A virus is an infectious microorganism made up of a protein-coated segment of nucleic acid (either DNA or RNA). A virus can not multiply by itself; it has to infect cells in order to make copies of itself. A virus frequently causes harm to the host organism by killing the host cell.
- HIV, SARS COV II, Herpes simplex, etc are well-known viruses that cause human disease.
- They differ from other living things as they only survive inside the host.
- The host cell is the one in which they reproduce.
- Viruses are described as acellular as they lack cell nucleus and cell wall.
| Table of Content |
Key terms: Virus, capsid, DNA, RNA, virion, nucleocapsid, capsomeres, nucleic acid.
What is a Virus?
[Click Here for Sample Questions]
Viruses are simple, noncellular organisms made up of DNA or RNA molecules encased in a protein sheath. Viruses are contagious agents that can only reproduce inside of their host organisms.
- Viruses can infect animals, plants and humans.
- Due to their microscopic structure and size, one can not visualize them with naked eyes.
- A microscope is required to observe viruses.
- A completely constructed structure called a virion is an infectious virus.
- The virion's major job is to get its DNA or RNA genome into the host cell.
Since viral genomes can be either DNA or RNA, single- or double-stranded, linear or circular, and can differ in length and in the amount of DNA or RNA molecules, they are incredibly diverse.
Viral Structure
[Click Here for Sample Questions]
Virion structures are extremely diverse, with broad variations in size, shape, and chemical make-up. Every virus has a nucleocapsid, which is a protein-coated shell around nucleic acid.
- Capsomeres proteins are used to construct the capsid.
- The nucleocapsid is made up of both the nucleic acid and the capsid.
- Some viruses have a membrane envelope.
- The membrane envelope is located externally near to the nucleocapsid.
- Viruses without an envelope are referred to as naked viruses.
- A viral genome is made up of DNA and/ or RNA, which can be single-stranded, double-stranded, linear, or circular.
Types of Viral Nucleic Acid
[Click Here for Sample Questions]
The type of nucleic acid present in viruses is the main factor that classifies viruses into different categories. Different types of nucleic acid in DNA and RNA viruses are tabulated below.
| Type of Nucleic Acid | Structure of Nucleic Acid |
|---|---|
DNA
| Linear, single stranded DNA Circular, single stranded DNA Linear, double stranded DNA Circular, double stranded DNA |
RNA
| Linear, single stranded, positive strand RNA Linear, single stranded, negative strand RNA Linear, single stranded, segmented RNA Linear, double stranded, segmented RNA |
Examples of DNA and RNA Viruses
Few examples of DNA and RNA viruses are as follows.
- DNA virus : Pox, Herpes virus, Adenovirus etc.
- RNA virus: Orthomyxo, Paramyxo, Togavirus etc.
Shape and Symmetry
[Click Here for Sample Questions]
A virus has a nucleocapsid structure. The symmetry can be either icosahedral (spherical shape) or helical (rod shape).
- The nucleocapsids of filamentous and pleomorphic viruses exhibit helical symmetry.
- A helical array of capsomeres is wrapped around a filament of nucleic acid to form a helical nucleocapsid.
- TMV is an example of a virus with helical symmetry.
- Many spherical viruses have nucleocapsids that exhibit icosahedral shape.
Classification of Viruses
[Click Here for Sample Questions]
Viruses are categorized into many taxonomic groupings depending on their host, virion structure and composition, mechanism of replication, and diseases they may cause.
A virus can be an animal virus, plant virus, bacterial virus, or insect virus depending on the nature of the host.
1. Animal Virus
A small infectious agent known as an animal virus is unable to reproduce outside of a living animal cell. Only one type of nucleic acid, either DNA or RNA, is present in animal viruses. A virus has to enter a host cell in order to create new viral particles via replication.
Examples: Hepatitis C, Chickenpox, and Shingles
2. Plant Virus
Plant viruses can be polyhedral or rod-shaped. Single strand of RNA makes up the majority of plant virus genomes. Tobacco mosaic virus (TMV) is the well-known plant virus. DNA genomes of plant viruses are rare. For instance, Cauliflower mosaic virus.
3. Bacterial Virus (Bacteriophage)
Viruses that infect bacteria are referred to as bacteriophages, also known as phage or bacterial viruses. Frederick W. Twort in Great Britain (1915) and Félix d' Hérelle in France (1917) separately discovered bacteriophages.
- Bacteriophages are regarded as the most prevalent biological agent on earth.
- They are widely distributed throughout the environment.
- Their size, appearance, and genetic organization are incredibly different.
- All phages include a nucleic acid genome wrapped in a shell of the phage-encoded capsid.
- It mediates the passage of the genetic material into the host cell.
4. Insect Virus
Insect viruses are living organisms that infect a specific host insect. Viruses interact with insect hosts in a variety of ways. The insect may serve as the virus's main host or serve as a vector for the virus to spread to different plant or animal hosts.
Examples: Ascovirus, Iridovirus, Polydnavirus, and Baculovirus.
Classification of viruses on the basis of their affinity to organs of the body
Viral infections can harm microorganisms, plants, animals, insects, and even humans. Initially, the classification of viruses was recorded based on how well they interact with the various bodily systems and organs. Some examples are as follows.
- Viruses that cause skin lesions (Measles, Chickenpox, and Smallpox).
- Viruses that impact the nervous system (Rabies, Polio).
- Viruses that affect the respiratory system (Influenza, the Common cold).
- Viruses that cause lesions in body organs (Hepatitis).
It was also suggested that viruses can be categorized according to epidemiological standards.
- Enteric fever – Adenovirus, Hepatitis virus, Reovirus, Picornavirus.
- Respiratory – Orthomyxovirus and Paramyxovirus.
Replication of Virus
[Click Here for Sample Questions]
Viral replication is the process by which they multiply into identical copies. In this process, a virus replicates the genomic material and expresses itself in the host. The steps of replication are as mentioned below.
- Attachment: The virus binds to the host cell
- Entry: The host cell receives a dose of genetic material.
- Replication: The host organelles produce new proteins and nucleic acids as the virus is controlling the cell's metabolism.
- Assembly: New virus particles get assembled from nucleic acids and proteins.
- Release: The host cell ruptures as a result of virus enzymes.
Followed by the assembly, viral copies are released outside the host cell. Other cells may become infected by these new viruses.
Viral Diseases and Causative Agents
[Click Here for Sample Questions]
Viruses are known to cause disease in plants and animals. Some of the viral diseases and causative agents are described below.
| Viral diseases | Causative agent |
|---|---|
| 1. Hepatitis | Hepatitis A,B,C,D,E |
| 2. Influenza | Influenza virus |
| 3. Measles | Measles virus |
| 4. AIDS | Human immunodeficiency (HIV) virus |
| 5. Rabies | Rabies virus |
| 6. Neonatal herpes | Herpes simplex |
| 7. Common cold | Rhinovirus |
Economic Importance of Virus
[Click Here for Sample Questions]
The largest pool of genetic variation on Earth is found in viruses. The major economic advantage of viruses is that they can be employed as an alternative to antibiotics due to the high degree of antibiotic resistance,
- Protein is a component of viruses which can be utilized to produce a variety of proteins,
- For instance, vaccines, antigens and antibodies can be produced by viral proteins.
- Some viruses can be utilized to manage insect pests that harm plants.
- However, other viruses are significant plant pathogens.
- Bacteriophages were used in research on recombinant DNA technology.
- Viruses have a wide range of medical applications.
- It serves as carriers or vectors that deliver the treatment agents.
Also Read:
| Related Articles | ||
|---|---|---|
| Virology | Genome Sequencing | Immunity |
| MCQ on Virus | Difference between HIV and AIDs | Types of Pathogens |
Things to Remember
- Virus is an acellular organism that can not survive without the host.
- A capsid, which is a protein shell, surrounds a virus' DNA or RNA genome.
- Viruses proliferate by infecting their host cells.
- There are numerous varieties of viruses.
- Viruses come in a wide range of sizes, shapes, and life cycles.
- The flu, the common cold, and COVID-19 are a few examples of viral diseases.
- Bacteriophages are viruses that selectively infect bacteria.
- Virus reproduces either through the lytic cycle or the lysogenic cycle.
Sample Questions
Ques. What's the difference between viruses and bacteria? (2 marks)
Ans. The smallest, most primitive, and prokaryotic cells with free-floating genetic material are bacteria. These tiny, single-celled organisms might have a spherical, spiral, or rod shape.
Viruses are a collection of various molecules made up of genetic material (either single- or double-stranded DNA or RNA), a protein coating, and occasionally a covering of fat (an envelope).
Ques. What is a virion? (2 marks)
Ans. A virion is a whole, contagious viral particle made up of nucleic acid genome (DNA or RNA), which is encased in a protein coat called a capsid and occasionally surrounded by a lipid- and protein-based envelope. The virus can persist outside of a host cell and infect new cells in this form, known as a virion.
Ques. What are the structure and symmetry of Viruses? (3 marks)
Ans. RNA or DNA, makes up the major component of nucleic acids in viruses. Capsid, a protein coat envelops the nucleic acid's primary core. Numerous capsomere, or smaller units, make up the capsid itself.
Helix-shaped arrangement is found around coils of nucleic acid.
Icosahedral arrangement refers to the cube-shaped arrangement of spheroidal nucleic acid. Some viruses do not conform to icosahedral or helical symmetry.bacteriophage, the chickenpox virus, etc.
Ques. Give some examples of RNA and DNA Virus. (2 marks)
Ans. Examples of RNA and DNA viruses are as below.
DNA virus : Pox virus, Herpes virus, Adenovirus etc.
RNA virus: Orthomyxoviruses, Coronavirus, Toga virus etc.
Ques. Why are viruses regarded as neither living things nor non-living? (2 marks)
Ans. The distinguishing features of both living and non-living things can be found in viruses. They are similar to parasites in that they can only replicate inside a host. However, viruses may form crystals, unlike parasites or any other living thing. They remain inactive throughout this phase until they infect a new host and start the cycle afresh.
Ques. Describe the criteria used to classify viruses. (1 mark)
Ans. Viruses are divided into a variety of taxonomic groups based on the features of their host, the structure and makeup of the virion, the replication mechanism, and any diseases that may be responsible for.
Ques. Give a few examples of viral diseases. (1 mark)
Ans. AIDS, Ebola, Influenza, SARS, Smallpox etc are viral diseases.
Ques. What traits does a virus possess? (4 marks)
Ans. Viruses possess the following characteristics.
- Viruses are parasitic, tiny, non-cellular organisms that range in size from 20 to 300 mm.
- They lack biological components like cytoplasms and ribosomes that would allow them to survive on their own, but they do contain the genetic material to evolve and replicate, which they do by adhering to a host body.
- The protein-coated capsid that encases a virus' DNA or RNA genome.
- Some viruses are outwardly surrounded by a membrane envelope.
- The various cell organelles necessary for living, such as cells, cell membranes, cytoplasm, ribosomes, and many others, are absent from viruses.
Ques. What are enveloped viruses? (2 marks)
Ans. These viruses frequently enclose a piece of their host's cell membrane around themselves. The creation of a viral envelope uses the host cell's outer membrane or internal membranes like the nuclear or endoplasmic reticulum. Examples of enveloped viruses include the influenza virus, HIV, and varicella zoster virus.
Ques. How are viroids different from viruses? (2 marks)
Ans. Viruses create proteins, whereas viroids produce RNA. Viroids are made of nucleic acid without a protein coat, whereas viruses are made of nucleic acid that has a protein coat.
RNA can be copied in the cytoplasm or nucleus of viruses, whereas RNA or DNA molecules can only be copied in the nucleus of viroids. Viroids have a simpler structure than viruses.
For Latest Updates on Upcoming Board Exams, Click Here: https://t.me/class_10_12_board_updates
Do Check Out:
Comments