What are Viruses?

A virus is neither living nor non-living, so how do we define viruses? 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.

Structure of Virus

Diagram of a virus

These microbes belong to the family of viridae and genus of virus. 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 protein. 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.

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 with a protein coat – capsid and have a nucleic acid core comprising of 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.

Classification of Virus

Viruses can be classified primarily on their phenotypic characteristics such as:

  • Morphology

Viruses are classified based on the chemical composition, size and shape, genome structure and modes of replication. Helical morphology is found in nucleocapsids of different pleomorphic and filamentous viruses. Helical nucleocapsids include a set of helical capsid proteins coated over a filament of nucleic acid. Many capsomeres arrangement are effective in identification and many consists of an outer envelope as well.

  • Chemical Composition

The whole genome may cover either one molecule of nucleic acid (monopartite genome) or different segments of nucleic acid (multipartite genome). The different kinds of genome call for various replication techniques.

  • Structure and Function

Viruses are tiny, obligate intracellular entities that either has DNA or an RNA genome. It is also 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. 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.

Types of Viruses

Based on the type of host, the viruses are further classified into three types:

  1. Animal viruses
  2. Plant viruses
  3. Bacteriophage

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 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 etc.


The virus which infects bacterial cells is known as bacteriophage. There are many varieties of bacteriophages, such as DNA virus, RNA virus, etc.

Stay tuned with BYJU’S to learn more about the virus, its structure, and its features.

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