A virus is non-cellular, infectious, parasite, made up of genetic material and protein that can invade and reproduce by itself within the living cells of bacteria, plants and animals.

These microbes belongs to the family of viridae and Genus of virus. The name virus is generally originated from a Latin word which means poison. 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 nanometres to 250 nanometres. Therefore, they can only be seen with an electron microscope

Many virus have either DNA or RNA as a 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 term virus was discovered and coined by the Dutch microbiologist named Martinus Willem Beijerinck in the year 1897. The study of viruses is called as virology.

Characteristic Features 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 and are active within host cells. These viruses causes several infections and reproduce within the host cell by using the enzymes and raw materials.

Classification of Virus


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 includes a set of helical capsid proteins coated over a filament of nucleic acid. Many capsomeres arrangement are effective in identification and many consists 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, and contain either DNA or RNA genome and surrounded by a protective protein (consisting of virus) coating. It can be seen as genetic element and characterized by a combined evolution of virus and the host. Viruses mainly depend on host that deliver the complex metabolic machinery of prokaryotic or eukaryotic cells for propagation. The main task of virion is to carry its DNA or RNA genome to the host cell which can be transcribed by the host cell. The viral genome is packed in a capsulated symmetric protein. The protein associated with nucleic acid known as nucleoprotein produces the nucleocapsid with the genome.

Types of Virus

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

  • Animal viruses

These virus infect by invading the cells animals including humans. Few examples of animal viruses includes: influenza virus, mumps virus, rabies virus, poliovirus etc.

  • Plant viruses

These virus infect plants by invading the plant cells. Few examples of plant virus include: potato virus, tobacco mosaic virus, beet yellow virus and turnip yellow virus etc.

  • Bacteriophage

The virus which infect bacterial cells are 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.

Practise This Question

Identify the correct sequence of events in the viral replication process
I. Eclipse II. Maturation III. Adsorption
IV. Assembly V. Penetration VI. Lysis