What Are Viruses? Discover the Classification and Properties of Viruses (2024)

“Viruses are infectious agents that replicate inside the body of a host.”

Table of Contents

• • Explanation
  • Structure
  • Properties
  • Classification
  • Virus Reproduction
  • Diseases
  • Economic Importance of Virus

• • Frequently Asked Questions About Viruses

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 a living organism or non-living. A virus can be an infectious agent which only replicates within a host organism. 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 lackthe required cellular machinery. Therefore, itenters and attaches itself to a specific host cell,injects its genetic material, reproduces byusing the host genetic material and finallythe host cell splits open, releasing the newviruses.

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.

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Structure and Functionof Viruses

Viruses are tiny and smaller in its size, ranging between 30-50 nm. Viruses do not contain cells and usually lack a 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 structure is packed in a capsulated symmetric protein. The protein associated with nucleic acid (also known as nucleoprotein) produces the nucleocapsid with the genome.

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Bacteriophage and HIV

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 living 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 and the nucleic acid with 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 virus is known as virology.

Further Reading:Microbes

Properties of Viruses

1. They are non-cellular organisms, which is enclosed in a protective envelope.
2. The presence of spikes helps in attaching the viruses to the host cell.
3. These viruses do not grow, neither respire nor metabolize, but they reproduce.
4. They are surrounded by a protein coat – capsid and have a nucleic acid core comprising DNA or RNA.
5. 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.

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Classification of Viruses

Viruses can be classified primarily on their phenotypic characteristics, core content, chemical composition, capsid structure, size, shape, modes of replication and other viral genome structures.

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 the1970s, for which he was awarded the Nobel Prize.

The below virus information describes the classification of viruses based on their different criteria.

Classification based on the presence of nucleic acid

DNA virus

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, Herpes virus, etc.

RNA virus

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

Viruses come in different shapes, from basic helical and icosahedral shapes to more intricate ones. The classification based on different shapes and symmetry of viruses are as follows:

1. Complex virus. E.g Poxvirus
2. Radial symmetry virus. E.g.Bacteriophage
3. Cubical or icosahedral symmetry shapedvirus. E.g. Reovirus, Picornavirus
4. Rod or Spiral shaped or helical symmetry virus. E.g. Paramyxovirus, orthomyxovirus

Read more: Virus Nomenclature

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.

1. Replication within the cytoplasm of the host cell.
   E.g. All RNA viruses except the Influenza virus.
2. Replication within the nucleus and the cytoplasm of the hostcell.
   E.g. Influenza virus, Poxvirus, etc.
3. Replication within the nucleus of the hostcell.
   All DNA viruses except Pox virus.
4. Replication of the virus through the double-stranded DNA intermediate.
   E.g. All DNA viruses, Retrovirus and some tumour causing RNA virus.
5. Replication of the virus through a single-stranded RNA intermediate.
   E.g. All RNA viruses except Reovirus and tumour-causing RNA viruses.

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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. Replication of plant viruses is obligate and does not happen without a host. Well-known examples of plant virus include the potato virus, tobacco mosaic virus (TMV), beet yellow virus, and turnip yellow virus, cauliflower mosaic virus, etc.

Bacteriophage

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 page, etc.

Explore more:Bacteriophage

Insect virus

The virus which infects insectsis known as Insect virus, also called the viral pathogen ofinsects. Theseviruses are considered as a powerful biocontrol agentin the landscape of modern agriculture. AscovirusvirionsandEntomopox virus, are best examples for insect virus.

Classification based on the mode of transmission

1. Airborne infections – Transmission of the virus through the air into the respiratory tract. E.g. Swine flu, and Rhinovirus.
2. Fecal oral route – Transmission of the virus through the contaminated water or food.
   E.g. Hepatitis A virus, Poliovirus, Rotavirus.
3. Sexually transmitted diseases – Transmission of the virus through sexual contacts with the infected person. E.g. Retrovirus, human papillomavirus, etc.
4. Transfusion-transmittedinfections- Transmission of the virus through the blood transfusion.
   E.g. Hepatitis B virus, Human Immunodeficiency Virus, etc.
5. Zoonoses-Transmission of the virus through the biting of infected animals, birds, and insects to human. E.g. Rabies virus, Alpha virus,Flavivirus, Ebola virus, etc.

Also Read:Virology