Model Answer
0 min readIntroduction
Prions and viroids represent unique challenges to our understanding of infectious agents, as they deviate significantly from the conventional virus-host paradigm. Discovered in the 1980s, these acellular entities are responsible for several neurodegenerative diseases in animals and humans. While both are infectious and cause disease, they differ fundamentally in their composition and mode of replication. Understanding these differences is crucial for developing effective diagnostic and therapeutic strategies. This answer will delineate the nature of prions and viroids, contrasting their characteristics and detailing prion transmission pathways.
Prions: Infectious Proteins
Prions, short for "proteinaceous infectious particles," are misfolded proteins that have the ability to induce normal proteins to adopt the same misfolded shape. This misfolding leads to the formation of aggregates that accumulate in the brain, causing neurodegenerative diseases. The prion protein (PrP) exists in two forms: a normal, cellular form (PrPC) and an abnormal, infectious form (PrPSc). PrPSc is resistant to degradation and triggers the conversion of PrPC to PrPSc, initiating a chain reaction.
Viroids: Infectious RNA
Viroids are the smallest known infectious agents, consisting solely of a small, circular, single-stranded RNA molecule. Unlike viruses, they lack a protein coat. Viroids replicate autonomously within host cells, utilizing the host's RNA polymerase to amplify their RNA genome. They primarily infect plants, causing a variety of diseases characterized by stunted growth and leaf abnormalities.
Key Differences: Prions vs. Viroids
The fundamental difference lies in their composition. Prions are composed entirely of protein, while viroids are composed solely of RNA. This difference dictates their replication mechanisms. Prions propagate by converting normal proteins, whereas viroids replicate using the host's enzymatic machinery. Furthermore, prions primarily affect animals (including humans), while viroids are known to infect only plants.
| Feature | Prions | Viroids |
|---|---|---|
| Composition | Misfolded protein (PrPSc) | Small, circular RNA |
| Protein Coat | Absent | Absent |
| Replication | Conversion of normal proteins (PrPC to PrPSc) | Host RNA polymerase-mediated replication |
| Host Range | Animals (including humans) | Plants |
| Disease Examples | Creutzfeldt-Jakob Disease (CJD), Bovine Spongiform Encephalopathy (BSE) | Potato Spindle Tuber Disease, Citrus Exocortis Disease |
Prion Transmission
Prion transmission can occur through several routes:
- Sporadic: Prions arise spontaneously due to the misfolding of normal proteins. This is the most common form of CJD in humans.
- Inherited: Genetic mutations in the PRNP gene (encoding PrP) increase the likelihood of misfolding and prion disease.
- Acquired: This occurs through exposure to prion-contaminated materials. Examples include:
- Iatrogenic: Transmission through medical procedures, such as contaminated surgical instruments or corneal transplants.
- Consumption of contaminated meat: Variant CJD (vCJD) is linked to the consumption of beef from cattle infected with BSE ("mad cow disease").
Prions are remarkably resistant to conventional sterilization methods, such as autoclaving and irradiation, requiring specialized procedures for inactivation.
Conclusion
In conclusion, prions and viroids represent fascinating and challenging infectious agents. While both are acellular and cause disease, their fundamental differences in composition – protein versus RNA – dictate their replication strategies and host ranges. Prion transmission, particularly through sporadic, inherited, and acquired routes, poses significant public health concerns, necessitating stringent surveillance and preventative measures. Further research into the mechanisms of prion misfolding and viroid replication is crucial for developing effective therapies and mitigating the risks associated with these unique pathogens.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.