• These enzymes are essential for infection and growth

Carbohydrates

• Carbohydrates have been detected in some viruses like Influenza virus
• However, their function remains unknown

Characteristics or Properties of Viruses

• Viruses are obligatory, acellular, non-cytoplasmic infectious agents
• Viruses possess only one type of nucleic acid–DNA or RNA
• Viruses are the smallest and most primitive organisms, visible only with an electron microscope
• Viruses infect all types of organisms
• Viruses are abundant in almost every ecosystem
• Viruses multiply by replication inside host cells
• Viruses lack their own metabolism
• Viruses undergo genetic change through mutation, genetic drift, recombination, or reassortment
• Viruses can be crystallized and precipitated
• Viruses have definite races or strains
• Viruses are host-specific, with narrow or broad host ranges
• Viruses are effective in small doses
• Viruses resist antibiotics, germicides, and extreme conditions
• Virus shapes vary considerably
• Virus size and complexity vary, ranging from 20-700 nanometers
• Viruses are transmissible from diseased to healthy organisms
• Viruses are considered both living and non-living; they are inactive outside host cells but active inside cells

Classification or Diversity of Viruses

• The Lwoff et al. (1962) classification system is widely used
• The classification system is based on virion properties like nucleic acid type, capsid shape, host range, size, synthesis site, presence/absence of envelope, and means of transmission

Classification of Viruses Based on Nucleic Acid Type

• Double-stranded RNA viruses, e.g., Reovirus
• Double-stranded DNA viruses, e.g., Herpes virus
• Single-stranded DNA viruses, e.g., Coliphage, Influenza virus
• Single-stranded RNA+ viruses, e.g., Tobacco Mosaic Virus, Picornavirus
• Single-stranded RNA- viruses, e.g., Rhabdovirus

Classification of Viruses Based on Capsid Shape

• Helical viruses (e.g., Tobacco mosaic virus)
• Polyhedral viruses (e.g., Polio virus)
• Spherical viruses (e.g., Influenza virus)
• Binal viruses (Bacteriophage or Phage)

Classification of Viruses Based on Host Range

• Host range is the spectrum of organisms a virus can infect
• Some viruses are host-specific, while others infect multiple hosts
• Host range depends on receptors on the host cell's surface
• Four major groups of viruses: animal, plant, fungal (myco), and bacterial (bacteriophages)

Classification of Viruses Based on Size

• Viruses range from approximately 2.5nm to 300nm
• The largest virus (T4) contains 77 genes
• The smallest viruses (Qβ and MS2) contain only 3 genes

Classification of Viruses Based on Synthesis Site in Host Cell

• Viral synthesis occurs in the nucleus or cytoplasm
• Influenza virus synthesized in the nucleus
• Para-influenza virus is synthesized in the cytoplasm

Classification of Viruses Based on Envelope Presence

• Enveloped viruses have viral envelopes like Influenza, Herpes, and Pox viruses
• Naked viruses lack viral envelopes like Coliphage ϕ X 174 and Coliphage fd

Classification of Viruses Based on Transmission Means

• Plant and animal viruses are transmitted by insect vectors
• Some are transmitted by fungi and nematodes
• Some are transmitted through the blood and saliva of vertebrate animals

Taxonomy of Viruses

• Initially, viruses were named after their host range or affected organ
• By the 1960s, this naming system was inadequate so a new system was needed
• The International Committee on Taxonomy of Viruses (I.C.T.V.) was established in 1966
• The system has five hierarchical levels: Family, Subfamily, Order, Genus, and Species
• Family names end with -viridae
• Subfamily names end with -virinae
• Order names end with -virales
• Generic names end with -virus
• Specific names are English words
• Using the I.C.T.V. system, the virus causing AIDS is classified as follows:
  • Family: Retroviridae (Retrovirus)
  • Subfamily: Lentivirinae
  • Order: Retrovirales
  • Genus: Lentivirus
  • Species: Human Immunodeficiency Virus (HIV)

Uses of Viruses

• Cause human diseases like common cold, influenza, chicken pox, ebola fever, and HIV/AIDS
• Cause plant diseases like mosaic diseases, cocoa swollen shoot disease, and citrus tristeza disease
• Virulence describes a virus's ability to cause disease, which can be endemic, epidemic, or pandemic
• Essential for regulating salt water and freshwater ecosystems
• Important for transferring genes between species, increasing diversity and driving evolution
• Can be used as carriers for genetically modified genomes in nanotechnology
• Important in molecular and cell biology
• Can be modified to encapsulate materials for biotechnology and biomedicine
• Used in virotherapy to treat diseases by targeting cells and DNAs
• Used as scaffolds for surface modifications in material science
• Synthetic viruses develop vaccines for human and animal diseases
• The ability of viruses to cause death and devastation has led to concerns of biological warfare
• Genetic engineering methods can modify genomes for gene transfer, leading to more productive organisms
• Used for biological control of insect and vertebrate pests by causing disease
• Less toxic than conventional pesticides, but have limitations such as limited range, slow effects, high cost, and low environmental stability