What is the Difference Between Virus and Viroids

The main difference between virus and viroids is that a virus is a small infectious agent, which can only replicate inside living cells whereas viroids are the smallest form of infectious agents, which only infect plants. Furthermore, the virus is a nucleoprotein particle whose nucleic acid can be either DNA or RNA while viroids are RNA particles. Moreover, the virus consists of a protein coat known as the capsid while viroids lack a protein coat.

Viruses and viroids are two forms of infectious particles, which can reproduce exclusively inside a host cell.

Key Areas Covered

1. What is a Virus
     – Definition, Structure, Importance
2. What are Viroids
     – Definition, Structure, Importance
3. What are the Similarities Between Virus and Viroids
     – Outline of Common features
4. What is the Difference Between Virus and Viroids
     – Comparison of Key Differences

Key Terms

Capsid, Host, Infectious Agents, Nucleic Acids, Virions, Viroids, Virus

What is a Virus

A virus is a non-living, small, infectious agent, which can only replicate inside a host cell. Generally, viruses are not equipped with the cellular mechanisms required by the replication including DNA replication and protein synthesis. Therefore, they have to depend on a host cell to replicate their nucleic acids and to synthesize their protein coat. Hence, viruses are obligate, intracellular parasites, which invade cells, causing diseases.

Figure 1: Structure of a Virus

Furthermore, outside the host cells, viruses occur as independent particles known as virions. A virion consists of genetical material, which can be either DNA or RNA. Therefore, based on the type of nucleic acids present in the genome, we can classify viruses as DNA viruses and RNA viruses. Also, their DNA and RNA can be either single-stranded or double-stranded.

Moreover, a virion consists of a protein coat, surrounding the genetic material known as the capsid. Thus, based on the shape of the capsid, viruses are classified as helical, icosahedral, prolate, and complex viruses. Some of the virions consist of an envelope made up of lipids surrounding the capsid. In general, the size of the most viruses ranges between 20 and 300 nm.

What are Viroids

Viroids are subviral agents known as the smallest infectious particles, which are even smaller than a virus. Also, they can only reproduce inside a host, especially a plant cell. Generally, viroids do not contain a capsid or an envelope. Therefore, viroids do not produce any proteins during their replication inside the host. The main structural component of a viroid is a short strand of circular, single-stranded RNA molecule whose size ranges from 246 to 467 nucleobases.

Figure 2: Putative Secondary Structure of the PSTV (Potato Spindle Tuber Viroid)

Moreover, viroids spread during either plant propagation via cutting or tubers, through seeds or due to the manual mishandling with contaminated implements. They infect plants such as potatoes, cucumbers, tomatoes, chrysanthemums, avocados, and coconut palms. Unfortunately due to the crop failure resulted by viroid infections, millions of dollars are going to lose each year.

Similarities Between Virus and Viroids

• Virus and viroids are two types of small, infectious agents.
• They are non-living particles, and their replication occurs only inside a living cell.
• Also, they lack cellular machinery for their replication.
• However, both particles contain nucleic acids.
• Besides, both cause diseases in their host.

Difference Between Virus and Viroids

Definition

A virus refers to a small infectious agent that replicates only inside the living cells of other organisms while viroids refer to the infectious particles smaller than any of the known viruses, serving as an agent of certain plant diseases. Thus, this is the main difference between virus and viroids.

Significance

Also, an important difference between virus and viroids is that a virus is a nucleoprotein particle while viroids are RNA particles.

Size

Furthermore, a virus is a small particle while the viroids are smaller than viruses.

Type of Nucleic Acid

Moreover, viruses contain either DNA or RNA as their nucleic acids while viroids contain a short strand of circular, single-stranded RNA as their nucleic acid. Hence, this is another difference between virus and viroids.

Protein Coat

Additionally, one other difference between virus and viroids is that the virus contains a protein coat surrounding its nucleic acid while viroids do not contain a protein coat.

Type of Host

Besides, different types of viruses can infect different forms of hosts including animal, plant or bacterial cells but, viroids infect only plant cells.

Production of Proteins during Replication

The virus produces proteins during their replication while viroids do not produce proteins during replication. Thus, this is also a difference between virus and viroids.

Examples

Some of the examples of viruses are EBV, adenoviruses, hepatitis B, influenza A, etc. while some of the examples of viroids are potato spindle tuber viroid, Avsunviroidae, etc.

Conclusion

A virus is a small, infectious particle whose replication exclusively occurs inside a living cell as the virus does not contain cellular mechanisms for DNA replication or protein synthesis. Viruses are made up of a nucleic acid, either DNA or RNA surrounded by a protein coat known as the capsid. Generally, they can infect all types of living cells. In comparison, viroids are the smallest infectious particles, which are also smaller than viruses. They are made up of a single-stranded RNA molecule, and they do not contain a capsid. Significantly, viroids can only infect plant cells. Therefore, the main difference between a virus and viroids is the structure and the type of host.

References:

1. Ronca, Debra. “How Are Viruses, Viroids and Prions Related?” HowStuffWorks Science, HowStuffWorks, 8 Mar. 2018, Available Here.

Image Courtesy:

1. “Virus stucture simple” By GrahamColmTalk – I created this work entirely by myself. (CC BY-SA 3.0) via Commons Wikimedia  
2. “PSTviroid” By Author: Jakub Friedl (user kyknos) – Source file with layers (gimp xcf file): (CC BY-SA 3.0) via Commons Wikimedia