5.8 Prions and Viroids

This section outlines acellular infectious particles that are simpler and smaller than viruses, challenging the traditional view that infectious agents require both nucleic acids and proteins.

Prions

Definition: Prions are infectious protein particles, smaller than viruses, that are unique for containing no nucleic acids (neither DNA nor RNA).

Composition: Composed of a single, abnormally folded protein encoded by a cellular gene.

Resistance: They are significantly more resistant to inactivation by heat and ultraviolet light than viruses.

Mechanism of Disease: Prion proteins, primarily in brain cells, induce normal proteins to misfold, causing them to clump together. This leads to neurodegeneration, memory loss, and permanent brain damage.

Diseases: They cause fatal neurodegenerative diseases:

• Kuru in humans
• Mad Cow Disease (Bovine Spongiform Encephalopathy) in cattle

Transmission: Spreads through body fluids, as well as direct and indirect contact.

Treatment: There is currently no treatment or cure for prion diseases.

Viroids

Definition: Viroids are pathogens consisting of a short, circular, single-stranded RNA molecule.

Composition: They are "naked" RNA molecules, lacking a protein coat (capsid) or envelope.

Replication:

• The viroid's RNA does not code for any protein.
• It uses the host cell's enzyme, RNA polymerase II, to replicate its own RNA, using its RNA as a template.

Catalytic Properties: Some viroids are ribozymes - RNA molecules with catalytic properties that allow them to self-cleave and process their replicated RNA.

Diseases:

• Human: The only known human disease caused by a viroid is Hepatitis D.
• Plant: They are a significant cause of plant diseases, such as potato spindle tuber disease and cucumber pale fruit disease.

Transmission: Spread in plants via mechanical damage, seeds, pollen, and biological vectors (like insects).

Comparison of Prions and Viroids

Biological Significance

The existence of prions and viroids demonstrates that nucleic acids and proteins are not always required together for an agent to be infectious and pathogenic, expanding our understanding of disease. These particles represent the simplest known infectious agents, even simpler than the .