The key difference between virus and viroids is that virus is an obligate parasite composed of either RNA or DNA genome and a protein capsid, while viroids are infectious particles composed of single-stranded circular RNA molecules.
There are different types of infectious agents that cause disease to plants, animals and other organisms. Among them, bacteria, fungi, protozoans, viruses, viroids, and prions are well known infectious agents. Viruses and viroids are very small particles. Both types are obligate parasites. A virus comprises an RNA or DNA genome surrounded by a protein capsid. However, viroids are single-stranded circular RNA molecules. They do not contain protein capsids. Likewise, there are many significant differences between virus and viroids. Hence, this article is on the investigation of the difference between virus and viroids.
CONTENTS
1. Overview and Key Difference
2. What is a Virus
3. What are Viroids
4. Similarities Between Virus and Viroids
5. Side by Side Comparison – Virus vs Viroids in Tabular Form
6. Summary
What is a Virus?
A virus is an obligate intracellular parasite. It lives and replicates inside a living cell. Viruses can invade and infect animals, plants, fungi, protists and also microorganisms like bacteria and archaea. A virus comprises an outer protein coat and an inner nucleic acid core. The outer protein coat, also known as a capsid, contains subunits called capsomeres. The inner nucleic acid core contains either RNA or DNA. In addition to the protein capsid, some viruses have a one more covering composed of lipids called envelope. They are termed as enveloped viruses, while others who lack an envelope are naked viruses.
Figure 01: Virus
The virus structure also contains different types of projections. These projections are mainly glycoproteins. Some appear as spikes as they are thin, long projections while others are peplomers, which are wider projections. Coronavirus has peplomer projections which have a similar shape to a cloverleaf. Adenovirus contains a spike type of projections which are thin and long. Apart from projections, protein coats, envelopes and nucleic acids, some viruses also possess other additional structures. For example, Rhabdoviruses consists of a protein lattice called the matrix just below their envelope. M protein is the main protein which makes the matrix, and it provides rigidity to the virus.
Viruses do not have the ability to generate energy. But, the main function of viruses is to deliver or transfer its viral genome into the host cell, allowing transcription and translation to take place within the host.
What are Viroids?
Viroid is an infectious single-stranded circular RNA particle. The first viroid identified was Potato Spindle Tuber Viroid (PsTVd). Up to now, thirty-three species of viroids have been identified. Viroids do not contain a protein capsid or envelope. They only contain RNA molecules. Since viroids are RNA particles, the ribonucleases can digest the viroids. The size of the viroid is smaller than a typical virus particle. Moreover, viroids need a host cell for reproduction. During reproduction, they produce only single-stranded RNA molecules.
Figure 02: Viroid Infection
Viroids do not cause human or animal diseases. They infect higher plants, causing diseases such as potato spindle tuber disease, chrysanthemum stunt disease, etc. These infectious RNA particles are responsible for crop failures and the loss of millions of money in agriculture annually. Potato, cucumber, tomato, chrysanthemums, avocado and coconut palms fall victim to viroid infections frequently. Viroid infections transmit by cross-contamination followed by mechanical damage of the plant. Some viroid infections are transmitted by aphids and leaf to leaf contact.
What are the Similarities Between Virus and Viroids?
- Both virus and viroids are infectious particles.
- They are very small particles.
- Some viruses contain single-stranded RNA similar to viroids.
- Both virus and viroids need a host to reproduce and replicate.
- Some viruses and all viroids infect plant species.
- Plant viruses and viroids are easily transmitted from one plant to another along with vegetative plant propagules.
- Furthermore, plant RNA viruses and viroids replicate through a double-stranded RNA intermediate.
- Accurate diagnosis is the key to the control of viruses and viroids.
What is the Difference Between Virus and Viroids?
A virus is an infectious particle composed of the nucleic acid genome and a protein capsid. On the other hand, viroids are infectious particles composed only of single-stranded RNA molecules. So, this is the key difference between virus and viroids. Furthermore, viruses infect all types of living organisms, including animals and humans, while viroids infect higher plants only. Therefore, this is a significant difference between virus and viroids.
Moreover, viroids are smaller than viruses. Furthermore, another significant difference between virus and viroids is that the viroids lack protein capsids; hence, they do not produce proteins during the replication.
Below infographic shows more details regarding the difference between virus and viroids.
Summary – Virus vs Viroids
Viruses and viroids are infectious particles that are very small. Viruses contain both a nucleic acid genome and a protein coat. But, viroids have only single-stranded RNA molecules. They lack a protein capsid. Thus, this is the key difference between virus and viroids. Furthermore, they also differ in their host organisms. Viruses infect all types of organisms while viroids infect only higher plants.
Reference:
1.“Viruses and Viroids.” Viruses and Viroids – an Overview | ScienceDirect Topics, Available here.
2. Sengbusch, Peter v. “Plant Viruses and Viroids.” Botany Online: Plant Viruses and Viroids, Available here.
Image Courtesy:
1. “1812092” (CC0) via Pixabay
2. “Viroids- how it do” By Liquiddeer – Own work (CC BY-SA 4.0) via Commons Wikimedia