What is a Virus?
Viruses are non-cellular, microscopic infectious agents that can only replicate inside a host cell. From a biological perspective, viruses cannot be classified either as living organism nor non-living. This is due to the fact that they possess certain defining characteristic features of living organisms and non-living entities.
In a nutshell, a virus is a non-cellular, infectious entity made up of genetic material and protein that can invade and reproduce only within the living cells of bacteria, plants and animals.
For instance, a virus cannot replicate itself outside the host cell. This is because viruses lack the required cellular machinery. Therefore, it enters and attaches itself to a specific host cell, injects its genetic material, reproduces by using the host genetic material and finally the host cell splits open, releasing the new viruses.
Viruses can also be crystallized, which no other living organisms can do. It is these factors that lead to viruses being classified in the grey area – between the living and non-living.
Structure and Function of Viruses
Viruses are tiny and smaller in its size, ranging between 30-50nm. They usually lack cell wall but are surrounded by a protective protein coating called the capsid. It can be seen as a genetic element and is characterized by the combined evolution of the virus and the host. They contain either RNA or DNA as the genetic material
Viruses mainly depend on a host to deliver the complex metabolic machinery of prokaryotic or eukaryotic cells for propagation. The main task of the virus is to carry its DNA or RNA genome to the host cell, which then can be transcribed by the host cell. The viral genome is packed in a capsulated symmetric protein. The protein associated with nucleic acid (also known as nucleoprotein) produces the nucleocapsid with the genome.
Diagram of a Virus: (Left) A virus that infects bacteria is called a bacteriophage. (Right) HIV has a long incubation period – roughly ten years.
These microbes belong to the family viridae and genus virus. Viruses could not be placed in any of the kingdoms because they are practically neither live nor dead. The term virus was coined by the Dutch microbiologist, Martinus Willem Beijerinck in the year 1897. It is derived from Latin, which means poison or venomous substance.
Once a susceptible cell is infected, a virus can start the cell machinery to generate more virus. Viruses are composed of a core of DNA or RNA surrounded by a protein coat. They are very small and their size ranges from 20 nanometers to 250 nanometers. Therefore, they can only be seen with an electron microscope
Many viruses have either DNA or RNA as the genetic element. The nucleic acid can have single or double strands. The whole infectious virus, called as virion has nucleic acid and an outer shell of proteins. The simplest virus includes DNA or RNA for encoding four proteins and the most complex encodes 100-200 proteins.
The study of viruses is called as virology.
Further Reading: Microbes
Properties of Viruses
- They are non-cellular organisms, which is enclosed in a protective envelope.
- The presence of spikes helps in attaching the viruses to the host cell.
- These viruses do not grow, neither respire nor metabolize, but they reproduce.
- They are surrounded with a protein coat – capsid and have a nucleic acid core comprising of DNA or RNA.
- They are considered both as living and non-living things. These viruses are inactive when they are present outside of host cells but become active within host cells. These viruses cause several infections and reproduce within the host cell by using the enzymes and raw materials.
Classification of Viruses
Viruses can be classified primarily on their phenotypic characteristics, core content, chemical composition, capsid structure, size, shape, genome structure and modes of replication.
The Baltimore classification is the most commonly used for studying the system of virus classification. This system was developed by an American biologist David Baltimore in the 1970s, for which he was awarded Nobel Prize.
The below flowchart describes the classification of viruses based on their different criteria.
Classification based on the presence of nucleic acid
The virus, having DNA as its genetic material. There are two different types of DNA virus
- Single-stranded (Ss) DNA virus: e.g. Picornaviruses, Parvovirus, etc.
- Double-stranded (Ds) DNA virus: e.g. Adenovirus, Herpesvirus, etc.
The virus, having RNA as its genetic material. There are two different types of RNA virus
- Double-stranded (Ds) RNA virus: e.g. Reovirus, etc.
- Single-stranded (Ss) RNA virus. It is further classified into two Positive sense RNA (+RNA) and Negative sense RNA (-RNA). Poliovirus, Hepatitis A, Rabies virus, Influenza virus are examples of single-stranded RNA virus.
Classification based on the structure or symmetry
- Complex virus. E.g Poxvirus
- Radial symmetry virus. E.g.Bacteriophage
- Cubical or icosahedral symmetry shaped virus. E.g. Reovirus, Picornavirus
- Rod or Spiral shaped or helical symmetry virus.E.g. Paramyxovirus, orthomyxovirus
Classification based on the replication properties and site of replication
Here, viruses invade into the host cell, where it replicates and assembly within the cell organelles.
- Replication within the cytoplasm of the host cell.
E.g. All RNA virus except the Influenza virus.
- Replication within the nucleus and the cytoplasm of the host cell.
E.g. Influenza virus, Poxvirus, etc.
- Replication within the nucleus of the host cell.
All DNA viruses except Pox virus.
- Replication of the virus through the double-stranded DNA intermediate.
E.g. All DNA virus, Retrovirus and some tumour causing RNA virus.
- Replication of the virus through a single-stranded RNA intermediate.
E.g. All RNA virus except Reovirus and tumour-causing RNA viruses.
Classification based on the host range
Based on the type of host, there are four different types of viruses:
- Animal viruses
These viruses infect by invading the cells of animals, including humans. Prominent examples of animal viruses include the influenza virus, mumps virus, rabies virus, poliovirus, Herpes virus, etc.
- Plant viruses
These viruses infect plants by invading the plant cells. Well-known examples of plant virus include potato virus, tobacco mosaic virus, beet yellow virus, and turnip yellow virus, cauliflower mosaic virus, etc.
The virus which infects bacterial cells is known as bacteriophage. There are many varieties of bacteriophages, such as DNA virus, MV-11, RNA virus, λ phage, etc.
Explore more: Bacteriophage
- Insect virus
The virus which infects insects is known as Insect virus also called as the viral pathogen of insects. These viruses are considered as a powerful biocontrol agent in the landscape of modern agriculture. Ascovirus virions and Entomopox virus, are best examples for insect virus.
Classification based on the mode of transmission
- Airborne infections – Transmission of the virus through the air into the respiratory tract. E.g, Swine flu, and Rhinovirus.
- Faecal oral route – Transmission of the virus through the contaminated water or food.
E.g. Hepatitis A virus, Poliovirus, Rotavirus.
- Sexually transmitted diseases – Transmission of the virus through sexual contacts with the infected person. E.g. Retrovirus, human papillomavirus, etc.
- Transfusion-transmitted infections- Transmission of the virus through the blood transfusion.
E.g. Hepatitis B virus, Human Immunodeficiency Virus, etc.
- Zoonoses -Transmission of the virus through the biting of infected animals, birds, and insects to human. E.g. Rabies virus, Alphavirus, Flavivirus, Ebola virus, etc.
List of Viral Diseases
Following is a list of virus diseases that have made a significant socio-economic impact in the last few decades.
- AIDS (Acquired Immunodeficiency Syndrome)
- SARS (Severe Acute Respiratory Syndrome)
- Small Pox (Now eradicated)
Explore more about Viral Diseases
Important Questions about Viruses
1. What is a Virus in Biology?
A virus is a biological entity that can only reproduce within a host. Anatomically, viruses possess nucleic acids (DNA or RNA) which are encased within a protective protein coat. These entities are able to infect all forms of life, ranging from bacteria to humans, and consequently, they bring about a multitude of diseases in their host.
2. State the basis of classification of viruses.
Typically, classification of viruses mainly depends on their phenotypic characteristics such as morphology, chemical composition, structure, and function.
3. List the types of viruses In biology.
Based on their host, viruses can be classified into three types, namely, animal viruses, plant viruses, and bacteriophages.
4. State a few examples of viral diseases.
- Small Pox
5. Why are viruses neither considered living, nor non-living?
Viruses possess trademark characteristics of both living and non-living entities. For instance, they can only reproduce within a host, just like a parasite. But unlike parasites or any other living organisms, viruses can be crystallized. During this stage, they remain dormant, until they enter another host, restarting the cycle all over.
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