Viruses, Viroids, Prions And Lichens (original) (raw)
Last Updated : 23 Jul, 2025
Viruses, viroids, prions, and lichens represent unique and diverse biological organisms, each with distinct structures and roles in nature. These were not included in the **Five Kingdom Classification given by Whittaker, due to their unique features.
In this article, we will read about viruses, viroids, prions, and lichens and their differences.
Table of Content
Viruses
Viruses are non-cellular organisms and are in an inactive state while outside the host body. They reproduce and replicate when they get inside the host body. This process often kills the host. Some of the most common diseases caused by viruses are Mumps, measles, Rubella, etc.
The most popular disease caused by the virus is the **Mosaic Disease of Tobacco. An experiment carried out by M.W. Beijerinck introduced a new pathogen called "virus" and "_Contagium Vivum Fluidum" (infectious living fluid). W.M. Stanley demonstrated that viruses could be crystallized and that crystals are mostly made up of proteins. Outside of their unique host cell, they are dormant. Viruses are parasitic to other organisms.
**Structure of Virus
The structure of virus is given below:
Virus Structure
- Viruses, outside the host, can crystallize and this crystal layer is consist of protein.
- The virus also contains genetic material (either RNA or DNA) which is infectious.
- There is a protein coat over the virus called **capsid made of small capsomere which is arranged in a polyhedral structure.
- A viruses size ranges from 30 nm to 50 nm
- The head of the virus contains the genetic material which is either DNA or RNA.
Bacteriophage
Viroids
After the discovery of viruses, scientists found another pathogen that was smaller than a virus. It didn't have a protein layer like a virus. It was discovered by T.O. Diener in 1971.
**Also Read: Viroids – Structure, Characteristics, Features, and Diseases
**Structure of Viroid
The viroid structure is given below:
- Smallest infectious agents (246-401 nucleotides, ~100 nm).
- Viroid only consists of circular RNA which is not protected by any layers of protein like a virus.
- They only infect plants (eg. Potato spindle tuber disease).
- Replicate within host plant cells using host machinery.
- The structure of viroid is never similar
- As same as viruses, these viroids infect the host cell to reproduce and slowly kill the host during the process.
**Also Read: Difference between Virus and Virion
Prions
These are abnormally folded proteins, which can transmit several diseases. The word prion is derived from "proteinaceous infectious particle". They were discovered by American biologist Stanley B. Prusiner.
**Structure of Prions
The strcuture and characteristics of prion are given below:
- They are similar in size to viroid (significantly smaller than viroid)
- Prions generate aberrant protein clumps known as amyloids, which accumulate in infected tissue and are linked to tissue damage and cell death.
- Prions are abundant in one's body but the one that is dangerous are structurally different.
The most common disease caused by Prions is bovine spongiform encephalopathy (**BSE), Cr–Jacob disease (CJD), etc.
**Also Read: Apoptosis
Lichens
Lichens are symbiotic relationship between algae and fungi in which algae prepare food and fungi provides shelter. In this relationship, algae are known as **photobionts, and fungi are known as **mycobionts. Lichens have properties different from those of their component organisms.
**Also Read: Difference Between Fungi and Lichens
Lichen
**Structure of Lichen
The structure of lichen is given below:
- **Thallus: The main body of the lichen, which can be crustose (crust-like), foliose (leaf-like), or fruticose (shrub-like).
- **Symbiotic Partners: Composed of a symbiotic relationship between a fungus (mycobiont) and a photosynthetic partner (photobiont), which is either an alga or cyanobacterium.
- **Cortex: The outer protective layer of the thallus, made up of tightly packed fungal cells.
- **Medulla: The inner layer beneath the cortex, consisting of loosely packed fungal hyphae.
- **Photobiont Layer: Located just below the upper cortex, where the algal or cyanobacterial cells are embedded and perform photosynthesis.
- **Rhizines: Root-like structures in some lichens, made of fungal hyphae, that help anchor the lichen to its substrate.
Types of Lichens
There are different types of lichens:
- **Fruticose- Grows outside of a standing branch tree
- **Foliose- This species has two defined upper and lower sides.
- **Squamulose- A lichen that is composed of small, often overlapping "scales" called squamulose.
- **Leprose- This lichen has the physical appearance of dust.
- **Gelatinous- This lichen has the physical appearance of jelly
- **Crustose- a crust that strongly appears.
Difference Between Viruses, Viroids, Prions And Lichens
The difference between viruses, viroids, prions, and lichens are given below:
| **Feature | **Viruses | **Viroids | **Prions | **Lichens |
|---|---|---|---|---|
| Nature | Infectious agents composed of nucleic acid (DNA or RNA) and protein coat (capsid) | Infectious agents composed of small, circular RNA molecules | Infectious proteins that cause abnormal folding of normal proteins | Symbiotic association between fungi and photosynthetic organisms (algae or cyanobacteria) |
| Size | 20-300 nanometers | 246-401 nucleotides in length | Very small, typically around 10 nm | Macroscopic, visible to the naked eye |
| Genetic Material | DNA or RNA | RNA | No nucleic acid, only protein | N/A, composed of fungal and algal/cyanobacterial cells |
| Replication | Requires a host cell to replicate, using host's machinery | Replicates autonomously in the host plant cells | Induces abnormal folding in normal proteins, does not replicate like typical pathogens | Reproduces through symbiotic relationship, either sexually or asexually |
| Infects | Plants, animals, bacteria, archaea | Plants | Animals, mainly affecting nervous system | Not an infectious agent, but a mutualistic organism |
| Disease Example | Influenza, HIV, COVID-19 | Potato spindle tuber disease | Creutzfeldt-Jakob disease, Mad cow disease | No disease caused but contributes to ecological balance and nutrient cycling |
| Treatment | Antiviral drugs, vaccines | No specific treatment | No known cure, only preventive measures | Not treated, as they are beneficial organisms |
Conclusion - Viruses, Viroids, Prions And Lichens Difference
In summary, viruses, viroids, prions, and lichens are distinct biological organisms with unique structures and roles in nature. Viruses are infectious agents with genetic material and a protein coat, causing various diseases in plants, animals, and humans. Viroids are simpler, consisting only of RNA, and primarily infect plants. Prions are misfolded proteins that induce neurodegenerative diseases in animals. Lichens, in contrast, are beneficial symbiotic associations between fungi and photosynthetic organisms, contributing to ecological balance. Understanding these differences enhances our knowledge of their biological significance and impacts.
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