Viruses, Viroids, Prions, and Lichens – Definition, Characteristics & Differences

Viruses, Viroids, Prions, and Lichens: We all have heard about SARS-CoV-2, commonly known as covid -19. It is a deadly virus. Do you know what exactly viruses are? Can we see them? No, we cannot. Do they have any structure? Yes, they do have. Though we cannot see viruses, viroids, prions, etc through naked eyes, they all have specialized structures and characters.

On the other hand, lichens can be seen through naked eyes. Viruses, Viroids, Prions, and Lichens do not have many similarities among them but one thing that is common in them is that they do not belong to the five-kingdom classification. So in this article, we will briefly introduce them to you.

What are Viruses?

Viruses are disease-causing, acellular, antigenic, infectious, obligatory parasitic, nucleoprotein microorganisms that are capable of being introduced into the living cells of specific organisms and have the power to multiply within the living host cells, lies in between living and non-living objects.

Fig: Different Examples of Virus

Virus Characteristics

Viruses are mainly infectious agents that range from 20-300nm in diameter. When a virus infects bacteria, they are called bacteriophages. The characteristics of the virus are described below:

Living Characteristics of Viruses

1. They can replicate within the host body.
2. In the host body, they can synthesize protein.
3. They can exhibit mutability and can infect other organisms.

Non-living Characteristics of Viruses

1. They cannot respond to external stimuli.
2. The molecules of viruses can be crystallized.
3. They do not have protoplasm.
4. Their body is not covered by the cell membrane.
5. They do not respire.
6. They have no power of locomotion.
7. They remain inactive outside the host body.

Structure of Viruses

A virus mainly contains four parts. These are:

1. Envelope

(a) It is the outer covering present in many viruses.
(b) It is made up of protein, lipids, and carbohydrates of the host.
(c) Many viruses develop spike or outgrowth, which is composed of glycoprotein. These spikes or outgrowths help them to attach to specific cell surfaces.
(d) Some common examples of enveloped viruses are HIV, Herpes virus, Vaccinia virus, etc.

2. Capsid

(a) It is a protein shell around the genetic material.
(b) The capsid, together with its enclosed nucleic acids, is called a nucleocapsid.
(c) Capsomeres are the morphological protein subunits. TMV contains 2130 capsomeres.
(d) These are rod-shaped helix or polygon-shaped structures.
(e) They protect the nucleic acids from digestion by enzymes, help to contain special sites on their surface, and also provide proteins that enable the virion to penetrate the host cell membrane.

3. Core or Nucleoid

(a) The central core of the virus is composed of nucleic acid called the nucleoid.
(b) They contain genetic material as DNA or RNA but never both.
(c) The majority of animal viruses contain double-stranded DNA as genetic material. Some animal viruses also contain single or double-stranded RNA.
(d) The majority of plant viruses contain single-stranded RNA as genetic material.

4. Enzymes

(a) They are rarely present in viruses and if present then they play a central role in the infection process.
(b) Lysozymes are found in Bacteriophages that make a small hole in the bacterial cell which allows viral nucleic acid to get in.
(c) Retrovirus is an RNA virus that possesses a reverse transcriptase enzyme that replicates inside the host cell as a DNA intermediate.

Fig: Structure of Virus

Function of Virus

The main function of a virus is to deliver its genome into the host cell so that the genome can be transcribed and translated by the host cell. During the release, the newly-created viruses are released from the host cell, either by causing the cell to break apart, waiting for the cell to die or by budding off through the cell membrane. Different diseases such as chickenpox, herpes, influenza, rabies, smallpox and AIDS (Acquired Immunodeficiency Syndrome), Polio, etc., are caused by the attack of the virus.

What are Viroids?

Viroids are infectious agents which are 100 times smaller than viruses, made up of naked, single-stranded RNA.

Fig: Viroid

Viroids Characteristics

The characteristics of viroids are described below:
1. They were discovered by T. O. Diener in 1971.
2. They are made up of naked, single-stranded, or circular RNA.
3. They do not have protein coats and have low molecular weight.
4. They are responsible for causing infections in plants only.
5. They cause diseases like Potato spindle tuber disease, Chrysanthemum stunt, Cucumber pale fruit, etc.

What are Prions?

The prions are the smallest, misfolded, proteinaceous, infectious particles made up of glycoproteins only.

Fig: Prions

Prions Characteristics

The characteristics of prions are described below:
1. They were discovered by S. Prusiner.
2. They are obligate parasites.
3. They are formed due to the mutation of the PRNP gene.
4. They are composed of prion protein (PrP) that accumulate in the brain and progressively damage and destroy brain cells.
5. They can cause neurodegenerative diseases.
6. They are not affected by proteases, nucleases, temperature up to 8000C, UV radiations, and formaldehyde.
7. They do not contain nucleic acid.
8. Some common diseases that are caused by prions are bovine spongiform encephalopathy in cattle, mad cows, etc.

Differences between Viruses, Viroids, Prions

The difference between viruses, viroids, and prions are:

What are Lichens?

Lichens are composite organisms made up of both algae (phycobiont) and fungi (mycobiont) which live together symbiotically. They are found throughout the world, growing on forest floors, the bark of trees, exposed rocks, etc.

Fig: Lichens

Lichen Characteristics

The characteristics of lichen are described below:
1. They are formed by the symbiotic association of algae and fungi.
2. The algal partner provides food to the fungal partner, and the fungal partner, in return, provides the function of absorption and retention of water.
3. Crustose, foliose, fruticose, squamulose, leprosy, and filamentous are different types of lichens based on the morphological structure of thalli.
4. They show all the three types of reproduction viz., vegetative, asexual, and sexual.

Economic Importance of Lichens

Some of the economic importance of lichens are mentioned below:
1. Lichen, like Cladonia rangifera can be served as food for reindeer and musk ox.
2. Some species of lichens like Lecanora, Parmelia, Umbilicaria and Cetraria can be used as food.
3. Lichens serve as an indicator of air pollution.
4. Lichens like Cetraria icelandica, Lobaria pulmonaria are used as tanning agents in leather industries.
5. The dye litmus, which is used in chemical laboratories as an acid-base indicator, can be obtained from the lichen Roccella montagnei and Lasallia pustulata.
6. Lichen can be used as medicine in the treatment of bile, Diarrhoea, fever, nervous disorders, skin diseases, whooping cough, lung diseases, etc., due to the presence of Lichenin and some astringent substances.

Summary

Viruses are disease-causing, acellular, antigenic, infectious, obligatory parasitic, nucleoprotein microorganisms that are capable of being introduced into the living cells of specific organisms and have the power to multiply within the living host cells. Viroids are infectious agents which are 100 times smaller than viruses, made up of naked, single-stranded RNA. The prions are the smallest, misfolded, proteinaceous, infectious particles made up of glycoproteins only. Lichens are composite organisms made up of both algae (phycobiont) and fungi (mycobiont) which live together symbiotically.

Frequently Asked Questions (FAQs) on Viruses, Viroids, Prions and Lichens

Q.1. What are viruses?
Ans: Viruses are disease-causing, acellular, antigenic, infectious, obligatory parasitic, nucleoprotein microorganisms that are capable of being introduced into the living cells of specific organisms and have the power to multiply within the living host cells.

Q.2. What are lichens?
Ans: Lichens are composite organisms made up of both algae (phycobiont) and fungi (mycobiont) which live together symbiotically.

Q.3. Does the five-kingdom classification include viruses, viroids, and lichens?
Ans: No, they are not included under the five-kingdom classification.

Q.4. What are prions?
Ans: The prions are the smallest, misfolded, proteinaceous, infectious particles made up of glycoproteins only.

Q.5. Are viruses, viroids, and prions living?
Ans: Viruses, viroids, and prions are not considered living.

We hope this article on the Viruses, Viroids, Prions, and Lichens helps you in your preparation. Do drop in your queries in the comments section if you get stuck and we will get back to you at the earliest.