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Virus Definition Biology

Viruses are microscopic, non-cellular infectious organisms that can only reproduce within a host cell. The name is derived from a Latin word that means "slimy liquid" or "poison." The virus cannot replicate outside the host cell because it lacks the necessary biological components.

Virus Definition Biology

Viruses are extremely small, with sizes between 30 and 50 nm. A protective protein layer is a capsid surrounding viruses, which do not typically have cells or a cell wall.

Virus Classification

Viruses are categorized based on their shape, chemical structure, and replication method.

1. Host Range-Based Classification

There are four main types of viruses, depending on the host:

  • Animal Virus

This virus spreads disease by infecting the cells of animals, including humans. Examples of animal viruses are the influenza virus, herpes virus, rabies virus, poliovirus, mumps virus, etc.

  • Plant Virus

Viruses that harm plants are referred to as plant viruses. Plant viruses are tiny infectious particles with a protein shell and a nucleic acid core. Plant viruses cannot replicate without a host. The potato virus, beetroot yellow and turnip yellow viruses, cauliflower mosaic virus, etc., are examples of plant viruses.

  • Bacteriophage

The term "bacteriophage" refers to the virus that attacks bacterial cells. Bacteriophages come in many forms, including DNA viruses, MV-11, RNA viruses, etc.

  • Insect Virus

An insect virus is a virus that affects insects. Insect viruses are commonly referred to as the viral pathogen of insects. These viruses are thought of as effective biocontrol agents in agriculture.

2. Classification Based on Mode of Transmission

  • Airborne Infections

Viruses spread by the respiratory route. For example, swine flu and rhinovirus.

  • Fecal Oral Route

The virus spreads via infected water or food. For example, rotavirus, poliovirus, and hepatitis virus.

  • Sexually Transmitted Diseases

The virus is spread by sexual activity with an infected individual. For example, retroviruses, human papillomaviruses, etc.

  • Transfusion-transmitted Infections

Infections that are spread by blood transfusions are known as transfusion-transmitted infections. For example, HIV, the Hepatitis B virus, etc.

  • Zoonotic Virus

The virus spread via animal bites from infected animals. For example, the rabies virus, the alphavirus, the flavivirus, the ebola virus, etc.

3. Structure-Based Classification

  • Cubical Virus
  • Icosahedral symmetry virus is another name for it.
  • Its examples are Reo and Picorna viruses.
  • Spiral Virus
  • It is referred to as a helical symmetry virus.
  • Its examples are Paramyxovirus and Orthomyxovirus.
  • Complex Virus
  • An example is the pox virus.
  • Radial Symmetry Virus
  • Its example is a bacteriophage.

4. Classification of Viruses Based on the Characteristics of Replication and the Location of Replication

  • Replication and assembly within the host's cytoplasm. For example, all RNA viruses excluding the influenza virus
  • Replication happens inside the host cell's nucleus. For example, all DNA viruses, excluding the pox virus
  • Replication happens inside the cytoplasm and nucleus of the host cell. For example, the influenza virus and the poxvirus.
  • The virus is replicated via a single-stranded RNA intermediate. For example, all RNA viruses, excluding cancer and reovirus-causing viruses.
  • The virus is replicated via a double-stranded DNA intermediate. For example, all DNA viruses, retroviruses, and several RNA viruses that cause cancers.

5. Classification Based on Nucleic Acid Content

1. RNA Virus

The genetic substance of an RNA virus is RNA. The common cold, influenza, SARS, MERS, Covid-19, hepatitis C, hepatitis E, Ebola viral disease, rabies, polio, and mumps are serious diseases caused by RNA viruses.

Types of RNA virus

  • Viruses with double-stranded RNA (dsRNA), such as reovirus, etc.
  • Viruses with single-stranded (ss) RNA, such as hepatitis A, rabies, polio, etc.

2. DNA Virus

This particular virus has DNA as its genetic material. Herpes, smallpox, hepatitis B, adenoviruses, and warts are some examples of DNA viruses.

Types of DNA virus

  • Viruses with single-stranded DNA (ssDNA), such as picornaviruses and parvovirus
  • Double-stranded DNA viruses, such as adenovirus

Economic Significance of Virus

  • Viruses are the main component in the vaccinations that fight viral illnesses. Injecting dead viruses into individuals as vaccines allows for controlling diseases like pox, polio, mumps, jaundice, and others.
  • Viruses include a protein, and this characteristic can be utilized to produce a variety of proteins, including vaccine antigens and antibodies.
  • Viruses have been utilized as templates for organizing materials on the nanoscale due to their size, shape, and chemical structures.
  • A tablespoon of seawater contains one million viruses. A virus can increase ocean photosynthesis and lower atmospheric carbon dioxide levels by around three gigatonnes of carbon each year.

Characteristics of Viruses

1. Non-living

Small infectious agents called viruses are also referred to as pathogens. They are not living things and will only grow when they come in contact with living cells. Most of the cellular organelles seen in other organisms are not present in viruses because they are not formed of cells. They require an outside force to survive, such as host animals or plants that provide them nutrition when no oxygen is available.

2. Size and shape

Viruses can be found in various sizes and shapes, ranging from basic spherical particles to more intricate structures. The proteins that make up a virus's capsid describe its shape. The capsid is made up of multiple capsomeres that are organized into a particular pattern.

3. Virus structure

Viruses are tiny infectious particles that can only replicate inside the cells of other species. A virus is made up of the following:

  • A core of viral genetic material is covered in a capsid, a protein covering. A protein subunit known as a capsomere is used to build the capsid.
  • The viral genome contains the genetic material required to create new viruses.
  • The envelope is the name given to the membrane's outside lipid-protein layer, but this property is not present in all membranes.

4. Reproduction

Viruses replicate using the lytic cycle and the lysogenic cycle. All viruses cannot reproduce on their own. They require living hosts like the human body, plants, or animals.

FAQ on Virus

1. Defined virus in simple words.


A virus is an infectious, non-cellular organism formed of genetic material and protein that can only enter and replicate inside the living cells of bacteria, plants, and animals.

2. How do viruses enter inside of the human body?


Viruses typically enter your body from your mucous membranes. These include your mouth, nose, eyes, penis, vagina, and anus.

3. How do viruses function?


Viruses go through several processes to infect cells and reproduce. The several processes are:

  1. Attachment.
  2. Entry.
  3. Replication
  4. Assembly.
  5. Release.

4. List out characteristics of viruses.


  1. Compared to bacteria, they are smaller and simpler.
  2. They can only be seen with an electron microscope because they are ultramicroscopic.
  3. They are dependent parasites who cannot survive on their own.
  4. They are not self-replicating.
  5. Viruses are made up of genetic material (DNA or RNA)
  6. Viruses are the source of several terrible diseases that affect people, animals, and plants.

5. What exactly are retroviruses?


It is a particular kind of RNA virus that attaches its genome to the host cell's DNA before infecting and altering the host cell's genome. In this instance, RNA viruses utilize their RNA template to generate DNA. It is a reverse of the standard transcription procedure; that's why it is referred to as Retroviruses, for, e.g., HIV.

6. What are a virus's habitats?


Phage refers to viruses that infect prokaryotic organisms. The phage impacts several species, such as soil, meats, fermented vegetables, animal intestines, and marine organisms.

7. Difference Between Viruses & Bacteria.


Virus Bacteria
1) Viruses are incredibly small organisms with RNA or DNA as their genetic material. 1) Bacteria are generally free-living organisms that often only have one biological cell.
2) They are extremely small. 2) Compared to viruses, they are larger.
3) Non-Cellular. 3) Single-Celled.
4) Consume nothing. 4) Use absorption to consume food.
5) Host-dependent reproduction. 5) Host-independent reproduction.
6) Ribosomes Absent. 6) Ribosomes Present.
7) Able to crystallize. 7) Unable to crystallize.
8) Replicate more rapidly than bacteria. 8) Replicate solver than viruses.
9) Antiviral medications help reduce reproduction, and vaccines prevent the spread but cannot stop it. 9) Antibiotics are utilized to treat them.
10) Examples: Common cold, influenza, and sore throat. 10) Examples: Strep throat, tuberculosis, and whooping cough.

8. List out few viral diseases.


  • AIDS
  • Ebola
  • SARS
  • Chikungunya
  • Influenza
  • Small Pox

9. What is a virus' main objective?


The main goal is to introduce its genome into a host cell so that it can be generated (via transcription and translation).

10. Differentiate RNA and DNA virus.


DNA virus RNA Virus
1) This virus has DNA as its genetic material. 1) This virus has RNA as its genetic material.
2) They are assembled in the nucleus. 2) They are assembled in the cytoplasm.
3) Their genomes are bigger. 3) Their genome size is noticeably smaller.
4) They exhibit a low rate of mutation. 4) They exhibit a higher rate of mutation.
5) They belong to the Baltimore categorization groups I, II, and VII. 5) They belong to Baltimore categorization groups III, IV, V, and VI.
6) Examples: Poxviruses, Herpesviruses, Papillomaviruses, etc. 6) Examples: Retroviruses, Reoviruses, Picornaviruses, etc.

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