Both viroids and prions are Strict intracellular, Subcellular infectious agents, that always requires a living host for replication. Besides there are many differences between Viroids and Prions.
Differences between Viroids and Prions
• Definition :
Viroids
- Viroids are infectious RNA particles without protein coat.
- Viroids are just virus-like.
- It was discovered by T. Diener.
- The first viroid discovered was PSTV (Potato spindle tuber viroid) and he called it as virus like or viroid.
- Viroids are the smallest known Infectious agents.
Prions
- Prions are infectious protein particles that is discovered by Stanley Prisoner in 1982.
- Prions cause Neurodegenerative diseases.
- Prions is actually a misfolded protein.
• Chemical Nature and Size
Viroids
- Viroids are made up of RNA.
- It is always small circular single stranded RNA molecule.
- The size of the genome is 5 to 10 times smaller than the smallest DNA and RNA viral genomes.
- Generally the size of viroids genome ranges from 246 to 463 nucleotides.
Prions
- Prions are actually misfolded proteins.
- It is called as PRPc (Prionprotein cellular)
- Prion protein is made up of 209 amino acids and the function is it is having a neuroprotective function.
- This prion (PRPc) is converted to PRPsc which is called as Prion Protein Scrapie; the disease that occurs in sheep.
- The structure of PRPsc is very stable with beta sheets and it accumulates in nerve cells as amyloids and ultimately kill neurons.
• The Replication
Viroids
- In viroid replication is done by an RNA-based rolling circle mechanism; using RNA polymerase 2 of the host.
- Viroid the RNA does not code for any protein but it can act as ribozyme. so it can effectively use the machinery of the host.
Prions
- In the case of prion, The PRPc is converted to PRPsc.
- The normal protein PRPc is converted to non-abnormal protein or disease causing protein PRPsc.
- There are two models proposed regarding the replication of Prion,
- One is heterodimer model and the second is fibrillar model; both suggest that conversion of PRPc to PRPsc is happening during the replication process.
• The Hosts and Examples
Viroids
- Viroids infects only plants.
- Potato spindle tuber viroid (PSTV) is causing potato spindle tuber disease.
- Other viroids diseases include :
- Tomato plant maco viroid (TPMV).
- Avocado sunblotch viroid (ASBV)
- Peach latent mosaic viroid (PLMV) etc.
Prions
- Prions causes Neurodegenerative diseases in animals.
- In cattle it causes bovine spongi form encephalopathy (BSE) which is called a "mad cow disease".
- In humans it causes neurodegenerative diseases like CJD disease and Kuru disease etc.
These are the basic differences between Prions and Viroids.
