What is Virus in Biology?
The word Virus comes from Latin word means ” Slimy Liquid” or ” Poison”. The viruses are basically small size infection causing agent. Which multiply in only living cells of Plants, Animals or Bacteria as well. Virus Structure is described below:
Virus Structure and composition of viruses vary from species to species. Viruses mainly consist of two components:
1) Nucleic Acid Genome
2) Protein Capsid
Protein Capsid covers the Nucleic Acid Genome. Collectively, Protein Capsid and Nucleic Acid Genome are known as Nucleocapsid.
Additionally, many animal viruses consist of Lipid Envelope. Viruses which contain lipid envelope are called Viron.
Classification of Viruses:
Viruses are classified into five main Groups on the basis of Nucleic Acid, Structure, Symmetry, Mode of Replication, Transmission and Host Range.
- Classification on the Basis of Nucleic Acid
- Classification on the Basis of Structure
- Classification on the Basis of Replication Site
- Classification on the basis of Mode of Transmission
- Classification on the basis of Host Range
Classification on the Basis of Nucleic Acid
On the basis of Nucleic Acid Viruses are of two Types:
- DNA Virus
- RNA Virus
DNA Viruses are those Viruses which contain DNA viral Genome. DNA Viruses are furtherly subdivided into two classes on the basis of either Single Strand DNA Genome or Double-Strand DNA Genome.
Double-stranded DNA virus:
These Viruses are such Viruses which contain Double-Strand DNA Genome in Capsid. For Example Adeno Virus, Herpes Virus
Single-stranded DNA virus:
These Viruses are such Viruses which contain Single Strand DNA Genome in Capsid. For Example Parvo Virus, φ174 Virus
RNA Viruses are those Viruses which contain RNA viral Genome. RNA Viruses are furtherly subdivided into two classes on the basis of either Single Strand RNA Genome or Double Strand RNA Genome.
Double-stranded RNA virus:
These Viruses are such Viruses which contain Double Strand RNA Genome in Capsid. For Example Reo Virus.
Single-stranded RNA virus:
These Viruses are such Viruses which contain Single Strand RNA Genome in Capsid. For Example Polio Virus, Influenza Virus. These Viruses are furtherly Subdivided into two types in the basis of either Positive-sense RNA (+RNA) or Negative-sense RNA (-RNA).
Positive-Sense RNA (+RNA)
Positive-sense RNA are those Viruses which contain positive (+RNA) Strand. For Example Polio virus, Hepatitis A
Negative-sense RNA (-RNA)
Negative-sense RNA are those Viruses which contain negative (-RNA) Strand. For Example: Rabies virus, Influenza virus.
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Classification of Virus on the Basis of Structure
On the basis of Structure, Viruses are Classified into following Subclasses:
Cubical viruses are those Viruses which contain Icosahedral Symmetry. For Example Reo Virus, Picorna Virus.
Spriral Viruses are hose which contain Helical symmetry. For Example Paramyxo Virus, Orthomyxo Virus.
Radial Symmetry virus:
These Viruses are those which have Radial Symmetry. For Example Bacteriophage.
Complex Viruses are those Viruses which consist of Complex Symmetry. These Virus Structure is very comples For Example Pox Virus.
Classification of Virus on the basis of Replication Site
Replication in Host Cytoplasm:
Replication of RNA Viruses take place in Cytoplasm of Host but in Influenza Virus this process did not occur.
Replication in Host Nucleus and Cytoplasm :
Replication of RNA Viruses takes place in Nucleus and Cytoplasm of Host. For Example Influenza Virus, Pox Virus
Replication in Host Nucleus:
Replication of all DNA viruses occur in Nucleus of Host Cell but this process did not occur in Pox virus.
Replication through Intermediate DNA :
Replication of all DNA viruses, Retrovirus, and some RNA virus occur through DNA as intermediates.
Replication through Intermediate RNA :
Replication of all RNA viruses except Reo Virus and Tumor causing RNA viruses to occur through RNA as intermediate.
Classification of Virus on the Basis of Host Range:
On the Basis of Host Range Viruses has the following Classification:
Bacteriophages are that virus which infects Bacteria. For Examples λ phage, T2, T4, φ174, MV-11
Plant Virus is that virus that infects plants. For Example TMV, Cauliflower Mosaic Virus
3. Animal virus:
The animal virus is those that infect animals. For Example Polio Virus, Retro Virus, Herpes Virus, Adeno Virus
4. Insect virus:
Insect Virus is that Virus which infects insects. For Example Baculovirus, Sacbrood Virus Structure Virus, Entomopox Virus, Granulosis Virus
Classification of Virus on the basis of Mode of Transmission:
On the Basis of Mode of Transmission Viruses has the following types :
Respiratory Mode of Transmission :
The virus that is transmitted through Respiratory Route. For Example Swine flu, Rhino Virus.
Faeco Oral Mode of Transmission:
These Viruses are transmitted by Faeco-oral route. For Example Hepatitis A virus, Polio Virus, Rota Virus
Sexual Mode of Transmission
These Viruses are transmitted by sexual contacts. For Example Retro Virus.
Blood Transfusion Mode of Transmission
These Viruses are transmitted through blood transfusion. For Example Hepatitis B Virus, HIV.
10 Unique Characteristics of Viruses
- Viruses are ultramicroscopic pathogens and able to cause diseases both in plants and animals.
- Viruses have a simple unit system composed of the modest structure.
- Viruses are usually composed of nucleic acid that is surrounded by a protein coat.
- In the viruses’ nucleic acid can be either RNA or DNA but never exist both in the virus body.
- Being a single system unit virus have no cellular organization.
- Viruses have no machinery for any sort of metabolic activities.
- Viruses are obligate intracellular parasites.
- Viruses can easily multiply within their host cells but if they come outside the host cell they are completely inactive and unable to multiply.
- Viruses are very delicate to stimulants like radiation & chemical substances etc.
- Viruses are host specific and have antigenic possessions.
Are Viruses Living ?
Are Viruses Living or Non- Living?
Scientists are not certain about whether viruses are non-living or living. But in short, scientists practice a list of standards to determine if the Virus is alive.
Living characteristics of viruses
- Viruses have the ability to multiply inside a host plant or animal cell.
- They are pathogenic have the ability to cause diseases.
- Viruses possess of nucleic acids, proteins & enzymes.
- Viruses being living able to undergo mutation.
- Viruses genetic material DNA & RNA can replicate.
- Mutation is well-established by the availability of mutant forms in some viruses.
- They are delicate to stimulants like radiation, chemical substances etc.
- Viruses can multiply in the living cells of the host.
- The viruses possess antigenic property.
- Viruses can attack to a specific host.
Non-Living Characteristics of Viruses
- They are unable to multiply extracellularly.
- Viruses do not have any metabolic activity.
- In viruses no protoplasm present.
- Viruses can be crystallized.
Examples of Viruses
- Tobacco mosaic virus(TMV)
- T4 bacteriophage:
- Human Immunodeficiency Virus (HIV)