Comprehensive UPSC Science and Tech Study Material

Virus – A Detailed Overview

30/08/2024

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What is a Virus?

A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to bacteria and archaea. They are much smaller than bacteria and can only be seen with an electron microscope.

Structure of Viruses

Viruses come in all sizes and can be arranged into different groups based on what they look like and this is known as morphologies.
It is made of nucleic acid which is encapsulated in a protein shield known as capsid. These capsids are made up of protein structures known as capsomeres.
Some also possess a lipid ‘envelope’ taken from the host cell membrane during preparation of the virus.
The first level of capsid organization is based on the shape that can be used for the morphological differentiation.
A protein subunit which is encoded by a virus is likely to self-assemble to form a capsid often aided by the genome of the virus. Some viruses have segmented genome thatdecode to produce proteins that can help in the building of their capsid.

Classification of Viruses

Essentially, the primary methods of categorizing viruses include based on their phenotype, on molecular content, on composition and on the nature of their capsid.

Baltimore Classification

- Baltimore classification is perhaps the most used system in a classification of viruses.
- This system was invented by American biologist David Baltimora in the seventies wherein he received a Nobel Prize for the same. Bacteria’s classification is based on their common characteristics and the kind of nucleic acid which forms the virus’ genetic material.
- A unified taxonomy has been developed by the ICTV that divides the viruses into six realms, ten kingdoms, seventeen phyla, two subphyla, forty classes, seventy two orders, eight suborders, 264 families, one hundred and eighty two subfamilies, two thousand eight hundred and eighteen genera, eighty four subgenera and eleven thousand two hundred and seventy three species till the year 2022.

Classification Based on Nucleic Acid

Viruses can be classified based on the presence of nucleic acid:

DNA Viruses:

- Single-stranded (ss) DNA viruses: for example: Picornaviruses, Parvovirus
- Double-stranded (ds) DNA viruses: For example; adenoviruses, herpesviruses.

RNA Viruses:

- Double-stranded (ds) RNA viruses: e. g. Reovirus
- Single-stranded (ss) RNA viruses, further classified into positive sense (+RNA) and negative sense (-RNA): for example: Poliovirus, Hepatitis A, Rabies virus, Influenza virus

Classification Based on Structure
- Complex viruses
- Radial symmetry viruses
- Cubical or icosahedral symmetry viruses
- Rod or spiral shaped/helical symmetry viruses

Characteristics of Viruses

Viruses share several common features:Viruses share several common features:

They are parasitic life forms that are capable of invading hosts that include, for instance, human beings, plants or even animals.
They are composed of a small nucleic acid, which may be either DNA or RNA surrounded by a protein coat known as capsid.
It should also be mentioned that some viruses possess an envelope.
Another concept is that viruses are not capable of replicating themselves with no host.
These are far much smaller than bacteria with most of the viruses measuring between 20-400 nanometers.
Its nucleic acids can be single or double helix and can be either DNA or RNA.