Origin of Viruses
Virology is stated as study of viruses that are small, acellular entities which generally have only a single kind of nucleic acid and which should use metabolic machinery of the living host to reproduce.
Viruses are inert in extracellular environment and only come alive in contact with the living host cell. They replicate only in living cells. Viral nucleic acid has all information essential for programming infected cells to synthesize the number of virus specific macromolecules needed for production of viral progeny. Viral genome takes control of metabolism of host cell. Viruses are known to infect the wide range of host cells ranging from bacteria, mycoplasma, algae, invertebrates, all higher plants and animals.
Terms in Virology
Capsid: This is a protein shell or coat which surrounds nucleic acid of virus. Empty capsids may be result of replicative cycle of viruses with icosahedral symmetry.
Nucleocapsid: This refers to capsid together with surrounded nucleic acid.
Virion: This is the complete infectious unit or complete viral particle. The virion transfers nucleic acid from cell to cell.
Viriod: This refers to some naked genetic materials which are air-borne but lacks capsid. They also cause infection on contact with the living host cell. They generally cause plant infections.
Envelope: This is the lipid having membrane which surrounds some viruses. It is obtained during virus maturation by budding process through cellular membrane. Virus surrounded glycoproteins are exposed on surface of envelope.
General characteristics of Viruses
Chemical Composition of viruses
Chemical constituents of viruses comprise the following: nucleic acid, Proteins, lipids and carbohydrates. They are given below.
i) Viral protein: The major objective of viral protein is to help transfer of viral genome. They protect viral genome against action of nucleases of the host cell and participate in attachment of the virus particle to the susceptible cell during infection.
ii) Nucleic acid: Viruses have only single kind of nucleic acid, DNA or RN which encodes all genetic information essential for replication of virus. Viral nucleic acid is also referred to as viral genome. Viral genome can be single or double stranded, circular or linear, segmented or linear.
iii) Lipids: The number of different viruses has lipid envelopes as part of their structure. Lipid is needed when virus nucleocapsid buds through the cellular membrane during maturation. Phospholipid composition is determined by specific kind of cell membrane in budding process.
iv) Viral Carbohydrate: Virus envelopes have glycoproteins. On the contrary to lipids they are derived from host cell, the envelope glycoproteins are virus-coded though the sugars added to the virus glycoprotein reflect host cell in which the virus is developed.
Several thousands of virus have being categorized and have been found in all main groups of organisms from prokaryotes to eukaryotes. There are more than 2000 descriptions on virus infection bacteria alone. Taxonomy is a science of classification, very dynamic and substitution and subject to change on basis of available data. Important development in categorization of viruses is documented in the reports of International Committee on Nomenclature of Viruses (ICNV). In viral taxonomy, family name ends with the suffix VIRIDAE, the subfamily name with d suffix VIRINAE and the genus name with the suffix VIRUS.
Factors considered in classification of viruses
Symmetry in Virus
Use of electron microscope and x-ray diffraction methods have made and possible to determine differences in the basic morphology of viruses. Heavy metal stains like potassium phosphotungstate is utilized in highlighting viral surface structure. Heavy metal permeates the virus particle like the cloud and brings out surface structure of the viruses by virtue of negative staining. There are three basic symmetries in viral morphology namely Cubic or Icosahedral, Helical, Complex structures and Binal which comprises of both cubical and helical symmetry like T-even coliphage, a bacteriophage.
Categorization of DNA viruses
i) Parvoviridae: They are extremely small DNA viruses with particle size of approx 18-22nm in diameter with icosahedral symmetry of 32 capsomers. They are non-enveloped with single stranded DNA (ssDNA) of molecular weight (MW) of 1.5-2.0 × 106, G+C content of 41-53%. Physicochemical properties of the virion comprises MW of 5.5 = 6.2 106; S20W = 110-122; buoyant density of 1.39-1.42g/dm3 in cesium chloride (CsCl).
ii) Papoviridae: Name is derived from sigla 'PA'= papilloma; PO = polyoma and VA = vacuolating agent (SV40). Members of this family are genera polyomavirus and papillomavirus. Vrions are small in size of approx 40-55nm in diameter, heat stable, ether and acid resistant. They have the icosahedral symmetry with 72 capsomers in skew arrangement, although filamentous forms take place.
iii) Herpesviridae: The name was derived from Greek word "Herpes", "herpetos" signifying creeping, or crawling creature, from nature of herpes febrilis lesions in infected patients. There are 3 sub families in this family.
a) Herpes Simplex Virus group known as Alpha Herpesvirinae.
b) Cytomegalovirus group known as Beta Herpesvirinae.
c) Lymphnproliferative Virus group called Gamma Herpesvirinae
iv) Poxviridae: Name was derived from old English Poc, Pocc-, (Plural of pock) signifying Pustule or Ulcer as one of the symptoms shown on infected individuals. They are large, fairly pleomorphic, brick-shaped or ovoid virion, 230-450nm x 140-260nm, with external coat having lipid and tubular or globular protein structures, surrounding one or two lateral bodies and core that has the genome.
v) Hepadnaviridae: Name was derived from sigla 'HEPA' due to its affinity for liver cells that is heap tropism, DNA meaning the kind of genome that is deoxyribonucleic acid (DNA). They are spherical in shape, 40-4nm in diameter, with no surface projections. They are of 7nm in diameter and detergent sensitive.
Classification of RNA viruses
i) Picornaviridae: The picornavirus is a virus belonging to family Picornaviridae. Picornaviruses are non-enveloped, positive-stranded RNA viruses with the icosahedral capsid.
ii) Calciviridae: These group of viruses are like Picornaviridae but somewhat larger (approx 35-39nm) in diameter. Genome is the single-stranded positively sensed RNA. They are non-enveloped viruses.
iii) Reoviridae: Genera of this family are reovirus group (that is Orthoreovirus, Orbivirus, Coltivirus, Rotaviris, Aquareovirus); cytoplasmic polyhedrosis virus group (cypovirus); plant reovirus subgroup 1 (phytoreovirus); plant reovirus subgroup 2 (Fijivirus) and plant reovirus subgroup 3.
iv) Arboviridae: The sigla ARBO is derived from Arthropod (AR) - Borne (BO) viruses. The ecologic grouping of viruses with varied physical and chemical properties. All these viruses (approx 350 in number) have complex cycle comprising arthropods as vector which transmits viruses to vertebrate hosts by their bite.
v) Toagaviridae: Togaviridae family is a member of group IV of Baltimore classification of viruses. Genome is linear, single-stranded, positive sense RNA i.e. 10,000-12,000 nucleotides long.
vi) Arenaviridae: Name 'Arena' is derived from Latin word 'arenosus' meaning 'Sandy' due to appearance of viral particles in EM sections. Members of this family are:
(a) Lymphocytic Chroriomenigtis (LCM) that is Lassa, Mopeia, Mobala, and Ippy virus.
(b) Tacarbe Complex: that is Tacaribe, Junin, Amapari, Macupo, Parana, Pichinde, Latino and Flexal virus.
vii) Coronaviridae: They have petal-shaped projections arranged in fringe like solar corona. Name (Prefix "Corona") is derived from Latin word Crown from appearance of surface projections in negatively stained electron micrographs. Members of tile family are Human coronavirus, Mirune hepatitis virus, Porcine transmissible gastroenteritis virus, bovine coronavirus, canine coronavirus, Feline infectious peritonitis virus, Turkey, rat and Rabbit coronavirus.
viii) Retroviridae: They are enveloped viruses of approx 90 - 120nm in diameter whose genome has duplicate copies of high molecular weight, SSRNA of same polarity as viral mRNA. Enzyme is liable for reverse transcription of genomic RNA to DNA in infected host cells.
ix) Rhaboviridae: They are bullet shaped for those infecting vertebrates and invertebrates but are baciliform for those infecting plants. They are 100 - 430nm long and 45 - 100nm in diameter with surface projections (G - proteins), 5-10nm and long 3nm in diameter. The central axial channel is observed in thin section of virion.
x) Toroviridae: Name 'Toro" is derived from Latin word 'torus meaning lowest convex melding in base of a column. Other members are Breda virus (infecting cattle), Torovirus, infecting man and possibly carnivores like mustellids. They are pleomorphic, kidney, biconcave disk, and rod- shaped viral particles of approx 20 - 140nm diameter having elongated tubular capsid with helical symmetry.
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