Difference Between Prions and Viroids

Key Difference – Prions vs Viroids
 

Infectious particles cause diseases in plants, animals, and other organisms. There are different types of infectious agents such as bacteria, fungi, protozoans, viruses, viroids, prions, etc. Viroids and prions are small infectious particles which behave like virus particles. However, both types are structurally different from a typical viral particle. Viruses consist of two major parts: genetic material and protein capsid. Viroids and prions contain either genetic material or protein capsid. Viroids can be defined as small and naked infectious RNA molecules that cause diseases in higher plants. Prions can be defined as small proteinaceous particles that cause diseases in animals including humans.  The key difference between prions and viroids is that prions do not contain nucleic acids while viroids do not contain proteins.

CONTENTS

1. Overview and Key Difference
2. What are Prions 
3. What are Viroids
4. Similarities Between Prions and Viroids
5. Side by Side Comparison – Prions vs Viroids in Tabular Form
6. Summary

What are Prions?

A prion is an infectious protein particle made up of amino acid chains. They do not contain nucleic acids such as DNA or RNA. Most prions are smaller than viroids. Prions infect animals, causing neurological degenerative diseases such as mad cow disease (bovine spongiform encephalopathy) in cows, scrapie disease in sheep and goats, kuru and gerstmann-strausler-sheinker diseases in humans, creutzfeldt-jakob disease, etc. Kuru and mad cow diseases are very common and their symptoms include the loss of motor control and unusual behaviors. Prion diseases can arise in three different ways named, acquired, familial and sporadic. However, the primary method of prion infection in animals is through ingestion.

Prions have a very long incubation period in hosts. Since prions are proteins, they can be digested by proteinase K and trypsin. However, prions are resistant to ribonucleases. They are also highly resistant to heat, chemical agents, and irradiation. Prions are able to self-replicate. However, they are not considered as viruses. They serve as a separate infectious group.

What are Viroids?

A viroid is an infectious RNA particle formed from a single-stranded circular RNA. Viroids were first discovered and named by the plant pathologist Theodor O. Diener in 1971. The first viroid identified was Potato Spindle Tuber Viroid (PsTVd) and thirty-three species of viroids have been identified up to now. Viroids do not contain a protein capsid or an envelope. They are made up of only RNA molecules. Since viroids are RNA particles, they can be digested by ribonucleases. But unlike prions, viroids cannot be destroyed by proteinase K and trypsin. The size of the viroid is smaller than a typical virus particle. Viorids need a host cell for reproduction. Other than a single stranded RNA molecule, they do not synthesize proteins.

Viroids do not cause human diseases. They infect higher plants and cause diseases like potato spindle tuber disease, and chrysanthemum stunt disease. These infectious RNA particles are responsible for crop failures and subsequently, the loss of millions of money in agriculture annually. Potato, cucumber, tomato, chrysanthemums, avocado and coconut palms are plants which are commonly subjected to viroid infections. Viroid infections are transmitted by cross contamination followed by mechanical damage to the plant. Some viroid infections are transmitted by aphids and leaf to leaf contact.

What are the Similarities Between Prions and Viroids?

• Prions and viroids are pathogenic particles.
• Both types lack either one component of two major components (protein coat and nucleic acids) of viruses.
• Both particles are smaller than viruses.

What is the Difference Between Prions and Viroids?

Summary – Prions vs Viroids

Prions and viroids are infectious particles which cause diseases in animals and plants, respectively. Prions are small infectious protein molecules which cause diseases in animals. Prions do not contain nucleic acids. Viroids are plant pathogens that possess only a single stranded circular RNA molecule. Viroids do not encode or contain proteins. This is the difference between prions and viroids.

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References:

1. “Prions and Viroids – Boundless Open Textbook.” Boundless. Boundless, 26 May 2016. Web. Available here. 23 June 2017.
2. Diener, T. O., M. P. McKinley, and S. B. Prusiner. “Viroids and prions.”Proceedings of the National Academy of Sciences of the United States of America. U.S. National Library of Medicine, Sept. 1982. Web. Available here. 23 June 2017.

Image Courtesy:

1. “Prion subdomain-colored sec structure” By Cornu (talk) 19:04, 5 June 2009 (UTC) – Own work (CC BY 2.5) via Commons Wikimedia