Definition of Virus: Characteristics, Types and Generalized Structure

Table of Contents


  • Etymology and Historical Context
  • Definition of Virus

Characteristics of Virus

Living Characteristics

  • Possession of DNA or RNA
  • Reproductive Abilities
  • Mutation and Genetic Recombination
  • Disease-Causing Agents

Non-living Characteristics

  • Subcellular Structure
  • Lack of Respiration and Excretion
  • Crystallization

3. Classification of Virus

  • Based on Hosts
  • Animal virus
  • Plant Viruses
  • Bacteriophage 

Virus Based on Morphology

  • Spherical Virus
  • Tadpole Virus
  • Rod-shaped Virus

4. Generalized Structure of a Virus

  • Non-Cellular Nature
  • DNA or RNA Filament Core
  • Capsid
  • Capsomere

Definition of Virus 

Definition of Virus: Characteristics, Types and Generalized Structure

The word "virus" is derived from the Latin word "venome," meaning a poisonous substance. Initially, it was used to describe any disease-causing agent.

A virus can be defined as a non-cellular infectious entity containing RNA or DNA encased in a protein coat, reproducing only in a living cell.

Status and Characteristics of Virus.

Virus lie on the border line between living and non living things. They have both characteristics of living and non living things.

Living Characteristics of Virus.

  • Virus Posses DNA or RNA which is the basic Character of living things.
  • They have the ability to reproduce
  • They have undergo mutation and genetic recombination.
  • They are disease causing agents which have the ability to infect other living organisms.

Non living Characteristics of Virus.

  • They are sub cellular or non cellular structure.
  • They do not respire.
  • They do not excrete. 
  • The can be Crystallized

Classification of Virus.

Viruses can be broadly classified based on morphology and the types of host they infect.

Classification on the base of hosts.

Animal Virus.

They are the parasites of animals and human being and cause disease in them. They are usually consist of DNA Core protected by capsid. Some Common diseases of animals virus in human are Polio virus, Small pox, Measles, Mumps amd influenza etc.

Plant Viruses. 

They cause diseases in plants. A plant virus consists of RNA core surrounded by protective protein coat.
For example.
Tobacco mosaic Virus (TMV)

Definition of Virus: Characteristics, Types and Generalized Structure

Bacteriophage virus.

This virus is the parasite of bacteria which only infect bacteria.

Definition of Virus: Characteristics, Types and Generalized Structure

Classification on the base of Morphology/ shape.

1. Spherical Virus 

e.g Polio virus

2. Tadpole virus

e.g Bacteriophage

3.Rode shape virus

e.g. tobacco mosaic Virus

Generalized Structure of a Virus 

The viruses are non-cellular and possess a very simple structure. Each virus consists of a DNA or RNA filament forming a core. 


Definition of Virus: Characteristics, Types and Generalized Structure

The virus DNA is surrounded by a coat of protein called a capsid. This structure formed of nucleic acid and capsid is generally termed nucleocapsid.
The capsid is made up of proteins. These include basic proteins, the histones, histone-like proteins (adenoviruses) or small peptides and polyamines (bacteriophages)
Lipids and carbohydrates are also found in viruses. These include phospholipids, glycolipids, neutral fats, fatty acids, and cholesterol. Phospholipid is predominant in viral


The virus coats (capsids) are often built up of identical repeating units Called 
capsomeres. The capsomeres form highly symmetrical structures that can be crystallized and help to study their structure through X-ray crystallography and electron microscopy.
Most animal viruses such as herpes and influenza viruses, have an additional
lipoprotein envelope derived from the surface membrane of the host cell. However, all the
plant viruses and phages are naked.

Viral Genome

Definition of Virus: Characteristics, Types and Generalized Structure

The viral genome consists of nucleic acids. Bacteriophages and animal viruses
contain DNA whereas plant viruses contain RNA usually. Both DNA and RNA never exist in the same virion. Virions that contain only a single copy of the nucleic acid are called 
haploid. However, retroviruses are contains two identical RNA single strands. So they are
termed diploid.

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