The nature of viruses is regarded as intermediate between non-living entities and living organisms. It is very difficult to ascertain and come to a single conclusion. Some characters of viruses suggest their non-living nature whereas many other characters suggest their living nature.
The term ‘virus’ has been derived from Latin, which means poison or venom or viscous fluid. They remain inactive outside a living host but become active inside the host and multiply in it. They represent a transitional form of life between the living and the nonliving world.
It’s an important question in the chapter “Biological Classification” of Class 11 Biology NCERT. This article provides a detailed and to-the-point answer which will help you in your boards preparation or NEET exam preparation.
I’ll first explain both the living as well as non-living aspects of viruses in detail and then describe and elucidate some important features of viruses.
Are Viruses Living or Non-Living?
Viruses are intermediate between living and nonliving entities. I’ll be listing down the important characteristics that prove the nature of viruses as living or non-living respectively.
Viruses are non-living
The following characters indicate that they are non-living:
- Viruses do not have a complete cellular structure. They are not surrounded by a cell membrane or a cell wall.
- They do not show cellular metabolism.
- Absence of respiration.
- Absence of energy storing systems.
- They possess high specific gravity, unlike living organisms. They have high specific gravity which is found only in non-living objects.
- Viruses are active only when they are inside the living host’s cell. Outside the host, they are non-living. Thus, they do not have an independent existence.
- The postulates of Robert Koch are not true for the viruses. A virus cannot be grown in an in vitro condition in the lab.
- Absence of growth and division. Instead, different parts are synthesized separately.
- Ability to get crystallized. It can be stored as a crystal in a bottle for several years.
- Lack of protoplast.
- Inability to live independently of a living cell.
Viruses are living
The following characters indicate that they are living organisms:
- They possess genetic material (DNA or RNA), which determines their structure and development. Genetic material is being passed from generation to generation in the usual manner.
- Infectivity and host specificity. All viruses are intracellular obligate parasites and attack specific hosts. The bacteriophages recognise the real bacterial surface.
- They show the property of mutation. The most mutable virus is HIV or AIDS (Acquired Immune Deficiency Syndrome) virus followed by the Influenza virus.
- They show irritability and respond to environmental conditions such as heat, ultraviolet rays, humidity, drought, alcohol, etc.
- Ability to multiply or reproduce. They can grow inside the host and multiply enormously showing one of the most important properties of living organisms. They take over the biosynthetic machinery of the host cell and produce chemicals required for their multiplication.
- Viruses resemble living beings in being formed of organic macromolecules that occur only in living organisms.
- Some viruses like the Pox virus contain vitamins like Riboflavin and Biotin.
- The occurrence of enzyme transcriptase in most viruses.
- The occurrence of antigenic properties.
- Viruses are killed by autoclaving and ultraviolet x-rays.
- They have definite shape and morphology like that of a living organism.
- Responsible for a number of infectious viral diseases like the common cold, epidemic influenza, chickenpox, mumps, poliomyelitis, rabies, herpes, AIDS.
Important features of viruses:
Viruses are unique in many ways. Here are some of their important features:
- They cannot be termed as prokaryotes or eukaryotes as they do not have a cellular structure.
- They contain hereditary material in the form of DNA or RNA forming a core enclosed in a protein coat called the capsid.
- Viruses are much smaller than bacteria. They are considered to be the smallest living organisms. The size is about 20 nm to 300 nm, which is about 50 times smaller than bacteria.
- They exhibit properties of both living and non-living things. A virus has no metabolic activity of its own. It becomes active and multiplies when it infects (attacks) a specific living host cell. A virus can survive as a parasite only inside a living cell. So they are said to be intracellular obligate parasites.
- Biologists are not clear whether they are early primitive forms of living things or are highly evolved super parasites.
- Viral genomes are small and contain the information to code for a few proteins only. Viruses make use of the host machinery like enzymes, ribosomes, and other components to form more viral particles rather than the host cell constituents.
- The genetic material of viruses could be DNA or RNA. No virus contains both DNA and RNA.
- Influenza virus and smallpox virus contain DNA.
- Bacterial viruses (bacteriophages) contain double-stranded DNA.
- Mumps virus, measles virus, AIDS virus, Hepatitis B virus, herpes virus contain single or double-stranded RNA.
- Tobacco mosaic virus and most of the plant viruses contain single-stranded RNA.
- x 174 virus contains single-stranded DNA.
- The viruses are very specific about the host they infect. They have been classified into three categories according to the hosts they live in. These are bacterial viruses (also called bacteriophages) that infect bacterial cells, plant viruses that infect plant cells, and animal viruses that infect animal cells.
- When a virus infects a cell, it disrupts the host’s metabolism. The virus makes use of the metabolic machinery of the cell to replicate its own kind. The diseased host cell ultimately bursts open, releasing new viruses.
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