5.8.2 Viroids

Definition: Viroids are pathogens that consist of a short, circular, single-stranded RNA without a protein coat or envelope.

Key Points:

• Viroid RNA does not code for any protein.
• The replication mechanism involves an enzyme RNA polymerase II, which synthesizes new RNA using the viroid's RNA as a template.
• Some viroids are ribozymes, having catalytic properties which allow self-cleavage.
• The only human disease known to be caused by a viroid is hepatitis D.
• Viroids cause several plant diseases, e.g.:
• Potato spindle tuber disease
• Cucumber pale fruit disease
• Viroids spread via mechanical damage, seed, pollen, or biological vectors.

Characteristics of Viroids:

Diseases Caused by Viroids:

Comparison of Prions and Viroids

Comparison of Prions, Viroids, and Viruses

Intermediate Level

Prions in Detail

Diseases Caused by Prions:

Viroids in Detail

Diseases Caused by Viroids:

Comparison of Prions and Viroids

Important Points

• 1. The idea of an infectious agent that did not use nucleic acids and proteins together was considered impossible.
• 2. Pioneering work by Nobel Prize-winning biologist Stanley Prusiner has convinced the scientific community that prions exist.
• 3. Prions:
• - Prions are infectious protein particles, smaller than viruses, that contain no nucleic acids (neither DNA nor RNA).
• - Prions are much more resistant to inactivation by ultraviolet light and heat than are viruses.
• - Prions are composed of a single protein.
• - This protein is encoded by a single cellular gene.
• - Prion proteins on brain cells cause clumping, memory loss, neurodegeneration, and permanent damage.
• - Fatal neurodegenerative diseases, such as Kuru in humans and mad cow disease in cattle, were shown to be transmitted by prions.
• - Prions spread through body fluids, direct and indirect contact.
• - There is no treatment or cure of prion diseases.
• 4. Viroids:
• - Viroids are pathogens that consist of a short, circular, single-stranded RNA without a protein coat or envelope.
• - Viroid RNA does not code for any protein.
• - The replication mechanism involves an enzyme RNA polymerase II, which synthesizes new RNA using the viroid's RNA as template.
• - Some viroids are ribozymes, having catalytic properties which allow self-cleavage.
• - The only human disease known to be caused by a viroid is hepatitis D.
• - Viroids cause several plant diseases, e.g., potato spindle tuber disease, cucumber pale fruit disease, etc.
• - Viroids spread via mechanical damage, seed, pollen, or biological vectors.

Remember

• - Prions = infectious proteins; no nucleic acid
• - Viroids = infectious RNA; no protein coat
• - Prions = smaller than viruses
• - Viroids = smaller than viruses
• - Prions = Kuru, mad cow disease
• - Viroids = potato spindle tuber disease, hepatitis D
• - Prions = resistant to UV light and heat
• - Viroids = ribozymes (some)
• - Both = no treatment or cure

Common Mistakes

• 1. Confusing prions and viroids – Prions = protein; Viroids = RNA.
• 2. Thinking prions contain nucleic acids – Prions contain no nucleic acids.
• 3. Thinking viroids have a protein coat – Viroids have no protein coat.
• 4. Confusing prions and viruses – Prions are smaller and contain no nucleic acids.
• 5. Confusing viroids and viruses – Viroids are smaller and have no protein coat.
• 6. Thinking prions can be treated – There is no treatment or cure for prion diseases.
• 7. Thinking viroids only cause plant diseases – Viroids also cause hepatitis D in humans.
• 8. Forgetting the role of Stanley Prusiner – He discovered prions and won the Nobel Prize.
• 9. Thinking viroids code for proteins – Viroids do not code for any protein.
• 10. Confusing the transmission of prions and viroids – Prions = body fluids, contact; Viroids = mechanical, seed, pollen, vectors.
• 11. Thinking prions are alive – Prions are not alive; they are infectious proteins.
• 12. Forgetting the diseases – Kuru, mad cow disease (prions); potato spindle tuber disease, hepatitis D (viroids).

Quick Revision

• The idea of an infectious agent that did not use nucleic acids and proteins together was considered impossible, but pioneering work by Nobel Prize-winning biologist Stanley Prusiner has convinced the scientific community that prions exist.
• Prions:
• Prions are infectious protein particles, smaller than viruses, that contain no nucleic acids (neither DNA nor RNA).
• Prions are much more resistant to inactivation by ultraviolet light and heat than are viruses.
• Prions are composed of a single protein.
• This protein is encoded by a single cellular gene.
• Prion proteins on brain cells cause clumping, memory loss, neurodegeneration and permanent damage.
• Fatal neurodegenerative diseases, such as Kuru in humans and in cattle mad cow disease were shown to be transmitted by prions.
• Prions spread through body fluids, direct and indirect contact.
• There is no treatment or cure of prion diseases.
• Viroids:
• Viroids are pathogens that consist of a short, circular, single-stranded RNA without a protein coat or envelope.
• Viroid RNA does not code for any protein.
• The replication mechanism involves an enzyme RNA polymerase II, which synthesizes new RNA using the viroid's RNA as template.
• Some viroids are ribozymes, having catalytic properties which allow self-cleavage.
• The only human disease known to be caused by a viroid is hepatitis D.
• Viroids cause several plant diseases, e.g., potato spindle tuber disease, cucumber pale fruit disease, etc.
• Viroids spread via mechanical damage, seed, pollen, or biological vectors.

60-Second Revision

• Prions = infectious proteins; no nucleic acid
• Viroids = infectious RNA; no protein coat
• Prions = smaller than viruses
• Viroids = smaller than viruses
• Prions = Kuru, mad cow disease
• Viroids = potato spindle tuber disease, hepatitis D
• Prions = resistant to UV light and heat
• Viroids = ribozymes (some)
• Both = no treatment or cure
• Warning: Prions and viroids are not the same. Prions contain no nucleic acids. Viroids have no protein coat. Both are smaller than viruses. Prions cause brain diseases; viroids cause plant diseases and hepatitis D. There is no treatment or cure for either. Stanley Prusiner discovered prions and won the Nobel Prize. Viroids do not code for proteins. Some viroids are ribozymes.