Virus is an infectious agent of the submicroscopic size that cannot survive alone. Hence, it infects cells of other living organisms and replicates itself using the components of these host cells. Its biological structure is comprised of a fragment of nucleic acid that could either be DNA or RNA. The most commonly known examples of viruses are HIV Virus, Coronavirus, Ebola virus, etc.
In this article, you will learn about what is virus in biology, characteristics of virus, its structure, importance, reproduction and examples of virus.
What is Virus in Biology?
A virus is a unique entity that resides within a host’s body. These non-cellular entities, when outside a living cell, possess a dormant crystalline structure. On the other hand, when present inside a living form, their tendency is to make use of the host cell to create multiple copies. Typically, this leads to the destruction of the host cell, resulting in damage to the host organism. All living things, including bacteria and archaea, as well as plants, animals, and microbes, are infected by the virus. Viruses are not considered to be ‘ truly living ‘. That is why viruses are excluded from Whittaker’s taxonomy, which categorizes living things into five Kingdoms. The study of viruses is known as virology.
Virus Definition
Virus is a microscopic infectious agent that falls somewhere between living and non-living entities. Viruses are composed of genetic material, either DNA or RNA, enclosed in a protein coat called a capsid.
Characteristics of Virus
M.W. Beijerinek discovered that healthy plants get infected from the extract of an infectious tobacco and named the new pathogen a ‘virus’. W. M. Stanley demonstrated that the virus could be crystallized, and the crystal consisted of proteins. Following are few of its characteristics :
- Viruses are submicroscopic organisms. They are even smaller than bacteria.
- They are non-cellular and lack cell organelles.
- Viruses act as connecting links between the living and the non-living.
- Viruses are obligate parasites.
- Viruses contain protein and genetic material that could either be DNA or RNA but can never be both.
- They require a host cell to multiply as they cannot live on their own.
- They can infect all life forms- humans, plants, animals, and microorganisms.
Classification of Virus
Virus can be classified either on the basis of the nucleic acid they contain or on the basis of their structure.iruses can be classified based on various phenotypic characteristics, including nucleic acid type, morphology, mode of replication, chemical composition, host organisms, size, shape, and the type of disease they cause. Baltimore classification, created by virologist David Baltimore in 1971, is the most accepted classification for studying viruses.
Classification of Virus based on the type of nucleic acid
DNA virus
When the genetic material in the virus is DNA, it is known as the DNA virus. It can be categorized into Single-stranded (ss) and Double-stranded (ds) DNA . Some category-wise examples are:
- Single-stranded(ss): Parvovirus, Picornaviruses
- Double-stranded(ds): Herpesvirus, Adenovirus
RNA virus
When the genetic material in the virus is RNA, it is known as the RNA virus. It can be categorized into Single-stranded (ss) and Double-stranded (ds) RNA. Some category-wise examples are:
- Double-stranded(ds):Reovirus
- Single-stranded(ss): Poliovirus, Hepatitis A, Rabies virus, Influenza Single-stranded (ss) can be further subcategorized as:
- Positive sense RNA (+RNA)
- Negative sense RNA (-RNA)
Classification of Virus Based on the Structure
The following are the different virus types based on the structure, each given with some examples.
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Picorna virus, Reovirus
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Bacteriophage
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Orthomyxovirus, Paramyxovirus
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Pox virus
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Structure of Virus
Viruses are submicroscopic organisms that vary in size between 30-50 nm. Following are the keypoints regarding the structure of a virus:
- Their genetic material is protected by a protein covering called the capsid.
- The capsid is made up of small subunits called Capsomeres. These capsomeres are arranged in a geometric form (helical or polyhedral).
- Some viruses have an external lipid membrane called an Envelope.
- The envelope surrounds the capsid to carry out a defense mechanism against the immune response of the host. It also has receptor molecules through which viruses bind with the host cell and cause infection.
- Viruses are non-cellular and usually lack a cell wall.
- Virus lacks any cell organelles like mitochondria and ribosome, therefore depend on the host machinery for multiplication.
Virus Diagram
Example of a Bacterial Virus
Bacteriophage
Bacteriophage is a virus that infects the bacteria. It belongs to the family Viridae and the genus Virus. The bacteriophage ranges in size and shape from 20 to 250 nm. Either DNA or RNA makes up the bacteriophage’s genetic makeup. It is enclosed by the capsid, a protein covering that can be icosahedral, filamentous, or head-tail in shape.
Reproduction in Virus
Virus can reprouce through two ways- Lysogenic Cycle and the Lytic cycle. The process used by the majority of the virus to reproduce is known as Lytic Cycle. During Lytic infection, the virus penetrates the host cell, multiplies, and causes the cell to lyse or burst.
The steps of the Lytic cycle are:
- Attachment: Initially, the virus starts the attack by attaching itself to the host cell.
- Penetration: The genetic material is injected into the host cell by the virus.
- Genome Replication and Gene expression: The host cell’s metabolism gets under the control of the virus. Thus, the host cell’s organelles start producing new proteins and nucleic acids.
- Assembly: In this stage, nucleic acid and protein are assembled to form a new virus.
- Release: The attacked cell bursts due to viral enzymes, and the replicas of the virus are released to other surrounding cells to make them infected as well.
In a Lysogenic cycle, the virus stays in a dormant phase and does not immediately attack the body. It keeps replicating silently within the body. Only in case of triggers like stress, or temperature change, the cells burst and the virus particles spread throughout the body.
Importance of Virus
Viruses are known to cause fatal diseases in humans, but they also have significant roles in our daily life. The importance of the virus is outlined as follows:
- Viruse are utilsed to create vaccines which can be injected during the infant phase in humans to prevent diseases.
- Virus do not belong to either the living world nor the non-living world. Hence, they act as connecting links.
- Viruses are studied extensively as they help in understanding various processes that occur in molecular biology such as transcription, replication, immune mechanisms, etc.
- Virus can be used as vectors which help in target delivery of drugs.
- Viruses are thesedays, being used in the process of pest management.
- In nanotechnology, viruses can be utilized as carriers for genetically altered genomes.
Diseases caused by Virus
When a virus enters the human body, it spreads via replicating which results in a viral disease. The virus will intiate the body’s immune response which in case is weak, will result in the body getting ill. A typical viral infection is a cold. Following are some other viral diseases:
- AIDS
- Chickenpox
- Measles
- Mumps
- Herpes
- Smallpox
- Influenza
- Viral hepatitis
The viral diseases that had a significant impact in the recent past are:
- COVID-19
- Ebola
- SARS (Severe Acute Respiratory Syndrome)
- Chikungunya
- Influenza
Summary – Virus
A virus, in the field of biology, is a unique infectious agent that blurs the line between living and non-living entities. These submicroscopic entities consist of genetic material, DNA or RNA, encased in a protein coat, and some may have an outer lipid envelope. Viruses lack the cellular machinery found in living organisms and rely on host cells for replication. They are known for causing various diseases in humans and other organisms, including examples like HIV, Coronavirus, and Ebola.
In this article, you’ve learned about the nature of viruses, their characteristics, structure, and their pivotal role in diseases. Viruses are considered connecting links between the living and non-living, as they depend on host cells for their reproduction.
FAQs on Virus in Biology
1. What is virus?
A virus is an infectious agent of a submicroscopic size that does not have the capability to replicate alone and uses other living cells to create its copies. It consists of a fragment of nucleic acid (either DNA or RNA) enclosed by a protective coat of protein.
2. Why is virus considered a connecting link between living and non-living things?
Viruses are considered as connecting links between living and non-living organisms because of their characteristic features that are a combination of both living and non-living organisms.
3. Give few examples of diseases caused by virus.
Few examples of diseases caused by virus are :
Chickenpox
Measles
Mumps
Herpes
4. List three characteristics of virus.
Three characteristics of virus are :
- These are microscopic in nature and are invisible to the naked eye.
- Virus require a host to survive and reproduce.
- Their genetic material is either DNA or RNA.
5. What is the body structure of a virus ?
The genetic material which may be either DNA or RNA is covered by a proetin coat called capsid. The capsid is made up of small subunits called Capsomeres. Some of the virus also have an envelope outside the capsid.